Java J2ee Planet

Solution for the Error : An internal error occured during: "reload maven project"

2016-11-17T05:06:00.001-08:00

You might have faced this issue after changing the project name or may be changing project name in pom.xml. When you restart your eclipse it may not launch due to project metadata file is corrupted due to your .project name changes and Maven can not laod your project properly and will displays this error shown above and eclipse console will not be opened. To get rid off this issue follow the solution given below

Solution : Delete the workbench.xmi file from your workspace : C:\<userHome>\workspace\.metadata\.plugins\org.eclipse.e4.workbench
or if it does not get resolved just delete the .metadata folder from workspace and import your projects again.

DOM Parser Vs. SAX Parser

2016-11-11T04:22:00.000-08:00

Many developers sometimes confuse which is better to parse the XML document. Ofcourse myself also got doubt which parser I can use for this situation. Since I have read about DOM and SAX very long back. Just refreshing it again

DOM (Document Object Model) Parser:

Tree model parser(Object based) (Tree of nodes).
DOM loads the file into the memory and then parse the file.
Has memory constraints since it loads the whole XML file before parsing.
DOM is read and write (can insert or delete the node).
If the XML content is small then prefer DOM parser.
Backward and forward search is possible for searching the tags and evaluation of the information inside the tags. So this gives the ease of navigation.
Slower at run time.

SAX (Simple API for XML Parsing) Parser:

Event based parser (Sequence of events). Event based means..don't be confused with word "event". Here event means. (e.g. <something>), then it triggers the tagStarted event (actual name of event might differ). Similarly when the end of the tag is met while parsing (</something>), it triggers tagEnded. Using a SAX parser implies you need to handle these events and make sense of the data returned with each event.
SAX parses the file at it reads i.e. Parses node by node.
No memory constraints as it does not store the XML content in the memory.
SAX is read only i.e. can’t insert or delete the node.
Use SAX parser when memory content is large.
SAX reads the XML file from top to bottom and backward navigation is not possible.
Faster at run time.

Conclusion : DOM Parser is better over SAX Parser, if the XML document is small. If you don't know the size of a XML, I mean size is huge go for SAX Parser.

How Java Garbage Collector Works?

2016-11-11T04:16:00.000-08:00

The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java's dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected. (A more complete description of our garbage collection algorithm might be "A compacting, mark-sweep collector with some conservative scanning".)

The garbage collector runs synchronously when the system runs out of memory, or in response to a request from a Java program. Your Java program can ask the garbage collector to run at any time by calling System.gc(). The garbage collector requires about 20 milliseconds to complete its task so, your program should only run the garbage collector when there will be no performance impact and the program anticipates an idle period long enough for the garbage collector to finish its job. Note: Asking the garbage collection to run does not guarantee that your objects will be garbage collected. The Java garbage collector runs asynchronously when the system is idle on systems that allow the Java runtime to note when a thread has begun and to interrupt another thread (such as Windows 95). As soon as another thread becomes active, the garbage collector is asked to get to a consistent state and then terminate.

HTTP + REST Vs. AMQP + RPC

2016-11-11T04:06:00.000-08:00

REST, RPC - architecture patterns, AMQP - wire-level and HTTP - application protocol which run on top of TCP/IP
AMQP is a specific protocol when HTTP - general-purpose protocol, thus, HTTP has damn high overhead comparing to AMQP
AMQP nature is asynchronous where HTTP nature is synchronous
Both REST and RPC use data serialization, which format is up to you and it depends of infrastructure. If you are using python everywhere I think you can use python native serialization - pickle which should be faster than JSON or any other formats.
Both HTTP+REST and AMQP+RPC can run in heterogeneous and/or distributed environment

So if you are choosing what to use:

HTTP+REST or AMQP+RPC, the answer is really subject of infrastructure complexity and resource usage. Without any specific requirements both solution will work fine, but i would rather make some abstraction to be able switch between them transparently.

If you want to build HA infrastructure with minimal complexity I guess AMQP protocol is what you want.

Advantages of RESTful services are:

they well-mapped on web interface
people are familiar with them
easy to debug (due to general purpose of HTTP)
easy provide API to third-party services.

Advantages of AMQP-based solution:

damn fast
flexible
easy to maintain
easy to scale
cost-effective (in resources usage meaning)

Note, that you can provide RESTful API to third-party services on top of your AMQP-based API while REST is not a protocol but rather paradigm, but should think about it building your AQMP RPC api. We can doin a way to provide API to external third-party services and provide access to API on those part of infrastructure which run on old codebase or where it is not possible to add AMQP support.

If we have high-load project RabbitMQ is damn good piece of software and we can easily add any number of workers which run on different machines. Also it has mirroring and clustering out of the box. And one more thing, RabbitMQ is build on top of Erlang OTP, which is high-reliable,stable platform, it is good not only for marketing but for engineers too. I had an issue with RabbitMQ only once when nginx logs took all disc space on the same partition where RabbitMQ run.

How To Decompile Android APK file(.apk) and Get Source Code

2016-11-06T11:44:00.004-08:00

Sometimes you may be amazed to see beautiful android apps with great design and simplicity. You may want to look into the source code of that app and to design the same way they have done. Usually APK file is not readable because its in dex format (Dalvik excutable file) which can be read by dalvik compiler. So there is a trick I found on stackoverflow to decompile any apk file. Just follow the below steps to get source code of any android app, if you have got .apk file

Step 1:

Create a new folder in your pc and copy over the .apk file that you want to decode.
Now rename the extension of this .apk file to .zip (e.g. rename from filename.apk to filename.zip) and save it.
Now you can access the classes.dex files, etc. At this stage you are able to see drawables but not xml and java files, so continue.

Step 2:

Now extract this .zip file in the same folder (or NEW FOLDER).
Download dex2jar and extract it to the same folder (or NEW FOLDER).
Move the classes.dex file into the dex2jar folder.
Now open command prompt and change directory to that folder (or NEW FOLDER). Then write d2j-dex2jar classes.dex (for mac terminal or ubuntu write ./d2j-dex2jar.sh classes.dex) and press enter. You now have the classes.dex.dex2jar file in the same folder.
Download java decompiler, double click on jd-gui, click on open file, and open classes.dex.dex2jar file from that folder: now you get class files.
Save all of these class files (In jd-gui, click File -> Save All Sources) by src name. At this stage you get the java source but the .xml files are still unreadable, so continue.

Step 3:

Now open another new folder

Put in the .apk file which you want to decode
Download the latest version of apktool AND apktool install window (both can be downloaded from the same link) and place them in the same folder.
Open a command window.
apktool d myApp.apk (where myApp.apk denotes the filename that you want to decode)

Now you get a file folder in that folder and can easily read the apk's xml files.

Step 4:

It's not any step just copy contents of both folder(in this case both new folder)to the single one
and enjoy the source code...
Source: stackoverflow.com

Asynchronous Vs. Synchronous Communications

2016-11-06T10:24:00.000-08:00

Synchronous (One thread):

1 thread ->   |<---A---->||<----B---------->||<------C----->|

Synchronous (multi-threaded):

thread A -> |<---A---->|   
                        \  
thread B ------------>   ->|<----B---------->|   
                                              \   
thread C ---------------------------------->   ->|<------C----->|

Asynchronous (one thread):

         A-Start ------------------------------------------ A-End   
           | B-Start -----------------------------------------|--- B-End   
           |    |      C-Start ------------------- C-End      |      |   
           |    |       |                           |         |      |
           V    V       V                           V         V      V      
1 thread->|<-A-|<--B---|<-C-|-A-|-C-|--A--|-B-|--C-->|---A---->|--B-->|

Asynchronous (multi-Threaded):

 thread A ->     |<---A---->|
 thread B ----->     |<----B---------->| 
 thread C --------->     |<------C--------->|

Start and end points of tasks A, B, C represented by <, > characters.
CPU time slices represented by vertical bars |

Synchronized means "connected", or "dependent" in some way. In other words two synchronous tasks must be aware of one another, and one must execute in some way that is dependent on the other. In most cases that means that one cannot start until the other has completed. Asynchronous means they are totally independent and neither one must consider the other in any way, either in initiation or in execution.

As an aside, I should mention that technically, the concept of synchronous/asynchronous really does not have anything to do with threads. Although, in general, it would be unusual to find asynchronous tasks running on the same thread, it is possible, (see below for e.g.) and it is common to find two or more tasks executing synchronously on separate threads... No, the concept of synchronous/asynchronous has to do solely with whether or not a second or subsequent task can be initiated before the other (first) task has completed, or whether it must wait. That is all. What thread (or threads), or processes, or CPUs, or indeed, what hardware, the task[s] are executed on is not relevant. Indeed, to make this point I have edited the graphics to show this.

ASYNCHRONOUS EXAMPLE. In solving many engineering problems, the software is designed to split up the overall problem into multiple individual tasks, and then execute them asynchronously. Inverting a matrix, or a finite element analysis problem, are good examples. In computing, sorting a list is an example. The quick sort routine, for example, splits the list into two lists, and sorts each of them by calling itself recursively. In both of the above examples, the two tasks can (and often were) executed asynchronously. They do not need to be on separate threads. Even a machine with one CPU, and only one thread of execution can be coded to initiate processing of a second task before a first one has completed. The only criterion is that the results of one task are not necessary as inputs to the other task. As long as the start and end times of the tasks overlap, (possible only if the output of neither is needed as inputs to the other), they are being executed asynchronously, no matter how many threads are in use.

SYNCHRONOUS EXAMPLE. Any process consisting of multiple tasks where the tasks must be executed in sequence, but one must be executed on another machine (Fetch and/or update data, get a stock quote from a financial service, etc.). If it's on a separate machine it is on a separate thread, whether synchronous or asynchronous.

A funny explanation what Synchronous & Asynchronous process

Synchronous Execution
My boss is a busy man. He tells me to write the code. I tell him: Fine. I get started and he's watching me like a vulture, standing behind me, off my shoulder. I'm like "Dude, WTF: why don't you go and do something while I finish this?"
he's like: "No, I"m waiting right here till you finish it off". This is synchronous

Asynchronous Execution
The boss tells me to do it, and rather that waiting right there for my work, the boss goes off and does other tasks. When I finish my job I simply report to my boss and say: "I'm DONE!" This is Asynchronous Execution.

Source:stackoverflow.com

CASCADE Vs RESTRICT Vs NO ACTION Vs SET NULL in MYSQL

2016-02-18T12:05:00.000-08:00

The table containing the foreign key is called the referencing or child table, and the table containing the candidate key is called the referenced or parent table.

SET NULL : Sets the column value to NULL when you delete the parent table row.

CASCADE : CASCADE will propagate the change when the parent changes. If you delete a row, rows in constrained tables that reference that row will also be deleted, etc.

RESTRICT : RESTRICT causes you can not delete a given parent row if a child row exists that references the value for that parent row.
NO ACTION : NO ACTION and RESTRICT are very much alike. when an UPDATE or DELETE statement is executed on the referenced table, the DBMS verifies at the end of the statement execution that none of the referential relationships are violated. in short child row no concern if parent row delete or update.

Resolution for Error: javax.persistence.JoinColumn.foreignKey()Ljavax/persistence/ForeignKey

2016-02-16T07:48:00.001-08:00

Error: "javax.persistence.JoinColumn.foreignKey()Ljavax/persistence/ForeignKey"

I was trying to implement Hibernate Search functionality on my current project. I faced the above issue and resolved it with some research.

Root cause and Solution:
JoinColumn.foreignKey() was introduced with JPA 2.1, which was not implemented by Hibernate 4 until version 4.3. If you're using an older version of Hibernate 4, you will face this issue. Try to upgrade it to hibernate-jpa-2.1-api and also Hibernate to 4.3.x.

If you're already using Hibernate 4.3 then make sure you're also using hibernate-jpa-2.1-api to make sure the API and implementation match up.

Resolved: “org.hibernate.ObjectNotFoundException: No row with the given identifier exists” but it does exists

2016-02-16T07:41:00.001-08:00

Error: “org.hibernate.ObjectNotFoundException: No row with the given identifier exists” but it does exists.

I was facing this issue with my current project. There might be 2 possible reasons to occur this error.

Root Causes:
1. Whene there is relationship between two tables in the database, the main entity may not exists in the database, You can check it with session.get() method whether it is there or not.

2. There will be broken relationships. When entity X owns a relation with entity Y, when entity B is deleted while the Foreign Key(FK) in entity X is left untouched. So whenever Hibernate tries to load the link X->Y this error occurs.

My application root cause is 2. What is yours???

If Runnable interface is better than Thread class, then why we are using Thread class?

2015-12-23T07:52:00.001-08:00

If Runnable interface is better than Thread class, than why we are using Thread class? What is the need for Thread class?

When you extends Thread class, after that you can’t extend any other class which you needed. (As you know, Java does not allow multiple inheritance.)

If your class is Implementing the Runnable interface then you only override the run() .So this instance creates a separate Thread and every individual Thread runs separately but not as a single heavy Thread in your program. Another thing, Since Java does not support multiple inheritance, if you implement the Runnable you'll avoid problems of multiple extending, so if you implement Runnable interface you can extend any class that you are required other than Thread class.

However, the main difference is.

When you extends Thread class, each of your thread creates unique object and associate with it.
When you implements Runnable, it shares the same object to multiple threads.

How To Solve "Error: Could not find or load main class org.codehaus.classworlds.Launcher while building Maven 3.0.4"

2015-08-03T13:30:00.000-07:00

I was trying to solve this error in my Ubuntu. Since I need to build Hue /Spark Application and when run the "make apps" or "mvn --version" command on terminal, I was getting "

Error: Could not find or load main class org.codehaus.classworlds.Launcher while building Maven 3.0.4
This error ate all my day almost. Finally found a solution on one website. Just you need to uninstall old maven versions or existing maven. and

sudo apt-get purge maven maven2 maven3
sudo apt-add-repository ppa:andrei-pozolotin/maven3
sudo apt-get update
sudo apt-get install maven3

Thanks a lot to those guys for a great help.

Solution For Error: " Current configuration of the parser doesn't allow a maxOccurs attribute value to be set greater than the value 5,000."

2015-07-29T03:51:00.000-07:00

Currently I am working on a project where I need to transform Excel sheet data to an XML format in compliance with XSD given by my client.

When I am trying to parse the XSD using JAXB I got this error when doing mvn clean install.
Error: ""Current configuration of the parser doesn't allow a maxOccurs attribute value to be set greater than the value 5,000."

I found many solutions and route cause for this issue. You can find some information on Oracle site the route cause for the issue. It is because of the feature enabled for secure XML processing. This feature was enable in JAXP 1.3 which instructs parsers, transformers to behave in a secure fashion. You can find more information here: https://jaxp.java.net/1.4/JAXP-Compatibility.html#JAXP_security.

For me 2 solutions worked out:

Solution 1:
With code change or implemenetation:
//Create SchemaFactory using XSD file
SchemaFactory factory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING,false);
Schema schema = factory.newSchema(xsdSource);

Solution 2:

Another way is excluding the dependency of older version xerces jar that comes by default with in milyn-smooks-javabean jar and add dependency of newer version of xerces jar in pom.xml file of your project. Just like below.

.......
.........
<dependency>
           <groupId>org.milyn</groupId>
           <artifactId>milyn-smooks-javabean</artifactId>
           <version>1.5.1</version>
           <exclusions>
           
               <exclusion>
                   <groupId>xerces</groupId>
                   <artifactId>xercesImpl</artifactId>
               </exclusion>
           </exclusions>
    </dependency>

    <dependency>
        <groupId>xerces</groupId>
        <artifactId>xercesImpl</artifactId>
        <version>2.11.0</version>
    </dependency>

-------
---------

I used the the 2nd solution worked perfectly. How about you?? Please let me know in comments section your way of solution for this issue.

How To Change IPAddress of Linux/Ubuntu ?

2015-07-24T03:48:00.001-07:00

Recently I needed to change the existing ipaddress of a linux machine. Since we migrated to our machines in to new cloud servers.

Just follow the steps defined below:

On terminal window type the following command.

   sudo vi /etc/network/interfaces
                or
           sudo gedit /etc/network/interfaces

Then find the eth0 section and set the ipaddress as below

                 auto eth0
                 iface eth0 inet static
                 address 192.168.10.43
                 netmask 255.255.255.0
                 network 192.168.10.46
                 broadcast 255.255.255.0
             Note: Its my own ipaddresses, you have to define your own addresses.

In above file may have " iface eth0 inet dhcp". when you want to define static ip you have to change it to "static". "dhcp" is for dynamic ipaddress.
Then save and close the file, then run the restart the network with this command.

sudo /etc/init.d/networking restart

To ensure ipaddress has been changed just run the command.

ifconfig eth0

Is It Possible To Define All The Methods In An Abstract Class?

2015-07-20T08:08:00.000-07:00

It is possible create an abstract class with all concrete methods. But this class can’t be instantiated. Since it is an abstract class.

We may have situations where we don’t want to instantiate a class which is the base class for all the classes in the system or application.This base class may have lot of methods used for different modules in the application and we do not want to instantiate the object of the base class and want to use only the reference of the class.

An abstract class without any abstract methods should be a rare thing and you should always question your application design if this case arises. Normally you should refactor to use a concrete super class in this scenario.

One specific case where abstract class may justifiably have no abstract methods is where it partially implements an interface, with the intention that its subclasses must complete the interface.
To take a slightly contrived motoring analogy, a Chassis class may partially implement a Vehicle interface and provide a set of core methods from which a range of concrete Vehicle types are extended. Chassis is not a viable implementation of a Vehicle in its own right, so a concrete Car subclass would have to implement interface methods for functional wheels, engine and bodywork.

A class can be declared abstract even if it does not actually have any abstract methods. Declaring such a class abstract indicates that the implementation is somehow incomplete and is meant to serve as a superclass for one or more subclasses that will complete the implementation. Such a class cannot be instantiated.

Creating a Docker Group

2015-07-20T07:52:00.001-07:00

The docker daemon binds to a Unix socket instead of a TCP port. By default that Unix socket is owned by the user root and other users can access it with sudo. For this reason, docker daemon always runs as the root user.

To avoid having to use sudo when you use the docker command, create a Unix group called docker and add users to it. When the docker daemon starts, it makes the ownership of the Unix socket read/writable by the docker group.

Note: The docker group is equivalent to the root user;

To create the docker group and add your user:

Log into Ubuntu as a user with sudo privileges.

This procedure assumes you log in as the ubuntu user.

Create the docker group and add your user.

$ sudo usermod -aG docker ubuntu
Log out and log back in.

This ensures your user is running with the correct permissions.

Verify your work by running docker without sudo.

$ docker run hello-world