XMedekO's blog

Eclipse: Manual and Automatic Code Formatting

2012-01-20T09:27:00.000+01:00

I like automatic code formatting in the Eclipse IDE. For Java see the settings in Window -> Preferences -> Java -> Code Style -> Formatter. I Just hit CTRL+SHIFT+F time from time and let the IDE do the boring job for me.

Occasionally, I want the different result then the Eclipse Formatter produce. Especially when a method has many parameters. Sometimes I like to have all of them on one line, sometimes I like to break them into multiple times. Of course, the Eclipse automatic formatter cannot be configured to meet wishes.

The Eclipse formatter has a nice option On/Off Tags:

@formatter:off
@formatter:on

But it's tedious and ugly to write these tags into the code. (Even Code templates does not simply usage of On/Off Tags very much). Much more simpler solution is to append the Java comment // ant the and of line, which you want to break. For example, I have configured the Eclipse formatter to have all method parameters on a single line. But when I write

this.someMethod(param1, //
    param2, //
    param3, //
    param4, //
    param5);

Then the CTRL+SHIFT+F reformatting let this piece of the code as is.

Simple Javascript SCORM 2004 API

2011-10-06T14:24:00.002+02:00

Just a very very simple implementation of SCORM 2004 API for LMS (RTS) developers. It implements just basic SCORM 2004 features. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/misc/simple-javascript-scorm-2004-api.

Java zsync

2011-05-31T09:51:00.001+02:00

The very first implementation of zsync in Java: http://sourceforge.net/projects/jazsync/. It has no release yet, check out the source code from SVN.

ZK-DL Released!

2011-05-11T10:56:00.000+02:00

ZK-DL, an extension to the ZKoss framework (ZK 5), has been released under LGPL just a few weeks ago!. ZK_DL provides especially:

Integration with Spring.
Flexible listbox, combobox and lovbox (a kind of combobox) components with automatic sorting and searching build upon the Hibernate Criteria API.
And much more, see http://zk.datalite.cz/zk-dl.

The documentation is at http://zk.datalite.cz/zk-dl and the source code can be found at http://code.google.com/p/zk-dl/.

I have used DL-ZK with EJB3 on JBoss 5.1 successfully. So I had not taken the advantage of ZK-DL Spring integration (IMHO better than original ZK Spring integration). Anyway the ZK-DL listobox, combobox and lovbox components has greatly simplified our development and provided a rich yet simple interface to our users.

Many thanks to Jiří Bubník and Karel Čemus for their valuable help and cooperation. I wish DL-ZK long life and have many users!

JPA Queries - A Few Methods to Simplify the Life with JPA/JPQL

2011-03-10T16:38:00.010+01:00

The JPA Query.getSingleResult() method is usable only when the JPQL has exactly one result. Yeah, you can catch NoResultException or NonUniqueResultException exceptions, but the try ... catch block make the code less readable. So I have coded three static method to overcome this JPA deficiency:

public class Queries {
    /**
     * @param query
     * @return {@code true}, the query has one or more results, otherwise returns {@code false}.
     */
    public static boolean hasAnyResult(Query query) {
        query.setMaxResults(1);
        final List<?> list = query.getResultList();
        return list.size() > 0;
    }

    /**
     * @param query
     * @return The only one result or {@code null}, when the query has no result.
     * @throws NonUniqueResultException
     *             when the query has more than one result.
     */
    public static Object singleResultOrNull(Query query) {
        query.setMaxResults(2);
        final List<?> list = query.getResultList();
        final int size = list.size();
        if (size <= 0) {
            return null;
        }
        if (size > 1) {
            throw new NonUniqueResultException("result returns more than one element");
        }

        // return first element from the list
        return list.get(0);
    }

    /**
     * @param query
     * @return The first result or {@code null}, when the query has no result.
     */
    public static Object firstResultOrNull(Query query) {
        query.setMaxResults(1);
        final List<?> list = query.getResultList();
        final int size = list.size();
        if (size <= 0) {
            return null;
        }
        if (size > 1) { // should not happened, Hibernate bug?
            Logger.getLogger(Queries.class).error("firstResultOrNull more rows returned. setMaxResults(1) does not work?");
        }

        return list.get(0);
    }

    /**
     * Constructor. No instances.
     */
    private Queries() {
    }
}

The hasAnyResult is the boolean query method - you may test if something exists in database. The best is to use it with some JPQL query returning only id of an entity, e.g.:

Queries.hasAnyResult(em.createQuery("SELECT id FROM Person WHERE ..."));

I think the usage of the singleResultOrNull is obvious. Use it, when you are sure the result returns one or no result. The exception NonUniqueResultException should warn you that something is wrong, wither the query or the data.

The firstResultOrNull returns just the first result of the query. Typically, use it with queries with "ORDER BY" statement, like "get the Person with lowest/highest salary."

Oracle Like Decode in JasperReports

2011-02-15T15:17:00.003+01:00

Oracle SQL has a nice function DECODE. I am using Java Beans data source for JasperReports and sometimes I have missed such a function in JasperReports expressions. One can achieve the same functionality by a ternary operator ? :. But it is not nice for more complicated conditions. So I have created my own Java decode:

public static Object decode(Object value, Object... arg) {
  if (arg == null) {
    return value;
  }
  int n = arg.length;
  for (int i = 0; i < n; i+=2) {
    int j = i + 1;
    if (j >= n) {
      // only the default remains
      return arg[i];
    }
    if (equal(value, arg[i])) {
      return arg[j];
    }
  }
  // no match found, no default is specified
  return value;
}

Then import it into JasperReports *.jrxml report by:

<import value="static com.mypackage.MyClass.decode"/>

And use it in the report like:

"" + decode(count, 0, "no one", 1, "one", 2, "two", "too many: " + count);

ZK and Converting to a Negative Number

2011-01-25T20:28:00.000+01:00

When I was working with ZK Web Framework, I have needed to convert some number to the negatives for the databinding. I have created a new type converter NumberNegativeConverter. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/zk-web-framework/numbernegativeconverter.

Java: A Simple Assert Class for Design By Contract Checks

2010-12-03T15:29:00.005+01:00

Very often, the programmer are lazy to include design by contract checks into the code. That means argument checking in the beginning of the method, checking of a wrong state in the method body, etc. The Java assert construct cannot be used for this purpose, since it is switched off by default (one has tu run java -ea to switch on the Java asserts). To simplify the design by contract checks, I have made three simple static methods:

/**
 * A static class for simple run-time checks. The methods from this class may (and should) be used to check arguments
 * and code for design-by-contract.
 * <p>
 * These are the methods for lazy programmers. If you like to write more complicated messages (and God bless you for
 * that), use the classic {@code if (...) throw ... } way.
 * <p>
 * It is not possible to switch off these checks, do not use them for any memory or CPU consuming operations.
 * 
 * @author xmedeko
 * 
 */
public class Assert {

 /**
  * Check, that the condition is {@code true}.
  * <p>
  * Usage:
  * 
  * <pre>
  * Assert.state(i &lt;= maxIter, &quot;iteration number exceeded&quot;);
  * </pre>
  * 
  * @param check
  * @param message
  *            An error message.
  */
 public static void state(boolean check, String message) {
  if (check)
   return;
  throw new IllegalStateException(message);
 }

 /**
  * Check, that the object is not null. 
  * <p>
  * Usage:
  * 
  * <pre>
  * Assert.notNull(entity, &quot;entity&quot;);
  * </pre>
  * 
  * 
  * @param o
  * @param message
  *            An error message.
  */
 public static void notNull(Object o, String message) {
  if (o != null)
   return;
  throw new NullPointerException(message);
 }

 /**
  * Check, that the argument fullfills the given condition.
  * <p>
  * Usage:
  * 
  * <pre>
  * Assert.argument(from &gt; 0, &quot;from has to be greater then zero&quot;);
  * </pre>
  * 
  * @param check
  * @param message
  *            An error message.
  */
 public static void argument(boolean check, String message) {
  if (check)
   return;
  throw new IllegalArgumentException(message);
 }

 /** No instances. */
 private Assert() {
  throw new UnsupportedOperationException();
 }

}

Note, to check argument for not null value, do not use

Assert.argument(obj != null, "obj is null");

but use instead:

Assert.argument(obj != null, "obj");

Also, do not put any String concatenation or other operation into the argument. These are the methods for lazy programmers. If you like to write more complicated messages (and God bless you for that), use the classic if (...) throw ... way.

Update 29.3.2011: I have just found out that Spring frameworks has org.springframework.util.Assert class, too.

ZK Calendar Colors

2010-12-02T15:05:00.003+01:00

Unfortunately, colours for the ZK Calendar has to be specified in the Java. They cannot be set in a CSS file. Because writing hexadecimal colour codes directly in the code is error prone, I have created a simple enum constant with a set of ZK Calendar colors:

public enum ZkCalendarsColor  {
 GREY("#B4BAB7", "#72706F"),
 GREEN("#4CB052", "#0D7813"),
 KHAKI("#BFBF4D", "#88880E"),
 PURPLE("#B373B3", "#7A367A"),
 RED("#D96666", "#A32929"),
 BLUE("#668CD9", "#3467CE");
 
 private String contentColor;
 private String headerColor;
 
 private ZkCalendarsColor(String contentColor, String headerColor) {
  this.contentColor = contentColor;
  this.headerColor = headerColor;
 }

 public String getContentColor() {
  return contentColor;
 }

 public String getHeaderColor() {
  return headerColor;
 }
}

Fixing Line Terminators for Cygwin CVS

2010-10-14T10:24:00.004+02:00

I have to use a Windows operating system (unfortunately :-)) and CVS in my current job. Thanks to the Cygwin, I can work with Windows almost as effectively as in Linux. The CVS (CVSNT) installation in Windows and other CVS tools installed in on my Windows box (TortoiseCVS, Eclipse) stores the CVS files with CRLF line terminators (EOLs). The Cygwin CVS client requires the classic UNIX LT terminator. Fortunatelly, the Windows CVS clients can read the UNIX-way text files, too. So, I have creates a simple bash script to forx this problem:

#!/bin/sh
find . -type f -path '*/CVS/*' -print0 | xargs -0 dos2unix

Hope, someone may find it useful.

JasperReports PDF Font Mapping

2010-08-31T11:22:00.001+02:00

Are your font bold and italic font properties lost when you export your JasperReports report into the PDF? Do you wander, why PDF preview in the iReport has all fonts right? Check this link out, mate: http://sites.google.com/site/xmedeko/code/misc/jasperreports-pdf-font-mapping

JBoss and HSQLDB 2.0.0

2010-08-05T08:13:00.004+02:00

HSQLBD 2.0 GA has been released on 7 June 2010 with a lot of new cool features. I have needed some of them for my JBoss 5.1.0GA, so I have just replaced the library common/lib/hsqldb.jar and voilà, it runs almost without any problem (Hibernate/JPA usage) as before. I have made just one small change, see bellow. Even the HSQLDB Database Manager starts from JMX console. My personal feeling is that the version 2.0 is a little bit faster than 1.8.0, but I have not made any benchmarks.

The HSQLDB 2.0 upgrade notes:

In my import.sql I had to change all timestamp formats from '2010-08-06 8:00:00' to '2010-08-06 08:00:00', i.e. I had to zero pad the hours into two decimal places.

ZK 3.6 CSRF Protection

2010-07-21T13:59:00.003+02:00

Unfortunately, ZK Web Framework version 3.6 and earlier lack any CSRF protection. i have created a very basic event filter to tackle this issue. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/zk-web-framework/zk-3-6-csrf-protection. It is just a very first version, leave me a comment, if you have some suggestion.

Update 15th Sep 2010: Improved protection and logging.

ZK - Converting Null Values

2010-07-14T16:11:00.002+02:00

A common problem in database programming is to display some meaningful labels instead of NULL values in the list of database rows. Fortunately, ZK Web Framework, offers a comfortable TypeConverter class to address this issue. I have created a general type converter NullToLabelConverter to convert NULL values into any localised text. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/zk-web-framework/nulltolabelconverter.

ZK Logging in ZUL zscript Code

2010-05-19T12:57:00.003+02:00

I have created a very simple class to simplify getting Java logger for the ZUL file (see ZK Web Framework). It is targeted especially for the JBoss logging org.jboss.logging.Logger, but can be easily changed for java.util.logging or any other logging library. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/zk-web-framework/zul-logging.

ZK and Very Long Strings

2010-04-07T17:10:00.004+02:00

When I was working with ZK Web Framework, I have needed to display just a beginning of a very long String in a listbox. I have created a new type converter StringTrimConverter, which trims a string to the specified length. Hope, someone else may find it useful, see http://sites.google.com/site/xmedeko/code/zk-web-framework/stringtrimconverter.

JPA: Mapping from Enum to Char(1)

2010-03-11T10:54:00.006+01:00

Very often, enumerations are stored in SQL databases as char(1) columns. However, JPA annotation Enumerated allows mapping from Java Enum type into javax.persistence.EnumType.STRING (i.e. SQL type VARCHAR) or javax.persistence.EnumType.ORDINAL (i.e. SQL type INTERGER) only.

I have read a few rather complicated advices, how to make a mapping from general Java Enum type into SQL char(1) in Hibernate. After a while, I have found out, that when all constant names in the Enum have length just character, one can make an easy JPA mapping:

@Basic(optional = false)
@Enumerated(EnumType.STRING)
@Column(nullable = false, columnDefinition = "char(1) default 'A'")
private FruitType fruit_type = FruitType.A;

So, the FruitType may be:

public enum FruitType {
  /** Apples */
  A,
  /** Apricots */
  B,
  /** Cherries */
  C,
  /** Pears */
  P;
}

Well, it has a disadvantage, that "apricots" are represented by FruitType.B. But all modern IDE (like Eclipse) shows Javadoc when the developer types the code. Beside the simplicity, the other great advantage is, that the application developer knows that the "apricots" are represented by 'B' in the database. So, it is easier for the developer to write JPQL/HQL/SQL queries. When you a complicated JPA/Hibernate mapping from some type FruitTypeLong.APPRICOT to the 'B' in database, then it will be just confusing after some time to write JPQL/HQL/SQL queries.

Java Web Container: Hunting Redeploy Memory Leaks

2010-02-11T09:06:00.007+01:00

Using a ThreadLocal in a web application (WAR) may simplify the development, but one must have a very deep understanding of the web container (e.g. Tomcat) internals and Java class loading to avoid memory leaks when WAR is redeployed. See

Problem with static ThreadLocal: http://blog.arendsen.net/index.php/2005/02/22/threadlocals-and-memory-leaks-revisited/ and once more at http://www.jroller.com/tackline/entry/fixing_threadlocal.
Problem with inner classes and ThreadLocal: http://crazybob.org/2006/02/threadlocal-memory-leak.html.

The problem might be hidden deep in any library used by the application. To check, if the redeploy memory leak occurs is simple with Eclipse Memory Analyzer (MAT). Just redeploy the WAR several times, make a heap dump (e.g. by jmap or jconsole). Then open the heap dump in MAT and click the link Duplicate Classes: List classes loaded by more than one class loader. If there are some duplicated classes, then you have a memory leak.

Tip: how to clean ThreadLocals in a servlet: http://www.dewavrin.info/?p=196

Eclipse: An error occurred while collecting items to be installed

2010-01-20T09:33:00.005+01:00

I have tried to update my Eclipse 3.5 Galileo plugins, and all what I have seen was:

An error occurred while collecting items to be installed
No repository found containing: org.eclipse.net4j.jms.api/osgi.bundle/1.0.0.v200806180305

I could not find the solution for a couple of hours. Finally, BINGO!, reinstalling EclipseLink packages solved it.

Just go to Help -> Install New Software ->, choose Galile update site Galileo - http://download.eclipse.org/releases/galileo and search for all installed EclipseLink packages. (You must uncheck Hide items that are already installed also.) The installed packages have grey icon. Just check them, press Next, and then Finish. Restart the Eclipse and then try to update plugins again.

Eclipse: Java Decompiler - Really Nice Plugin

2009-10-16T15:39:00.002+02:00

Why the method throws an exception? What happens when I pass a null parameter? Hot the hell is the class designed? These question I have often asked when I had was working with a library without sources. But now I have found a really nice Eclipse plugin http://java.decompiler.free.fr/jd-eclipse/. The update site is http://java.decompiler.free.fr/jd-eclipse/update.

Now I may press F3 button and see the decompiled source immediately. I just miss a possibility to debug the code without source.

Linux: How to Convert Binary File into Byte Array

2009-09-16T16:21:00.011+02:00

Sometimes, a programmer needs to embed a content of a binary file into a program, usually as an array of bytes. For instance, I needed to embed an icon into a Java code, to be really sure the icon is always available for the program. So I have saved an icon in a GIF format and then used the od command. But the result was an unusable list of octal numbers. After a minute with playing, I have found a feasible solution

od -A n -v -t d1 image.gif | sed -e 's/^ *//g' -e 's/  */,/g' -e 's/$/,/g'

I have copied a result into my Java code as

private static final byte[] MISSING_ICON_BYTES = new byte[] { /* copy result here */ };
public static final ImageIcon MISSING_ICON = new ImageIcon(MISSING_ICON_BYTES);

Ant: Removing a File from ZIP or JAR

2009-09-15T11:40:00.015+02:00

I was facing a common problem with building a Java project by Ant: how to remove one file, and how to replace another file in a JAR (or ZIP) file. Many folks on the web, e.g. here and here, advice to unzip the JAR, delete and replace what's needed, and then create JAR again. But I have found out a simpler solution using a <zipfileset> resource pattern:

<delete file="new.jar">
<zip file="new.jar">
  <zipfileset src="old.jar">
    <exclude name="META-INF/replace.properties"/>
    <exclude name="META-INF/remove.properties"/>
  </zipfileset>
  <zipfileset file="file-to-replace.properties" fullpath="source-file-to-be-replaced.properties"/>
</zip>

You can use any of Ant resources. I personally like zipfileset, because it allows to specify the fullpath attribute, which is the simple way how to change a name of a source file. The <exclude> element is like "delete file" command in this example.

The Ant jar task can be used instead of the zip task, too. But then it has to be specified the META-INF/MANIFEST.MF also, otherwise the jar task creates a new one.

The zip task has one unpleasant "feature". It does not create the file new.jar, if it is newer (has greater timestamp) than the file old.jar. So, I recommend to delete the new.jar file first.

Java Swing: Flexible Message Dialog

2009-08-27T10:00:00.009+02:00

JOptionPane makes life of a developer much easier when one need to show simple message or input dialogs. However, when a huge String or a String with many line breaks is passed to it's showMessageDialog() method, then the message dialog may the screen. For the user, it's better, when the very large messages are wrapped in an JScrollPane. It is possible to pass JScrollPane as an message parameter into to JOptionPane, so far so good. Unfotunatelly, it's not possible to limit the maximum size of the dialog. Finally, I have finished with this solution:

JTextArea textArea = new JTextArea();
textArea.setEditable(false);
// better to keep it true, so the user is able to copy the message to the clipboard
//textArea.setFocusable(false);
textArea.setOpaque(false);
textArea.setText(message);
textArea.setCaretPosition(0); // jump to the beginning
JScrollPane scrollPane = new JScrollPane(textArea);
scrollPane.setBorder(null); // no border, please
Dimension scrollPaneSize = scrollPane.getPreferredSize();
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
scrollPaneSize.width = Math.min(scrollPaneSize.width, Math.max(screen.width / 2, 260));
scrollPaneSize.height = Math.min(scrollPaneSize.height, Math.max(screen.height / 2, 125));
scrollPane.setPreferredSize(scrollPaneSize);
JOptionPane jOptionPane = new JOptionPane(scrollPane, JOptionPane.ERROR_MESSAGE);
JDialog dialog = jOptionPane.createDialog(parent, "Error");
// it could be nice to let the user resize the dialog, but then the scrollPane always shows the scroll bar
//dialog.setResizable(true);
dialog.setVisible(true);
dialog.dispose();

The size of the scrollPane grows or shrinks. It's maximum is screen size divided by two, or (260, 125) points (just in case the screen is too small :-)). I have found out, that (260, 125) is roughly the size of the empty massage dialog.

Eclipse: WTP Plugin and Name of Deployment

2009-06-26T10:12:00.014+02:00

I have a WTP project in Eclipse called in like myproject_war. When I have deployed it into the JBoss server, then the deployment name was myproject_war.war. I wanted to deploy it as myproject.war only. I could not find any property in the "Properties" dialogue to change the deploument name, so I have searched my filesystem for "myproject_war" string. So, I have discovered the file
.settings/org.eclipse.wst.common.component in the project directory. I have changed the attribute name of the wb-module module there, from

<wb-module name="myproject_war">

<wb-module name="myproject">

Now, the project is deployed as myproject.war.

Java: Standalone Pattern and Lazy Initialization with Concurrent Access

2009-06-19T09:10:00.015+02:00

Whats wrong with this piece of code?

public class MyClass {
  private static MyClass instance = null;
  public static MyClass getInstance() {
    if (instance == null) instance = new MyClass();
    return instance;
  }
}

Nothing, if you have single thread application only. Otherwise add a magic word synchronized:

public class MyClass {
  private static MyClass instance = null;
  public static synchronized MyClass getInstance() {
    if (instance == null) instance = new MyClass();
    return instance;
  }
}

Now, it's more safe, but a little bit slower. I was thinking about speeding it up by double check locking:

public class MyClass {
  private static MyClass instance = null;
  public static MyClass getInstance() {
    if (instance != null) return instance;
    synchronized (MyClass.class) {
      // yes, one more check in synchronized block
      if (instance == null) instance = new MyClass();
    }
    return instance;
  }
}

Danger! There's hidden snag in this code. Fortunately, I have stumbled upon the See Bill Pugh's page http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html explaining what's wrong with this piece of the code. Briefly, the construction of the object instance = new MyClass() is not an atomic operation. So a thread can get the instance variable while some other thread is still processing the constructor of the same instance object. You can find some old web pages advising to use double check locking in a wrong way, e.g. http://www.javaworld.com/javaworld/javatips/jw-javatip67.html. Bill Pugh's research has changed the memory model in Java 1.5, so there is a new keyword volatile which solves the hidden snag:

public class MyClass {
  private static volatile MyClass instance = null;
  public static MyClass getInstance() {
    if (instance != null) return instance;
    synchronized (MyClass.class) {
      // yes, one more check in synchronized block
      if (instance == null) instance = new MyClass();
    }
    return instance;
  }
}

Java has some nice semantic and syntactic sugar for multi-threaded programming. But this simple example shows, that one should not venture into the Java multi-threaded programming without deeper study of the Java internals and common concurrent programming principles.