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Scala Pattern Matching & Decomposition

2014-03-30T21:48:00.001-05:00

In this blog post, I will cover various pattern matching variants that I learned in my journey to learn Scala.

Matching constant values
Wildcard matcher
Variable matcher pattern
Variables and constants (also know as Stable identifier pattern)
Matching types
Matching types and restricting the data with guards
Matching types(tuples and other objects) & decompostion
Matching lists & decomposing lists
Matching sequences, ranges and decomposing them
Pattern matching in catching exceptions
Define multiple vals with patterns
Pattern matching in generators
Decomposing values based on Regular Expressions

I would like introduce val types, tuples, singleton object, case class to help us with the examples for each variant.

// Constants
val TheGood = "The Good"
val TheBad = "The Bad"
val theUgly = "The Ugly"

// A tuple
val tuple = (1, "It is me")

// A singleton object
object Singleton

// A case class
case class Container(x:Any)

// Singleton object with constants
object Constants {
val pi = "3.14"
val PI = 3.14
}

Matching constant values:

The following function matches values if x is 3.14 or "3.14". For all other values it raises scala.MatchError exception indicating that, the pattern matching couldn't fall in any case listed.

def matchConstant(x:Any) = x match {
case 3.14 => "PI"
case "3.14" => "PI value as String"
}

If you invoke the above function with various values as below:

scala> matchConstant(3.14)
res1: String = PI

scala> matchConstant("3.14")
res2: String = PI value as String

scala> matchConstant(2)
scala.MatchError: 2 (of class java.lang.Integer)

Wildcard matcher:

We got scala.MatchError as there is no case, that could match the input integer 2. We can fix this by using wildcard matcher pattern so that we can prevent the MatchError.

Added wildcard mather to the matchConstant function:

def matchConstant(x:Any) = x match {
case 3.14 => "PI"
case "3.14" => "PI value as String"

// Wildcard matcher
case _ => "Unknown"
}

Now invocation of matchConstant with integer parameter 2 results as "Unknown"

scala> matchConstant(2)
res5: String = Unknown

Variable matcher pattern

Certain times you may want to match any value you pass-in. It can be achieved using wildcard. Another alternate way to match is variable pattern. The following example illustrate the variable pattern

def matchAnything(x:Any) = x match {
// Variable pattern
case n => s"Matched value $n"
}

Invoking the above function:

scala> matchAnything(1)
res40: String = Matched value 1

scala> matchAnything("1")
res41: String = Matched value 1

scala> matchAnything(Singleton)
res42: String = Matched value Singleton$@1d20b21

Variables and constants (also know as Stable identifier pattern)

The val types identifers and singletons fall under this category. Scala language enforces few rules to match stable identifiers.

The identifier for constants must start with a capital letter. For example, TheGood and TheBad adhere to this rule.
If the constant identfier didn't start with a capital letter, we need to use backtick(`) . For example theUgly
If the constant identifer didnt' start with a capital letter and it qualified the pattern matching will work. For example, Constants.pi is qualified.
import Constants._
def matchStableIdentifiers(x:Any) = x match {
case TheGood=> "The Good"
case TheBad=> "The Bad"
case `theUgly` => "The ugly"

// case `pi` => s"Matched with backticks pi value $x"
case Constants.pi => s"Constants.pi has value $x"
case PI => s"PI value $x"
case Singleton => "The singleton"

// Wildcard pattern
case _ => s"Any value $x"
}

Let us invoke the matchStableIdentifiers function

scala> matchStableIdentifiers("The Good")
res19: String = The Good

scala> matchStableIdentifiers("The Bad")
res20: String = The Bad

scala> matchStableIdentifiers("The Ugly")
res21: String = The ugly

scala> matchStableIdentifiers(3.14)
res22: String = PI value 3.14

scala> matchStableIdentifiers("3.14")
res23: String = Constants.pi has value 3.14

scala> matchStableIdentifiers(Singleton)
res24: String = The singleton

scala> matchStableIdentifiers("No where to go!")
res25: String = Any value No where to go!

Matching types:

The pattern matching can be applied to based on type of the object. This is similar to instanceof operator in Java.

In the following example we match x to case b:Boolean if x hold a boolean type value. Similarly if x holds a value of double type then case d:Double =>... will be matched.

def matchByType(x: Any) = x match {
case b:Boolean => s"A boolean value: $b"
case d:Double => s"An double value: $d"
case _ => "Unknown type"
}

Let us invoke matchByType function

scala> matchByType(true)
res26: String = A boolean value: true

scala> matchByType(false)
res27: String = A boolean value: false

scala> matchByType(1)
res28: String = Unknown type

scala> matchByType(1.0)
res29: String = An double value: 1.0

Matching types and restricting the data with guards:

Let us extends pattern matching to match specific values of a specific type. In the following example we use pattern matching to print a number is even or odd.

def matchValueOfTypeWithGuard(x: Any) = x match {
case x:Int if x % 2 == 0 => s"The input is an even number : $x"
case x:Int if x % 2 != 0 => s"The input is a odd number : $x"
case _ => "Unknown type"
}

Let us invoke the above function with various values:

scala> matchValueOfTypeWithGuard(2)
res30: String = The input is an even number : 2

scala> matchValueOfTypeWithGuard(3)
res31: String = The input is a odd number : 3

scala> matchValueOfTypeWithGuard("R")
res32: String = Unknown value

Matching types(tuples and other objects) & decompostion:

Let me show examples of another variant in matching types and also decompose them.

Following example, matches a tuple and decomposes it to extract objects used in constructing the tuple.

def matchByTypeAndDecompostion(x : Any) = x match {

// Matches a tuple and extracts objects used in contructing the tuple
case (x, y) => s"A tuple with values $x and $y"

case (x, y, z) => s"A tuple with values $x, $y and $z"

// Contructor decomposition pattern
case Container(x) => s"Container with value: $x"

case _ => "Unknown type"
}

Let us invoke the above function with various values:

scala> matchByTypeAndDecompostion((1, (1,2)))
res36: String = A tuple with values 1 and (1,2)

scala> matchByTypeAndDecompostion((1, (1,2), (1,2,3)))
res37: String = A tuple with values 1, (1,2) and (1,2,3)

scala> matchByTypeAndDecompostion(Container(3))
res38: String = Container with value: 3

scala> matchByTypeAndDecompostion(Container("Hello"))
res39: String = Container with value: Hello

Matching lists & decomposing lists:

Following examples demonistrate pattern matching List objects and also decomposing the list objects

def matchList(x:Any) = x match {
// matches a List(1,2,3)
case List(1, 2, 3) => "Matched List(1, 2, 3)"

// Matches a list with only one element and extracts the element by decompising the list
case x :: Nil => s"Only one element in the list $x"

// Matching a list with single element and extracting the element, can be achieved using constructor decomposition
//case List(x) => s"Only one element in the list $x"

// Matches a list with head as integer 2
case 2 :: xs => s"A list with head 2 and tail $xs"

// Matches a list that has integer 3 followed by 4 and other elements.
// Decomposes list and extract rest of the list excluding the first two elements
case List(3, 4, xs @ _*) => s"A list with 3, 4 and tail $xs"

// Decomposing a list as head and tail
case x :: xs => s"A list with head $x and tail $xs"

// Matching a empty list or null
case nil => "Empty List"
// Matching a empty list using contructor decomposition??
//case List() => "Empty List"
}

Let us invoke the function to match a specific case

scala> matchList(List(1,2,3))
res49: String = Matched List(1, 2, 3)

scala> matchList(List(1))
res50: String = Only one element in the list 1

scala> matchList(List(2,3,4))
res51: String = A list with head 2 and tail List(3, 4)

scala> matchList(List(3,4,5,6,7,8,9))
res52: String = A list with 3, 4 and tail List(5, 6, 7, 8, 9)

scala> matchList(List(4,5,6,7,8,9))
res53: String = A list with head 4 and tail List(5, 6, 7, 8, 9)

scala> matchList(List())
res54: String = Empty List

scala> matchList(null)
res55: String = Empty List

Matching sequences, ranges and decomposing them:

Following example illustrates matching sequences, ranges and decompose them.

def matchSequence[E](x: Seq[E]) = x match {
case x +: Nil => s"Only element is head: $x"
case _ :+ x => s"Last element $x"
case nil => "Empty Sequence"
}

Let us invoke the above function with ranges and sequences

scala> matchSequence( 1 to 3)
res56: String = Last element 3

scala> matchSequence( 1 to 3 take 1)
res57: String = Only element is head: 1

scala> matchSequence( 1 to 3 take 0)
res58: String = Empty Sequence

scala> matchSequence(Seq(1,2,3,4))
res64: String = Last element 4

scala> matchSequence(Seq(1))
res65: String = Only element is head: 1

Pattern matching in catching exceptions

The following example demonistrates catching various exception using pattern matching.

def stringToPositiveIntOnly(s: String) = try {
val i = s.toInt
println(s"Input value $i")
assert(i > 0)
} catch {
case e: NumberFormatException => "Not a number"
case e:java.lang.AssertionError => "Assertion failed"
}

Let us invoke the above function with various values

scala> stringToPositiveIntOnly(null)
res84: Any = Not a number

scala> stringToPositiveIntOnly("-1")
Input value -1
res85: Any = Assertion failed

scala> stringToPositiveIntOnly("1")
Input value 1
res86: Any = ()

Defining multiple vals with patterns:

The following examples illustrate pattern matching and decompose them to multiple vals

scala> val (a, b) = (1, ("Ramesh", "NY"))
a: Int = 1
b: (String, String) = (Ramesh,NY)

Pattern matching in generators:

for ( keyAndValue <- -="" vector=""> "a", 2 -> "b") )
yield keyAndValue._2

for ( (k, v) <- -="" 1="" vector=""> "a", 2 -> "b") )
yield v

Decomposing values based on Regular Expressions:

Regular expression can be used to match certain values and decompse the input to extract interesting parts. Notice that the decomposition will be based on the groups matched by regular expression

// Regular Expressions with only one groups
val Yes = "(y|Y)es".r

// Checking regex works
Yes.pattern.matcher("yes").matches
Yes.unapplySeq("yes") will result: Option[List[String]] = Some(List(y))

def catchYes(s:String) = s match {
case Yes(x) => s
case _ => None
}

catchYes("yes")
catchYes("Yes")

// Regular Expressions with multiple groups
val Yes = "(y|Y)(es)".r

// Checking regex works
Yes.pattern.matcher("yes").matches
Yes.unapplySeq("yes") will result: Option[List[String]] = Some(List(y, es))

def catchYes(s:String) = s match {
// As regex has two groups we should have enough parameter to extract
case Yes(x,y) => s
case _ => None
}

catchYes("yes")
catchYes("Yes")

Take away from the talk "Simplicity in Scala Design" by Bill Venners

2014-03-30T21:18:00.004-05:00

Take away from the talk "Simplicity in Scala Design" by Bill Venners

Simplicity in Scala Design" by Bill Venners

- Design for busy teams. Make the user use the library with out reading about it. Just like the car manual no one will read it when renting the car.

- Make it obvious, guessable, or easy to remember.

- Design for readers and then for writers.
Make both parties happy. When you cannot favor reader. Make it hard to type when you want to discourage an specific method by choose a longer name

- Make errors impossible or difficult.
Enforce it via type errors if possible Let the user be specific when getting functionality based on contracts

- Document with examples.
  Provide samples examples in documentation. Provide good examples as people tend to copy.

- Minimize redundancy.
  There should be one -- and preferably only one -- obvious way to do it.
  In scalatest should vs must. Only one choice should have been there.

- Maximize consistency.
API should be consistent.
Consider Set, Map and Seq first two are immutable whereas Seq is not.
Providing variations won't leave a choice to readers. Where as writer can pick one.

- Minimize the magic.
Avoid implicits

"Remember what tools are for. They are for solving problems, not finding problems to solve"
- Terry Wall

Scala Pattern Matching & Decomposition

2012-10-31T20:46:00.004-05:00

Scala Pattern Matching & Decomposition

In this blog post, I will cover various pattern matching variants that I learned in my journey to learn Scala.

Matching constant values
Wildcard matcher
Variable matcher pattern
Variables and constants (also know as Stable identifier pattern)
Matching types
Matching types and restricting the data with guards
Matching types(tuples and other objects) & decompostion
Matching lists & decomposing lists
Matching sequences, ranges and decomposing them
Pattern matching in catching exceptions
Define multiple vals with patterns
Pattern matching in generators
Decomposing values based on Regular Expressions

I would like introduce val types, tuples, singleton object, case class to help us with the examples for each variant.

// Constants
val TheGood = "The Good"
val TheBad = "The Bad"
val theUgly = "The Ugly"

// A tuple
val tuple = (1, "It is me")

// A singleton object
object Singleton

// A case class
case class Container(x:Any)

// Singleton object with constants
object Constants {
    val pi = "3.14"
    val PI = 3.14
}

Matching constant values:
The following function matches values if x is 3.14 or “3.14”. For all other values it raises scala.MatchError exception indicating that, the pattern matching couldn’t fall in any case listed.

def matchConstant(x:Any) = x match {
    case 3.14   => "PI"
    case "3.14" => "PI value as String"
}

If you invoke the above function with various values as below:

scala> matchConstant(3.14)
res1: String = PI

scala> matchConstant("3.14")
res2: String = PI value as String

scala> matchConstant(2)
scala.MatchError: 2 (of class java.lang.Integer)

Wildcard matcher:
We got scala.MatchError as there is no case, that could match the input integer 2. We can fix this by using wildcard matcher pattern so that we can prevent the MatchError.
Added wildcard mather to the matchConstant function:

def matchConstant(x:Any) = x match {
    case 3.14   => "PI"
    case "3.14" => "PI value as String"

    // Wildcard matcher
    case _ => "Unknown"
}

Now invocation of matchConstant with integer parameter 2 results as “Unknown”

scala> matchConstant(2)
res5: String = Unknown

Variable matcher pattern
Certain times you may want to match any value you pass-in. It can be achieved using wildcard. Another alternate way to match is variable pattern. The following example illustrate the variable pattern

def matchAnything(x:Any) = x match {
    // Variable pattern
    case n => s"Matched value $n"
}

Invoking the above function:

scala> matchAnything(1)
res40: String = Matched value 1

scala> matchAnything("1")
res41: String = Matched value 1

scala> matchAnything(Singleton)
res42: String = Matched value Singleton$@1d20b21

Variables and constants (also know as Stable identifier pattern)
The val types identifers and singletons fall under this category. Scala language enforces few rules to match stable identifiers.

The identifier for constants must start with a capital letter. For example, TheGood and TheBad adhere to this rule.
If the constant identfier didn’t start with a capital letter, we need to use backtick() . For example.,theUgly`

If the constant identifer didnt' start with a capital letter and it qualified the pattern matching will work. For example, Constants.pi is qualified
import Constants._
def matchStableIdentifiers(x:Any) = x match {

  case TheGood=> "The Good"
  case TheBad=> "The Bad"
  case `theUgly` => "The ugly"

  // case `pi` =>  s"Matched with backticks pi value $x"
  case Constants.pi => s"Constants.pi has value $x"
  case PI => s"PI value $x"
  case Singleton => "The singleton"

  // Wildcard pattern
  case _ => s"Any value $x"

}

Let us invoke the matchStableIdentifiers function

scala> matchStableIdentifiers("The Good")
res19: String = The Good

scala> matchStableIdentifiers("The Bad")
res20: String = The Bad

scala> matchStableIdentifiers("The Ugly")
res21: String = The ugly

scala> matchStableIdentifiers(3.14)
res22: String = PI value 3.14

scala> matchStableIdentifiers("3.14")
res23: String = Constants.pi has value 3.14

scala> matchStableIdentifiers(Singleton)
res24: String = The singleton

scala> matchStableIdentifiers("No where to go!")
res25: String = Any value No where to go!

Matching types:
The pattern matching can be applied to based on type of the object. This is similar to instanceof operator in Java.
In the following example we match x to case b:Boolean if x hold a boolean type value. Similarly if x holds a value of double type then case d:Double =>… will be matched.

def matchByType(x: Any) = x match {
    case b:Boolean => s"A boolean value: $b"
    case d:Double => s"An double value: $d"
    case _ => "Unknown type"
}

Let us invoke matchByType function

scala> matchByType(true)
res26: String = A boolean value: true

scala> matchByType(false)
res27: String = A boolean value: false

scala> matchByType(1)
res28: String = Unknown type

scala> matchByType(1.0)
res29: String = An double value: 1.0

Matching types and restricting the data with guards:
Let us extends pattern matching to match specific values of a specific type. In the following example we use pattern matching to print a number is even or odd.

def matchValueOfTypeWithGuard(x: Any) = x match {
    case x:Int if x % 2 == 0 => s"The input is an even number : $x"
    case x:Int if x % 2 != 0 => s"The input is a odd number : $x"
    case _ => "Unknown type"
}

Let us invoke the above function with various values:

scala> matchValueOfTypeWithGuard(2)
res30: String = The input is an even number : 2

scala> matchValueOfTypeWithGuard(3)
res31: String = The input is a odd number : 3

scala> matchValueOfTypeWithGuard("R")
res32: String = Unknown value

Matching types(tuples and other objects) & decompostion:
Let me show examples of another variant in matching types and also decompose them.
Following example, matches a tuple and decomposes it to extract objects used in constructing the tuple.

def matchByTypeAndDecompostion(x : Any) = x match {

    // Matches a tuple and extracts objects used in contructing the tuple
    case (x, y) => s"A tuple with values $x and $y"

    case (x, y, z) => s"A tuple with values $x, $y and $z"

    // Contructor decomposition pattern
    case Container(x) => s"Container with value: $x"

    case _ => "Unknown type"
}

Let us invoke the above function with various values:

scala> matchByTypeAndDecompostion((1, (1,2)))
res36: String = A tuple with values 1 and (1,2)

scala> matchByTypeAndDecompostion((1, (1,2), (1,2,3)))
res37: String = A tuple with values 1, (1,2) and (1,2,3)

scala> matchByTypeAndDecompostion(Container(3))
res38: String = Container with value: 3

scala> matchByTypeAndDecompostion(Container("Hello"))
res39: String = Container with value: Hello

Matching lists & decomposing lists:
Following examples demonistrate pattern matching List objects and also decomposing the list objects

def matchList(x:Any) = x match {
    // matches a List(1,2,3)
    case List(1, 2, 3) => "Matched List(1, 2, 3)"

    // Matches a list with only one element and extracts the element by decompising the list
    case x :: Nil => s"Only one element in the list $x"

    //  Matching a list with single element and extracting the element, can be achieved using constructor decomposition
    //case List(x) => s"Only one element in the list $x"

    // Matches a list with head as integer 2
    case 2 :: xs  => s"A list with head 2 and tail $xs"

    // Matches a list that has integer 3 followed by 4 and other elements. 
    // Decomposes list and extract rest of the list excluding the first two elements
    case List(3, 4, xs @ _*) => s"A list with  3, 4 and tail $xs"

    // Decomposing a list as head and tail
    case x :: xs => s"A list with head $x and tail $xs"

    // Matching a empty list or null
    case nil => "Empty List"
    // Matching a empty list using contructor decomposition??
    //case List() =>  "Empty List"
}

Let us invoke the function to match a specific case

scala> matchList(List(1,2,3))
res49: String = Matched List(1, 2, 3)

scala> matchList(List(1))
res50: String = Only one element in the list 1

scala> matchList(List(2,3,4))
res51: String = A list with head 2 and tail List(3, 4)

scala> matchList(List(3,4,5,6,7,8,9))
res52: String = A list with  3, 4 and tail List(5, 6, 7, 8, 9)

scala> matchList(List(4,5,6,7,8,9))
res53: String = A list with head 4 and tail List(5, 6, 7, 8, 9)

scala> matchList(List())
res54: String = Empty List

scala> matchList(null)
res55: String = Empty List

Matching sequences, ranges and decomposing them:
Following example illustrates matching sequences, ranges and decompose them.

def matchSequence[E](x: Seq[E]) = x match {
    case x +: Nil => s"Only element is head: $x"
    case _ :+ x => s"Last element $x"
    case nil => "Empty Sequence"
}

Let us invoke the above function with ranges and sequences

scala> matchSequence( 1 to 3)
res56: String = Last element 3

scala> matchSequence( 1 to 3 take 1)
res57: String = Only element is head: 1

scala> matchSequence( 1 to 3 take 0)
res58: String = Empty Sequence

scala> matchSequence(Seq(1,2,3,4))
res64: String = Last element 4

scala> matchSequence(Seq(1))
res65: String = Only element is head: 1

Pattern matching in catching exceptions
The following example demonistrates catching various exception using pattern matching.

def stringToPositiveIntOnly(s: String) = try {
    val i = s.toInt
    println(s"Input value $i")
    assert(i > 0)
} catch {
    case e: NumberFormatException => "Not a number" 
    case e:java.lang.AssertionError => "Assertion failed"
}

Let us invoke the above function with various values

scala> stringToPositiveIntOnly(null)
res84: Any = Not a number

scala> stringToPositiveIntOnly("-1")
Input value -1
res85: Any = Assertion failed

scala> stringToPositiveIntOnly("1")
Input value 1
res86: Any = ()

Defining multiple vals with patterns:
The following examples illustrate pattern matching and decompose them to multiple vals

scala> val (a, b) = (1, ("Ramesh", "NY"))
a: Int = 1
b: (String, String) = (Ramesh,NY)

Pattern matching in generators:

for ( keyAndValue <- -="-" vector="vector"> "a", 2 -> "b") ) 
    yield keyAndValue._2

for ( (k, v) <- -="-" 1="1" vector="vector"> "a",  2 -> "b") )  
    yield v

Decomposing values based on Regular Expressions:
Regular expression can be used to match certain values and decompse the input to extract interesting parts. Notice that the decomposition will be based on the groups matched by regular expression

// Regular Expressions with only one groups
val Yes = "(y|Y)es".r

// Checking regex works
Yes.pattern.matcher("yes").matches
Yes.unapplySeq("yes") will result: Option[List[String]] = Some(List(y))

def catchYes(s:String) = s match {
    case Yes(x) => s
    case _ => None
}

catchYes("yes")
catchYes("Yes")


// Regular Expressions with multiple groups
val Yes = "(y|Y)(es)".r

// Checking regex works
Yes.pattern.matcher("yes").matches
Yes.unapplySeq("yes") will result: Option[List[String]] = Some(List(y, es))

def catchYes(s:String) = s match {
    // As regex has two groups we should have enough parameter to extract
    case Yes(x,y) => s
    case _ => None
}

catchYes("yes")
catchYes("Yes")

Scala - Concurrency - I

2012-08-05T12:31:00.000-05:00

Creating a Future using the global ExecutionContextExecutor

Creating a Future using a custom ExecutionContextExecutor

Successful scenario

Failure Scenario

On Completing the computation

There are very few examples on Scala Concurrency as most of the people using Scala Actors or Akka Actors as they provide more features that cover various use cases.

The SIP-14 provides most of the information related to Scala Concurrency. In this blog post, I'll cover few examples. I will blog on the concurrency as I discover more Scala Concurrency features.

Scala provides a global execution context to manage thread pools across an application. If needed we can provide own thread pools(Executors).

Creating a Future using the global ExecutionContextExecutor

import scala.concurrent.{future, Future}

import scala.concurrent.ExecutionContext.global

The following future will raise an exception or, will compute the sum.

val f: Future[Int] = future {

if ( System.currentTimeMillis % 2 == 0 ) {

throw new Exception("Exception")

}

(1 to 100000000).sum

}

In this case, the required context is implicitly available in the current scope as we imported "scala.concurrent.ExecutionContext.global"

Creating a Future using a custom ExecutionContextExecutor

import scala.concurrent.{future, Future}

import scala.concurrent.ExecutionContext

import java.util.concurrent.Executors

ExecutorContext provides various factory methods to create execution contexts. The following will use a scheduled thread pool to create the context and makes it implicitly available in the current scope of the execution.

implicit val ctx = ExecutionContext.fromExecutor(Executors.newScheduledThreadPool(10))

The following future will raise an exception or, will compute the sum.

val f: Future[Int] = future {

if ( System.currentTimeMillis % 2 == 0 ) {

throw new Exception("Exception")

}

(1 to 100000000).sum

}

Successful scenario

If you are interested only about a success use scenario use onSuccess callback as shown below:

f onSuccess {

case a:Int => a;

}

Failure Scenario

If you are interested only about a failure use scenario use onFailure callback as shown below:

f onFailure {

case e: Exception => e.printStackTrace

case _ => _

}

As a failure can happen for various reasons, it is better to provide default case statement. If not a MatchError will be thrown by onFailure callback.

On Completing the computation

Certain times, one may be interested to work with the result and error too. The Either[L,R] acts as a holder to the result of the computation or the error raised when doing the computation. In this case onComplete callback will help.

f.onComplete{

case Right(x) => "Result: " + x)

case Left(x) => "Error: " + x

}

In a later post, I will cover map, flatMap, filter and foreach combinators. These combinators will enable for comprehensions.

Scala - No checked exceptions

2012-08-05T02:46:00.001-05:00

Can the interface implementers choose the exceptions (checked exception vs runtime exception) they want to throw with out breaking the service contract?

Well, the answer is a yes!. It is possible in Java by leveraging generics.

 
interface Processor {
 public R process(I i) throws E;
} 

class StringProcessor implements Processor {
 public String process(String str)  {
  return "Hello! " + str ;
 }
}

class StringProcessor2 implements Processor {
 public String process(String str) throws Exception {
  return "Hello! " + str;
 }
}

public class Exceptions {
 public static void main(String ... args) throws Exception {
  Processor p = new StringProcessor();
  System.out.println(p.process("Ramesh"));
  Processor p1 = new StringProcessor2();
  System.out.println(p1.process("Ramesh");
 }
}

Converting the same interface contract to scala is impossible as scala didn't have
the concept of checked exception. When I posed this question, on Scala IRC <dobblego>
was kind enough to provide me the following trait.

trait Processor[I, R, E <: exception="exception">

   def process(i: I): Either[E, R]

This essentially has a benefit to carry the computation till you really need it.
And it is achieved using the Either construct. Either is place holder to carry error
on the left or to have the computation on the right.

Recent versions of scala provide a scala.util.Try to work with Either directly using the companion object.

import scala.util.Try
import scala.util.Try._
val result: Either[Exception, Int] = Try(Integer.parseInt(null))

Scala - Reflection - I

2012-08-04T08:08:00.005-05:00

1. Getting the class information

Value type class information

Reference type class information

Verify an object's type

2. Create an instance of a class

3. Create an instance of the companion object

Instantiating 1 Arity case class

Instantiating 2 Arity case class

3. Create Array of objects

In this blog post, I'll cover the following using Scala 2.10.0 Milestone 6.

1. Getting the class information

Java provides getClass method in java.lang.Object. In scala this functionality is available using getClass method declared as abstract in "scala.Any". The implementation for value types is in "scala.AnyVal" and for reference types it is in "scala.AnyRef" class.

Value type class information

1.getClass

Reference type class information

"sb".getClass

Invocation of getClass on null (null.getClass) results in a NullPointerException.

A reference to "ab" assigned to obj of AnyRef, the supertype of all reference types.

val obj:AnyRef = "ab"

A string "ab" assigned to str variable. Here the type is not mentioned and will inferred by the compiler.

val str = "ab"

Verify an object's type

Similar to instanceof operator in java, scala provided “isInstanceOf” method in "scala.Predef". We can invoke “isInstanceOf” on null too.

obj.isInstanceOf[java.lang.String]
str.isInstanceOf[java.lang.String]

To cast an object to appropriate type, scala provided “asInstanceOf” method in "scala.Predef". We can invoke asInstance of on null too.

val someStr = obj.asInstanceOf[java.lang.String]

2. Create an instance of a class

Getting the class object is the first step. The behaviour is same as String.class in java defined in “scala.Predef”.

val clazz: Class[String] = classOf[String]

or

val clazz = Class.forName("java.lang.String")

We create the instance using available constructors. Following will create a String object with "Ramesh" as value.

val user = clazz.getConstructor(clazz).newInstance("Ramesh").asInstanceOf[String]

3. Create an instance of the companion object

Scala case class produces a class and a companion object. As 0 arity case classes are deprecated, let us start with greater than 0 arity case classes.

Instantiating 1 Arity case class

Let us define 1 arity case class as below:

case class C1(name:String)

Get the reference of the companion object.

val c1Comp = Class.forName(classOf[C1].getName+"$").newInstance.asInstanceOf[Function1[String,C1]]

Invoke the apply method of the companion object with one parameter value "Ramesh"

val obj = c1Comp("Ramesh")

Let us see if the data is available or not.

obj._1

Instantiating 2 Arity case class

Let us define 2 artity case class as below:

case class C2(name:String, location: String)

Get the reference of the companion object.

val c2Comp = Class.forName(classOf[C2].getName+"$").newInstance.asInstanceOf[Function2[String, String, C2]]

Invoke the apply method of the companion object with two parameter values "Ramesh" and "India".

val obj2 = c2Comp("Ramesh", "India")

obj2._1

obj2._2

3. Create Array of objects

The following creates a String Array of size 4.

val s4: Array[String] = classTag[String] newArray(4)

Scala - Exception Handling

2012-03-29T23:04:00.000-05:00

Imperative way

The try catch finally construct

scala> val result: Int = try { "s".toInt } catch { case e: Exception => 0 } finally { println("converted") }
converted
result: Int = 0

Functional way

Using Either

Gives more controll to the caller. If result is Left, then it is error. If it is Right, then the value
.
scala> val computedValue : Either[Exception, Int] = try { Right { "s".toInt } } catch { case e: Exception => Left(e) }
computedValue: Either[Exception,Int] = Left(java.lang.NumberFormatException: For input string: "s")

Caller can use pattern matching to work with the result Either[E,R]

scala> val result = computedValue match {
case Left(left) => println(left); 0
case Right(right) => right
}
java.lang.NumberFormatException: For input string: "s"
result: Int = 0

Caller can verify if an error is the result is an error

scala> if (computedValue.isLeft) {
computedValue.left.map { e => e.printStackTrace }
}
java.lang.NumberFormatException: For input string: "s"
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at scala.collection.immutable.StringLike$class.toInt(StringLike.scala:231)
at scala.collection.immutable.StringOps.toInt(StringOps.scala:31)
at $line122.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$.liftedTree1$1(:22)
at $line122.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$.(:22)
at $line122.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$.()
at $line122.$eval$.(:11)
at $line122.$eval$.()
at $line122.$eval.$print()
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:914)
at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:546)
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:577)
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:543)
at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:694)
at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:745)
at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:651)
at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:542)
at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:550)
at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:822)
at scala.tools.nsc.interpreter.ILoop.main(ILoop.scala:851)
at xsbt.ConsoleInterface.run(ConsoleInterface.scala:57)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:57)
at sbt.compiler.AnalyzingCompiler.console(AnalyzingCompiler.scala:48)
at sbt.Console.console0$1(Console.scala:23)
at sbt.Console$$anonfun$apply$2$$anonfun$apply$1.apply$mcV$sp(Console.scala:24)
at sbt.TrapExit$.executeMain$1(TrapExit.scala:33)
at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)
res70: Any = Left(())

Scala built-in contructs to work with exceptions.

You need to import the following:

scala> import scala.util.control.Exception._

Ignoring exception

Exceptions should not be ignored. May be useful to handle usecases like Close a stream silently

scala> val result = ignoring(classOf[java.lang.NumberFormatException]) { "x" toInt }
result: AnyVal = ()

scala> val result = ignoring(classOf[java.lang.NumberFormatException]) { "1" toInt }
result: AnyVal = 1

Ignore the exception and return a default value on error using failAsValue

Compensates with a value on NumberFormatException

scala> val result = failAsValue(classOf[java.lang.NumberFormatException])( 0 ) { "".toInt }
result: Int = 0

Catch all exceptions using allCatch

scala> val result = allCatch opt { "x".toInt }
result: Option[Int] = None

scala> val result = allCatch opt { "3".toInt }
result: Option[Int] = Some(3)

Giving more controll to the caller using either

Using control structures to handle exceptions instead of try...catch...finally constructs

scala> val computedValue = catching(classOf[java.lang.NumberFormatException]) either { "s".toInt }
computedValue: Either[Throwable,Int] = Left(java.lang.NumberFormatException: For input string: "s")

Using either converting the result to Option

Some or, None on exception. The identity is defined in scala.Predef

scala> val result = catching(classOf[java.lang.NumberFormatException]) either { "1".toInt } fold( => None, identity() )
result: Any = 1
Preferred when you interested in the value and dont need to know the exception if any. The following only capture NumberFormatException exception resulting None

scala> val result = catching(classOf[java.lang.NumberFormatException]) opt { "x".toInt }
result: Option[Int] = None

For comprehension

If exception, skip the execution using for comprehension

scala> val result = for ( a <- catching(classOf[java.lang.NumberFormatException]).opt { "".toInt }) {
if ( a % 2 == 0 ) println("Entered Even number")
else println("Entered Odd number")
}
scala> val result = for ( a <- catching(classOf[java.lang.NumberFormatException]).opt { "2".toInt }) {
if ( a % 2 == 0 ) println("Entered Even number")
else println("Entered Odd number")
}

andFinally

Similar to try .. finally construct

scala> val result = allCatch.andFinally( println("cleanup") ) opt { s.toInt }
The try catch finally

scala> import java.io._

scala> for( a <- allCatch.opt { new BufferedWriter( new FileWriter(s) ) } ) {
allCatch.andFinally { a.close() } {
a.write("ramesh")
}
}

withApply and apply

Exception or value

scala>val result = allCatch withApply { e => e } apply { "".toInt }
result: Any = java.lang.NumberFormatException: For input string: ""

Option or None
scala> val result = allCatch withApply { e => None } apply { Some("".toInt) }
result: Option[Int] = None

Either
scala> val result = allCatch withApply { e => Left(e) } apply { Right("".toInt) }
result: Product with Serializable with Either[Throwable,Int] = Left(java.lang.NumberFormatException: For input string: "
")

Multiple Exceptions

scala> val result = catching(classOf[java.lang.ArithmeticException], classOf[java.lang.NumberFormatException]) withApply
{ e => e } apply { 10 / "d".toInt }
result: Any = java.lang.NumberFormatException: For input string: "d"

scala> val result = catching(classOf[java.lang.ArithmeticException], classOf[java.lang.NumberFormatException]) withApply
{ e => e } apply { 10 / "0".toInt }
result: Any = java.lang.ArithmeticException: / by zero

handling

Compute block and handle exception using function

scala> val result = handling(classOf[ArithmeticException], classOf[NumberFormatException]) by { e => e } apply { 10 / "d
".toInt }
result: Any = java.lang.NumberFormatException: For input string: "d"

failing

Value of None similar to Option but return value instead of Some

scala> val result = failing(classOf[ArithmeticException], classOf[NumberFormatException]) apply { 10 / "d".toInt }
result: Any = None

scala> val result = failing(classOf[ArithmeticException], classOf[NumberFormatException]) apply { 10 / "2".toInt }
result: Any = 5

ultimately

Similar to try finally construct. Helpful to handle usecases like closing resources etc

scala> ultimately { println("Finally") } { "" toInt }
Finally
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at scala.collection.immutable.StringLike$class.toInt(StringLike.scala:231)
at scala.collection.immutable.StringOps.toInt(StringOps.scala:31)
at $anonfun$2.apply$mcI$sp(:20)
at $anonfun$2.apply(:20)
at $anonfun$2.apply(:20)
at scala.util.control.Exception$Catch.apply(Exception.scala:88)
at .(:20)
at .()
at .(:11)
at .()
at $print()
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:914)
at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:546)
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:577)
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:543)
at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:694)
at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:745)
at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:651)
at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:542)
at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:550)
at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:822)
at scala.tools.nsc.interpreter.ILoop.main(ILoop.scala:851)
at xsbt.ConsoleInterface.run(ConsoleInterface.scala:57)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:57)
at sbt.compiler.AnalyzingCompiler.console(AnalyzingCompiler.scala:48)
at sbt.Console.console0$1(Console.scala:23)
at sbt.Console$$anonfun$apply$2$$anonfun$apply$1.apply$mcV$sp(Console.scala:24)
at sbt.TrapExit$.executeMain$1(TrapExit.scala:33)
at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)

Scala - REPL

2012-03-22T06:16:00.000-05:00

First step

Set SCALA_HOME and go ahead...
Open command prompt
scala -help
scala -nc -e 'println(util.Properties.versionString)'
Note: nc: no compilation daemon: do not use the fsc offline compiler

Scala Read-Evaluate-Print Loop(REPL)

Start REPL by typing scala at the command prompt...
Note: Notice version of scala and jvm
What is available
- Help
  scala>:help
- Power mode
  scala>:power
- Help in power mode
  scala>:help
- You can start scala in power mode by passing -Dscala.repl.power in the startup script
  cat scala/2.9.1/bin/scala
- Quit :quit

REPL in action

Basic stuff
scala>1+2
scala>val name = "ramesh"
scala>pritnln(name)

Auto Completion

packages
scala>scala.
scala>scala.sys
scala>scala.sys.env
both class methods and instance methods
scala>val name = "ramesh"
scala>name.to
This is possible via reflection

Introspection

Type of method
scala>def add(a:Int, b:Int) = a+b
scala>:type add
Type of variable
scala>val name = "Ramesh"
scala>:type name

Companion objects

class and static class
scala>class Test(name: String)
defined class Test
scala>object Test { val t = new Test("Ramesh")}
defined module Test
warning: previously defined class Test is not a companion to object Test.
Companions must be defined together; you may wish to use :paste mode for this.
scala>:paste
```
class Test(name: String)
object Test { val t = new Test("Ramesh")}
```
ctrl+d

Disassemble the code

javap
Welcome to Scala version 2.9.1.final (Java HotSpot(TM) Client VM, Java 1.7.0_02) . Type in expressions to have them evaluated. Type :help for more information.
scala> case class A(n:Int) defined class A
scala> :javap A :javap unavailable on this platform.
scala> :javap A$ :javap unavailable on this platform.
Some issue with JDK 1.7 see https://issues.scala-lang.org/browse/SI-4936

Try other JDK
Welcome to Scala version 2.9.1.final (Java HotSpot(TM) Server VM, Java 1.6.0_27). Type in expressions to have them evaluated. Type :help for more information.
scala> :javap A

Compiled from ""
public class A extends java.lang.Object implements scala.ScalaObject,scala.Product,scala.Serializable{
    public scala.collection.Iterator productIterator();
    public scala.collection.Iterator productElements();
    public int n();
    public A copy(int);
    public int copy$default$1();
    public int hashCode();
    public java.lang.String toString();
    public boolean equals(java.lang.Object);
    public java.lang.String productPrefix();
    public int productArity();
    public java.lang.Object productElement(int);
    public boolean canEqual(java.lang.Object);
    public A(int);
}

scala> :javap A$

Compiled from ""
public final class A$ extends scala.runtime.AbstractFunction1 implements scala.ScalaObject,scala.Serializable{
    public static final A$ MODULE$;
    public static {};
    public final java.lang.String toString();
    public scala.Option unapply(A);
    public A apply(int);
    public java.lang.Object readResolve();
    public java.lang.Object apply(java.lang.Object);
    public A$();
}

Other option to bring up console using ""-Xprint:typer"" jvm option

Working with files

Create a file and loading file
scala>:sh notepad fib.scala

Save file with contents

def fib(n:Int): Long =  {
    n match {
        case 0 => 0
        case 1 => 1
        case n => fib(n-1) + fib(n-2)
    }
}

Load the file
scala>:load fib.scala
execute
scala>fib(8)

Loading file continued....

What happens if n is negative. For example -1?

def fib(n:Int): Long =  {
    require(n >=0 , "Input cannot be negative")
    n match {
        case 0 => 0
        case 1 => 1
        case n => fib(n-1) + fib(n-2)
    }
}

Reload the file. Quick way to do so is
scala>:replay

Simple Utility

Wrappers
scala>:wrap

You can use how much time it took to compute something...
scala>:power
scala>:paste

def timed[T](body: => T): T = {
    val start = System.nanoTime
    try {
        body
    } finally {
        println(" [3" + scala.util.Random.nextInt(8) + "m " + (System.nanoTime * start) + " nanos elapsed." + Console.RESET)
    }
}

scala>:wrap timed
scala>1 to 10000 sum

Scala phases...

scalac -Xshow-phases
Back to REPL
scala>val x = ?[List[_]]
scala>x.

Imports and implicits

Import
scala>:imports
scala>object MyImplicits { implicit def string2Bigdecimal(s:String)=BigDecimal(s) }
scala>import MyImplicits._
scala>:implicits
scala>:imports

Working with jar files

Loading a jar file
scala>:cp c:/repository/lib/commons-lang-2.6.0.jar
scala>import org.apache.commons.lang.StringUtils
scala>StringUtils.isBlank(" ")
Note: module load wont work to load the libraries before starting scala repl
module load ossjava/commons-lang/2.4.0

Shell commands

Linux box
scala>:sh ls -alth
res7 foreach println
scala>scala.sys.process.Process(List("ls")) !
Windows box.
scala> :sh cmd /c dir C:\
res7: scala.tools.nsc.interpreter.ProcessResult = cmd /c dir C:\ (22 lines, ex it 0) scala> res7.show
The behaviour of console evolved with new releases of scala

History

History
scala>:history
Search the history
scala>:h? wrap
Then get the history
scala>:history 100

Scala Interpreter

scala>:power

scala>val x=11

scala>val y = 12

scala>intp.interpret("x+y")

Other tools

sbt * Simple Build Tool
If you have sbt setup, to bring up the console:
sbt console
Change the JDK
:cp C:\Java\jdk1.7.0_02\lib ools.jar

Thank you

Production Ready Tomcat

2010-03-25T15:39:00.001-05:00

Production Ready Tomcat - Optimizations, Security aspects

To disable directory listing for the entire tomcat server go to $CATALINA_HOME/conf/web.xml and set the listings parameter to false.

<servlet>
<servlet-name>default</servlet-name>
<servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>0</param-value>
</init-param>
<init-param>
<param-name>listings</param-name>
<param-value>false</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

Switch off application reloading by setting "reloadable" to "false" gives a small run-time performance increase.

Prevent Locking of Exploded Wars by setting antiResourceLocking="true"

By default tomcat uses the MemoryRealm for authentication. Alter the default username and password in tomcat-users.xml with more appropriate ones.

<?xml version='1.0' encoding='utf-8'?>
<tomcat-users>
  <role rolename="tomcat"/>
  <role rolename="manager"/>
  <role rolename="admin"/>
  <user username="tomcat" password="tomcat" roles="tomcat"/>
  <user username="admin" password="XXXX" roles="admin,manager"/>
</tomcat-users>

Make the tomcat-users.xml as readonly by adding readonly="true" to the <Resource> tag in server.xml

Move to http://svn.apache.org/repos/asf/tomcat/trunk/modules/jdbc-pool/ instead of any other connection pool.

       <dependency>
               <groupId>org.apache.tomcat</groupId>
               <artifactId>juli</artifactId>
               <version>6.0.24</version>
       </dependency>
       <dependency>
               <groupId>org.apache.tomcat</groupId>
               <artifactId>jdbc-pool</artifactId>
               <version>1.0.8.5</version>
       </dependency>

Swallow System.out and System.err and direct them to a log file

Rotation of logs

logging.properties file:

.handlers = 1catalina.org.apache.juli.FileHandler

1catalina.org.apache.juli.FileHandler.level = FINE

1catalina.org.apache.juli.FileHandler.directory = ${catalina.base}/logs

1catalina.org.apache.juli.FileHandler.prefix = catalina.

1catalina.org.apache.juli.FileHandler.pattern = ${catalina.base}/logs/catalina.%g.log

1catalina.org.apache.juli.FileHandler.limit = 20000000 # No more than 5x20mb files

1catalina.org.apache.juli.FileHandler.count = 5

Tuning Sun JVM settings for Tomcat.Set JAVA_OPTS variable to catalina.sh.

Try to adopt these settings :

-server

Better for server side applications

-Djava.awt.headless=true

Headless mode is a system configuration in which the display device, keyboard, or mouse is lacking. Sounds unexpected, but actually you can perform different operations in this mode, even with graphic data.

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/tomcat/dumps

Prepare thread dumps when a OutOfMemoryError occours. This way you can analyze the thread dumps to figure out the issue.

Memory related tuning parameters:

-Xms

initial heap size is 1/64th of the machine's physical memory

-Xmx

maximum heap size is 1/4th of the machine's physical memory (up to 1GB max).

-XX:PermSize
-XX:MaxPermSize

Garbage Collection Tuning Parameters:

-XX:+UseParallelGC parallel (throughput) garbage collector, or
-XX:+UseConcMarkSweepGC concurrent (low pause time) garbage collector (known as CMS)
-XX:+UseSerialGC serial garbage collector (for smaller applications and systems)

See http://java.sun.com/performance/reference/whitepapers/tuning.html

Apache Tomcat - Apache Log4j

2010-03-25T12:17:00.001-05:00

Configuring Apache Tomcat Logging using Apache Log4j

I will be covering how to use log4j instead of the default logging provided by tomcat based on the reference http://tomcat.apache.org/tomcat-6.0-doc/logging.html

Objective

Using log4j to customize the logging in apache-tomcat
Separate apache-tomcat logs from the application logs

Using log4j to customize the logging in apache-tomcat

Create a file called log4j.properties with the following content and save it into $CATALINA_HOME/lib.

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">

</layout>

</appender>

</layout>

</appender>

</logger>

</logger>

</logger>

<root>

<appender-ref ref="ROLL" />

<appender-ref ref="CONSOLE" />

</root>

</log4j:configuration>

Download Log4J (v1.2 or later) and place the log4j jar in $CATALINA_HOME/lib.
Build or download the additional logging components. See the extras components documentation for details.
Replace $CATALINA_HOME/bin/tomcat-juli.jar with output/extras/tomcat-juli.jar.
Place output/extras/tomcat-juli-adapters.jar in $CATALINA_HOME/lib.
Delete $CATALINA_BASE/conf/logging.properties to prevent java.util.logging generating zero length log files.
Start Tomcat

Separate apache-tomcat logs from the application logs

The start with following log4j.properties can be used in your web application to divert all the logs to application specific file (app_demo.out).

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">

</layout>

</appender>

</layout>

</appender>

</logger>

<root>

<appender-ref ref="ROLL" />

<appender-ref ref="CONSOLE" />

</root>

</log4j:configuration>

@TODO

Changing the log levels for various loggers when the app is running

Recurrsion using parallel processing

2010-02-25T22:35:00.001-05:00

Recurrsion using parallel processing

As I was google around I found the Concurrencer: a tool for retrofitting parallelism into sequential Java code. One of the cool thing of this tool is, it can convert the recurrsive program into a parallel program. I learned about ForkJoin framework from concurrency-interest mailinglist. So I quickly wrote a program to see how the performance will be boosted.

The ForkJoin framework will be part of JDK7.0. But the following instructions could help to work with ForkJoin framework on J2SE 6.0. First create a maven2 project and under the root create a lib folder. Download the ForkJoin framework jar "jsr166.jar" from Doug Lea website. Install jsr166.jar as a maven dependency using the following command:

mvn install:install-file -DgroupId=jsr166 -DartifactId=jsr166 -Dversion=166 -Dpackaging=jar -Dfile=jsr166.jar

The following program shows a recurrsive program to generate the fibonacci number and the also using parallel processing. ForkJoinPool is a ExecutorPool that orchestrates the threads.

Fibonacci.java

1    package com.blogspot.ascendant.forkjoin; 
2     
3    import java.util.concurrent.ForkJoinPool; 
4    import java.util.concurrent.RecursiveAction; 
5     
6    public class Fibonacci { 
7     
8        public static void main(String... args) { 
9     
10           final int input = 40; 
11           System.out.printf("Input: %d\n\n", input); 
12    
13           long nowSet1 = System.nanoTime(); 
14    
15           System.out.printf("Fibonacci using recursion: %d\n", fibRecurr(input)); 
16    
17           long laterSet1 = System.nanoTime(); 
18           System.out.printf("Computing time for recursion: %d nanoseconds\n\n", (laterSet1 - nowSet1)); 
19    
20           long nowSet2 = System.nanoTime(); 
21    
22           final int numberOfProcessors = Runtime.getRuntime().availableProcessors(); 
23           System.out.printf("Number of processors %d\n", numberOfProcessors); 
24    
25           final FibonacciParallel fp = new FibonacciParallel(input); 
26    
27           final ForkJoinPool es = new ForkJoinPool(numberOfProcessors); 
28           es.invoke(fp); 
29           es.shutdown(); 
30    
31           System.out.printf("Fibonacci using parallel processing: %d\n", fp.result); 
32    
33           long laterSet2 = System.nanoTime(); 
34           System.out.printf("Computing time for parallel processing: %d nanoseconds\n", (laterSet2 - nowSet2)); 
35       } 
36    
37    
38       /** 
39        * Recursive approach 
40        */ 
41       private static long fibRecurr(final int n) { 
42           if (n < 2) { 
43               return n; 
44           } else { 
45               return fibRecurr(n - 1) + fibRecurr(n - 2); 
46           } 
47       } 
48    
49    
50       /** 
51        * Recursive Task 
52        */ 
53       public static class FibonacciParallel extends RecursiveAction { 
54    
55           private static final long serialVersionUID = -6810496708906992778L; 
56           final int numberToCompute; 
57           volatile long result; 
58    
59           public FibonacciParallel(int numberToCompute) { 
60               this.numberToCompute = numberToCompute; 
61           } 
62    
63           @Override 
64           protected void compute() { 
65               if (numberToCompute < 2) { 
66                   result = numberToCompute; 
67               } else { 
68                   FibonacciParallel future1 = new FibonacciParallel(numberToCompute - 1); 
69                   FibonacciParallel future2 = new FibonacciParallel(numberToCompute - 2); 
70    
71                   invokeAll(future1, future2); 
72    
73                   result = future1.result + future2.result; 
74               } 
75           } 
76       } 
77   } 
78    
79

As we are alternat version of java.util.concurrent package we need to alter the bootclasspath using -Xbootclasspath/a:/home/ramesh/Workspace/ascendant/forkjoin/lib/jsr166.jar option.

java -Xbootclasspath/a:/home/ramesh/Workspace/ascendant/forkjoin/lib/jsr166.jar com.blogspot.ascendant.forkjoin.Fibonacci

ramesh@ramesh-laptop:~$ grep "model name" /proc/cpuinfo
model name : Genuine Intel(R) CPU T2500 @ 2.00GHz
model name : Genuine Intel(R) CPU T2500 @ 2.00GHz

Here is the output on my laptop:
Input: 40

Fibonacci using recursion: 102334155
Computing time for recursion: 1919286038 nanoseconds

Number of processors 2
Fibonacci using parallel processing: 102334155
Computing time for parallel processing: 33836924737 nanoseconds

Process finished with exit code 0

As you see the results using parallel computing is not attractive. I guess this is due to the overload of the thread infrastructure. As the computation becomes more complex, this parallel processing should result better results.

Here is the completed project:
http://www.box.net/shared/egrj42i8xf

Update:

Other people also facing the same performance issue. See http://cs.oswego.edu/pipermail/concurrency-interest/2010-March/006913.html

Upcoming JDBC-Pool -

2010-02-12T18:30:00.001-05:00

Upcoming Tomcat JDBC-Pool an alternate to Jakarta DBCP Component

The Jakarta Commons DBCP internally uses Commons Pool along with other dependencies. It worked me pretty well all the time. As I recently encountered Tomcat JDBC-Pool at Grails.org. I thought of exploring it to see, if it works as a standalone projects. And certainly works with out tomcat and usage is very straight forward and also an inplace replacement to Commons DBCP with very minimum change i.e., use org.apache.tomcat.jdbc.pool.DataSource instead of org.apache.commons.dbcp.BasicDataSource.

You can download the jars from http://people.apache.org/~fhanik/jdbc-pool/v1.0.8.5/ or build
it from the svn repository located at http://svn.apache.org/repos/asf/tomcat/trunk/modules/jdbc-pool. As it is not available in any public standard maven2 repository I manually updated local repository using:

mvn install:install-file -DgroupId=org.apache.tomcat -DartifactId=jdbc-pool -Dversion=1.0.8.5 -Dpackaging=jar -Dfile=tomcat-jdbc.jar

For apache maven lovers, below are the dependenies required:

        <dependency>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>jdbc-pool</artifactId>
            <version>1.0.8.5</version>
        </dependency>

        <dependency>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>juli</artifactId>
            <version>6.0.20</version>
        </dependency>

JDBC properties in app.properties file looks a below:

# Oracle configuration
jdbc.driverClassName=oracle.jdbc.driver.OracleDriver
jdbc.url=jdbc:oracle:thin:@//<<Server>>:<<PORT>/<<Service Name>>
jdbc.username=<<Database User>>
jdbc.password=<<Database Password>>

Spring applicationContext.xml

<context:property-placeholder location="classpath:app.properties" />

<bean id="datasource" class="org.apache.tomcat.jdbc.pool.DataSource"
          destroy-method="close"
          p:driverClassName="${jdbc.driverClassName}"
          p:url="${jdbc.url}"
          p:username="${jdbc.username}"
          p:password="${jdbc.password}"
          p:initialSize="5"
          p:minIdle="5"
          p:maxIdle="10"
          p:maxActive="20"
          p:maxWait="6000"
          p:jmxEnabled="true"
          p:jdbcInterceptors="org.apache.tomcat.jdbc.pool.interceptor.ConnectionState;org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer;org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReportJmx(threshold=10000)"
          p:removeAbandoned="true"
          p:removeAbandonedTimeout="60"
          p:logAbandoned="true"
          p:testOnBorrow="true"
          p:testOnReturn="false"
          p:testWhileIdle="false"
          p:useEquals="true"
          p:fairQueue="true"
          p:timeBetweenEvictionRunsMillis="30000"
          p:minEvictableIdleTimeMillis="30000"
          p:validationInterval="1800000"
          p:validationQuery="select 1 from dual"
          p:accessToUnderlyingConnectionAllowed="true"
          />

I still need to see how it works when dealing with vendor specific JDBC features.

Concurrent URL checker using Completi...

2010-02-12T16:57:00.001-05:00

Concurrent URL checker using Completion Service

From javadocs:

"A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. Producers submit tasks for execution. Consumers take completed tasks and process their results in the order they complete. A CompletionService can for example be used to manage asynchronous IO, in which tasks that perform reads are submitted in one part of a program or system, and then acted upon in a different part of the program when the reads complete, possibly in a different order than they were requested.

Typically, a CompletionService relies on a separate Executor to actually execute the tasks, in which case the CompletionService only manages an internal completion queue. The ExecutorCompletionService class provides an implementation of this approach."

The program:

Takes set of urls as input. This is typically done by a producer.
The CallableUrlChecker class checks one url and produces the Result.

UrlChecker.java

1    package com.blogspot.ascendant.urlchecker; 
2     
3    import org.apache.http.HttpHost; 
4    import org.apache.http.HttpResponse; 
5    import org.apache.http.StatusLine; 
6    import org.apache.http.client.HttpClient; 
7    import org.apache.http.client.methods.HttpGet; 
8    import org.apache.http.impl.client.DefaultHttpClient; 
9    import org.apache.http.protocol.BasicHttpContext; 
10   import org.apache.http.protocol.ExecutionContext; 
11   import org.apache.http.protocol.HttpContext; 
12    
13   import java.util.Set; 
14   import java.util.TreeSet; 
15   import java.util.concurrent.*; 
16    
17   public class UrlChecker { 
18    
19       public static void main(String... args) { 
20    
21           final Set<String> urls = getUrlsToCheck(); 
22    
23           // The executor that acutually executes the Url verification.  
24           final ExecutorService pool = Executors.newFixedThreadPool(10); 
25    
26           // The completion service that manages completed task queue 
27           final CompletionService<Result> completionService = new ExecutorCompletionService<Result>(
28                   pool 
29           ); 
30    
31           // Submit the url check tasks to the completion service 
32           for (final String url : urls) { 
33               completionService.submit(new CallableUrlChecker(url)); 
34           } 
35    
36           Future<Result> futureResult; 
37           Result result; 
38    
39           // Total urls submitted 
40           final int totalUrlsToCheck = urls.size(); 
41    
42           int urlCounter = 0; 
43           try { 
44               while (urlCounter != totalUrlsToCheck) { 
45                   try { 
46                       // Retrieves and removes the Future representing the next completed task, 
47                       // waiting if none are yet present. 
48                       futureResult = completionService.take(); 
49    
50                       // May result a null... 
51                       if (futureResult != null) { 
52                           // get the result with a time wait... 
53                           result = futureResult.get(3, TimeUnit.SECONDS); 
54    
55                           // further process the result 
56                           System.out.println(urlCounter + " : " + result); 
57                       } 
58                   } catch (ExecutionException e) { 
59                       System.err.println("Failed to check the url: " + e.getMessage()); 
60                   } catch (InterruptedException e) { 
61                       Thread.currentThread().interrupt(); 
62                   } catch (TimeoutException e) { 
63                       System.err.println("Failed to check the url: " + e.getMessage()); 
64                   } 
65    
66                   urlCounter++; 
67               } 
68    
69           } finally { 
70               // Shutdown the executor pool if you are done 
71               pool.shutdown(); 
72           } 
73       } 
74    
75       /** 
76        * Set of urls to check 
77        * 
78        * @return non-null set 
79        */ 
80       private static Set<String> getUrlsToCheck() { 
81           final Set<String> urls = new TreeSet<String>(); 
82           for (int i = 0; i <= 5; i++) { 
83               urls.add(String.format("http://test%d.com", i)); 
84           } 
85           return urls; 
86       } 
87    
88       /** 
89        * The callable task that actually verifies the URL and prepares the result... 
90        */ 
91       public static class CallableUrlChecker implements Callable<Result> { 
92           private final String urlToCheck; 
93    
94           public CallableUrlChecker(String urlToCheck) { 
95               this.urlToCheck = urlToCheck; 
96           } 
97    
98           public Result call() throws Exception { 
99               HttpClient httpclient = new DefaultHttpClient(); 
100              HttpGet httpget = new HttpGet(urlToCheck);
101              HttpContext hc = new BasicHttpContext(); 
102   
103              Result result = new Result(); 
104              result.setUrl(urlToCheck); 
105              try { 
106                  HttpResponse response = httpclient.execute(httpget, hc); 
107                  HttpHost target = (HttpHost) hc.getAttribute( 
108                          ExecutionContext.HTTP_TARGET_HOST 
109                  ); 
110   
111                  StatusLine status = response.getStatusLine(); 
112   
113                  result.setStatusCode(status.getStatusCode()); 
114                  result.setStatusPhrase(status.getReasonPhrase()); 
115                  result.setRedirectUrl(target.toURI()); 
116   
117              } catch (Exception e) { 
118                  result.setStatusPhrase(e.getMessage()); 
119              } finally { 
120                  httpclient.getConnectionManager().shutdown(); 
121              } 
122   
123              return result; 
124          } 
125      } 
126   
127      /** 
128       * Result encapsulates the data that is returned after completion of the task 
129       */ 
130      public static class Result { 
131          private String url; 
132          private String redirectUrl; 
133          private int statusCode; 
134          private String statusPhrase; 
135   
136          public String getUrl() { 
137              return url; 
138          } 
139   
140          public void setUrl(String url) { 
141              this.url = url; 
142          } 
143   
144          public String getRedirectUrl() { 
145              return redirectUrl; 
146          } 
147   
148          public void setRedirectUrl(String redirectUrl) { 
149              this.redirectUrl = redirectUrl; 
150          } 
151   
152          public int getStatusCode() { 
153              return statusCode; 
154          } 
155   
156          public void setStatusCode(int statusCode) { 
157              this.statusCode = statusCode; 
158          } 
159   
160          public String getStatusPhrase() { 
161              return statusPhrase; 
162          } 
163   
164          public void setStatusPhrase(String statusPhrase) { 
165              this.statusPhrase = statusPhrase; 
166          } 
167   
168          @Override 
169          public String toString() { 
170              return "Result{" + 
171                      "url='" + url + '\'' + 
172                      ", redirectUrl='" + redirectUrl + '\'' + 
173                      ", statusCode=" + statusCode + 
174                      ", statusPhrase='" + statusPhrase + '\'' + 
175                      '}'; 
176          } 
177      } 
178  } 
179

The above solution can be improved with the following changes.

1) Producer and Consumer are in only class; and this can be separated. Run the producer in a separate thread and the consumer part in a separate thread.

2) Can leverage Fork/Join Framework

Maven2 Dependencies

HttpComponents.

<dependency>

<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpclient</artifactId>
<version>4.1-alpha1</version>

</dependency>

Download code for URLChecker - Java Concurrent Completion Service

Amazon Product Advertising API

2009-09-06T18:51:00.001-05:00

Product Advertising API Signed Requests - Java SOAP using Spring Web Services

Product Advertising API - https://affiliate-program.amazon.com/gp/advertising/api/detail/main.html/178-0006257-2456255

We are using Amazon Web Services to advertise products on one of my clients websites. We used Springframework as the dependency container and Spring Web Services as web service platform as we Spring-WS favors contract-first web services. With the recent changes in the API all the requests must be signed. And the sample code (http://developer.amazonwebservices.com/connect/entry.jspa?externalID=2479&categoryID=14) helped me as a reference to migrate our code base to support the signed request requirement.

In the below sections I will walk through the steps to make signed requests using Spring Web Services.

Project Setup:

We used maven2 as our build & reporting tool we followed the maven project directory structure and is as below:

awsprocess
└───src
    ├───main
    │   ├───java
    │   │   └───com
    │   │       └───ascendant76
    │   │           └───awsdemo
    │   │               ├───client
    │   │               └───service
    │   ├───resources
    │   └───webapp
    └───test
        ├───java
        └───resources

The project object model uses few plug-ins:

maven-compiler-plugin: To compile the code that is compatible with J2SE 5.0.

maven-surefire-plugin: To execute the unit test cases. I used TestNG(http://testng.org/doc/index.html) for the test cases and their by the testng dependency is imported.

        <dependency>
           <groupId>org.testng</groupId>
           <artifactId>testng</artifactId>
           <classifier>jdk15</classifier>
           <version>5.9</version>
       </dependency>

maven-jaxb2-plugin: Creates JAXB2 binding from XSD. The plug-in scans the xjb files and generates the bindings in our case it AWSECommerceService.xjb in the resources folder.

AWSECommerceService.xjb:

<?xml version='1.0' encoding='utf-8' ?>
<jxb:bindings version="2.0" xmlns:jxb="http://java.sun.com/xml/ns/jaxb"
   xmlns:xs="http://www.w3.org/2001/XMLSchema">

   <jxb:bindings node="/xs:schema"
       schemaLocation="AWSECommerceService.xsd">
       <jxb:globalBindings fixedAttributeAsConstantProperty="false"
           collectionType="java.util.ArrayList" typesafeEnumBase="xs:NCName"
           choiceContentProperty="false" typesafeEnumMemberName="generateError"
           enableFailFastCheck="true" generateIsSetMethod="true"
           underscoreBinding="asCharInWord"
           enableJavaNamingConventions="true">
       </jxb:globalBindings>
       <jxb:schemaBindings>
           <jxb:package name="com.ascendant76.awsdemo.model" />
       </jxb:schemaBindings>
   </jxb:bindings>
</jxb:bindings>

The AWSECommerceService.xsd is extracted manually from the web service contract schema from the WSDL located at http://ecs.amazonaws.com/AWSECommerceService/2009-07-01/AWSECommerceService.wsdl

Account Setup:

Login to amazon at http://aws-portal.amazon.com/gp/aws/developer/account/index.html?ie=UTF8&action=activity-summary. Goto Access Credentials section by following this URL https://aws-portal.amazon.com/gp/aws/developer/account/index.html?ie=UTF8&action=access-key. This page allows you to create new X.509 certificates. Select the Create option to create your certificates and download them. You will have two files pk-XXXX.pem and cert-XXXX.pem

Note: You can download the private key file (pk-XXXX.pem) at the time of certificate creation only and will not be available later. If you lose it, you have to create a new certificate. So keep it safe.

Now we need to convert the certificate into the PCKS12 format. We need openssl (http://www.openssl.org) to create the certificate in pcks12 format.

C:\openssl pkcs12 -export -in cert-XXXX.pem -inkey pk-XXXX.pem -name ramesh_cert -out ramesh_cert.p12

The pkcs12 file will be generated. Execute the following command to verify the generated certificate using the user name.

C:\jdk1.6.0_13\bin\keytool -v -list -storetype pkcs12 -keystore ramesh_cert.p12

Enter keystore password: ramesh

The output appears something like below:

Keystore type: pkcs12
Keystore provider: SunJSSE

Your keystore contains 1 entry

Alias name: ramesh_cert
Creation date: Aug 28, 2009
Entry type: keyEntry
Certificate chain length: 1
Certificate[1]:
Owner: CN=zzp69oziqnw6, OU=AWS-Developers, O=Amazon.com, C=US
Issuer: CN=AWS Limited-Assurance CA, OU=AWS, O=Amazon.com, C=US
Serial number: d61749a157
Valid from: Fri Aug 28 16:49:58 EDT 2009 until: Sat Aug 28 16:49:58 EDT 2010
Certificate fingerprints:
         MD5: 60:BF:22:AF:51:F8:5A:72:36:8F:E8:69:1D:35:8C:C0
         SHA1: D0:D9:5A:60:38:35:98:45:C3:41:CE:1A:8F:76:DC:39:80:3A:02:1B
**************************************************************************************

Once you verify the certificate generated is okay with the given user
name, copy the certificate generated above - ramesh_cert.p12 - into the
resources directory.

Codebase:

The dependency diagram of various collaborators is given below.

The AwsGateway is the sub-class of org.springframework.ws.client.core.support.WebServiceGatewaySupport is responsible for marshaling the java objects in to SOAP messages and sends to location mentioned in the service binding of the WSDL and in this case https://ecs.amazonaws.com/onca/soap?Service=AWSECommerceService. The WebServiceGatewaySupport delegates the responsibility to the Marshaller to marshal java request objects to SOAP messages and UnMarshaller to marshal the SOAP response back to Java object.

    
   <bean id="marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller">
       <property name="schema" value="classpath:AWSECommerceService.xsd" />
       <property name="contextPath" value="com.ascendant76.awsdemo.model" />
   </bean>

Spring-WS supports two types of messages factories to deal with SOAP messages org.springframework.ws.soap.axiom.AxiomSoapMessageFactory and org.springframework.ws.soap.saaj.SaajSoapMessageFactory. And I choose to use SAAJ (https://saaj.dev.java.net/nonav/spec-1.3/api/).

   
   <bean id="messageFactory" class="org.springframework.ws.soap.saaj.SaajSoapMessageFactory" />

Now the web service gateway definition looks as below:

    <bean id="awsGateway" class="com.ascendant76.awsdemo.service.AwsGateway">
       <constructor-arg value="${aws.accessKey}" />
       <property name="defaultUri" value="https://ecs.amazonaws.com/onca/soap?Service=AWSECommerceService" />
       <property name="marshaller" ref="marshaller" />
       <property name="unmarshaller" ref="marshaller" />
       <property name="messageFactory" ref="messageFactory" />
       <property name="interceptors">
           <list>
               <ref bean="signatureSecurityInterceptor" />
           </list>
       </property>
   </bean>

As security is an orthogonal concept, spring-ws handles it using interceptors. We use WSS4J for the authentication to produce signed requests. Amazon API rely on Timestamp and Signature.

    <bean id="signatureSecurityInterceptor" class="org.springframework.ws.soap.security.wss4j.Wss4jSecurityInterceptor">
       <property name="securementActions" value="Timestamp Signature" />
       <property name="timestampPrecisionInMilliseconds" value="true" />
       <property name="securementSignatureKeyIdentifier" value="DirectReference" />
       <property name="securementUsername" value="${aws.keystoreUser}" />
       <property name="securementPassword" value="${aws.keystorePassword}" />
       <property name="securementSignatureCrypto" ref="cryptoFactoryBean" />
       <property name="securementCallbackHandler" ref="passwordCallback" />
   </bean>

   <bean id="cryptoFactoryBean" class="org.springframework.ws.soap.security.wss4j.support.CryptoFactoryBean">
       <property name="cryptoProvider" value="org.apache.ws.security.components.crypto.Merlin" />
       <property name="keyStoreLocation" value="classpath:${aws.keystoreUser}.p12" />
       <property name="keyStorePassword" value="${aws.keystorePassword}" />
       <property name="keyStoreType" value="pkcs12" />
       <property name="keyStoreProvider" value="SunJSSE" />
   </bean>

   <bean id="passwordCallback" class="com.ascendant76.awsdemo.service.PasswordCallback">
       <constructor-arg index="0" value="${aws.keystorePassword}" />
   </bean>

To keep all the properties in one place we can use property place holder. The keystore password is being served from properties file:

    <context:property-placeholder location="classpath:app.properties" />

Change the properties files to reflect your credentials

# Amazon Associate Access Key
aws.accessKey=<<AWS ACCESSKEY comes here>>
aws.keystoreUser=<<KEY STORE comes here>>
aws.keystorePassword=<<KEY STORE password>>

Now at this stage we are ready to invoke the web service. Execute mvn clean test to execute the test cases.

Download the code
You can download the code from here: Download the Code

References:
http://www.openssl.org/docs/apps/pkcs12.html

Nandan Nilekani's ideas for India's future

2009-08-25T07:33:00.000-05:00

Interesting video by Nandan Nilekani and his ideas for India's future. The problem domain is mapped to the following categories.

* Ideas that have arrived
* Ideas in progress
* Ideas in conflict
* Ideas in anticipation

What I like in this presentation is the approach based on the above four fundamentals.

Don't mess with Java naming convensions - Java Generics

2009-08-06T12:44:00.000-05:00

Today I found the following program over the internet. First time when I saw it I thought it won't compile.

import java.util.ArrayList;

public class ListProvider {
public static ArrayList sayHello(ArrayList ar) {
return ar;
}
public static void main(String[] args) {
System.out.println(sayHello("I am a list"));
}
}

Now I rewrote the program following java naming conventions.

public class ListProvider {
public static T sayHello(T ar) {
return ar;
}
public static void main(String[] args) {
System.out.println(sayHello("I am a list"));
}
}

is a Generic Type. It is hiding java.util.ArrayList in the first program. Second program it is bit clean using T as type.

Checking the file association in Windows from Command line

2009-05-19T09:34:00.002-05:00

Checking the file association in Windows:

The "assoc" will let you know the file type associated with the extension
C:\>assoc .txt
.txt=txtfile

Now the "ftype" displays or modifies file types used in file extension associations
C:\>ftype txtfile
txtfile=%SystemRoot%\system32\NOTEPAD.EXE %1

12 days of christmas - Indian Version

2008-12-22T09:22:00.000-05:00

I was stumbled to see this video from my boss. A very unusual e-mail I can say. Check this one.

When two in Tango ...

2008-10-19T17:24:00.001-05:00

When you look in to my eyes...
.. eye lids stop flittering
.. heart beats fast
.. minds freezes the frame of yours in my eye lids

Let us tango together..
.. the way no one did
.. the way no one will
.. let the eyes be in to my eyes

Let us not stop this...
.. as long as the clock ticks
.. as long as the earth rotates
.. as long as we can with stand standing

If you feel it is the last step we make ..
.. let our eyes lock together
.. let the warm hug seize the air separting us
.. let the God feel jealous of us

Let the tago continue ...
Let the eyes lock one another...
Let the time ends ...

Appropriate English Usage 1

2008-10-19T16:33:00.002-05:00

When some one acknowledges me by saying "Thank you!", my immediate reply used to be "No problem!". Today I learned that it is not traditional and sounds less polite.

Yahoo Query Language (YQL)

2008-10-14T08:38:00.001-05:00

You may be interested of this Yahoo Query Language (YQL) . This is really intereting as they are not exposing their database tables but enabling their domain model ( structured data) using SQL. Behind the scenes they can map to their tables or what ever it could be.

Yahoo! makes a lot of structured data available to developers, primarily through its web services. These services require developers to locate the right URLs and documentation to access and query them which can result in a very fragmented experience. The YQL platform provides a single endpoint service that enables developers to query, filter and combine data across Yahoo! and beyond. YQL exposes a SQL-like SELECT syntax that that is both familiar to developers and expressive enough for getting the right data. Through the SHOW and DESC commands we enable developers to discover the available data sources and structure without opening another web browser.

Yahoo Query Language (YQL)

2008-10-14T08:38:00.000-05:00

No One Needs to Know by Shania Twain

2008-10-08T05:20:00.001-05:00

No One Needs to Know by Shania Twain

2008-10-08T05:20:00.000-05:00

OMG - I Will Kill Her

2008-10-07T05:50:00.001-05:00

Is that Agony or Jealousy?