Archive for ‘Eclipse’

August 20, 2013

Mutable variable capture in anonymous Java classes

by Jay Jonas

The Java compiler requires local variables of enclosing contexts referenced in anonymous classes (so-called captured variables) to be final. What if the anonymous class wants to alter the value of the variable, i.e. requires the variable to be mutable? This post shows different ways how to achieve that.

Mutability with an array

The most common way to solve this problem is to put the variable into an array of length one, make the array final and capture the array:

<br /><%%KEEPWHITESPACE%%>   JButton button = new JButton("Press me!");<br /><%%KEEPWHITESPACE%%>   final String[] message = new String[]{"Never been pressed"};<br /><%%KEEPWHITESPACE%%>   button.addActionListener(new ActionListener() {<br /><br /><%%KEEPWHITESPACE%%>      @Override<br /><%%KEEPWHITESPACE%%>      public void actionPerformed(ActionEvent e) {<br /><%%KEEPWHITESPACE%%>         message[0] = "Pressed";<br /><%%KEEPWHITESPACE%%>      }<br /><br /><%%KEEPWHITESPACE%%>   });<br />

The value is stored in the first slot of the array. The reference to the array is final but its element stays mutable.

While this approach works fine, it looks strange and will certainly raise questions for developers in your team which have never seen this construct before.

Mutability with a holder

The better way to make the message mutable is to put it into a holder class

<br /><%%KEEPWHITESPACE%%>    class Holder {<br /><%%KEEPWHITESPACE%%>        private T value;<br /><br /><%%KEEPWHITESPACE%%>        Holder(T value) {<br /><%%KEEPWHITESPACE%%>            setValue(value);<br /><%%KEEPWHITESPACE%%>        }<br /><br /><%%KEEPWHITESPACE%%>        T getValue() {<br /><%%KEEPWHITESPACE%%>            return value;<br /><%%KEEPWHITESPACE%%>        }<br /><br /><%%KEEPWHITESPACE%%>        void setValue(T value) {<br /><%%KEEPWHITESPACE%%>            this.value = value;<br /><%%KEEPWHITESPACE%%>        }<br /><%%KEEPWHITESPACE%%>    }<br />

and pass the holder into the inner class:

<br /><%%KEEPWHITESPACE%%>   JButton button = new JButton("Press me!");<br /><%%KEEPWHITESPACE%%>   final Holder mutableMessage = new Holder("Never been pressed");<br /><%%KEEPWHITESPACE%%>   button.addActionListener(new ActionListener() {<br /><br /><%%KEEPWHITESPACE%%>      @Override<br /><%%KEEPWHITESPACE%%>      public void actionPerformed(ActionEvent e) {<br /><%%KEEPWHITESPACE%%>         mutableMessage.setValue("Pressed");<br /><%%KEEPWHITESPACE%%>      }<br /><br /><%%KEEPWHITESPACE%%>   });<br />

This solution states more clearly why the message has been wrapped. If the holder is implemented as a generic utility class, this solution is not more verbose than the one with the array. In case you don’t want to implement the Holder class yourself, you can also reuse the MutableObject from Apache Commons or the Holder from Google Guava. One could argue that the solution with the array is faster (creating an array is usually faster than instantiating a class), but in most cases the performance loss will be negligible.

// from at

August 5, 2013

Running javah tool from Eclipse

by Jay Jonas

The javah produces C header files and C source files from a Java class that are needed to implement native methods. The javah -jni is used to generate a C header file containing the function prototype for the native method implementation.

The javah can be configured as an external tool like this: click Run |External Tools |External Tools Configurations. This will invoke External Tools Configurations wizard.

Select Program Type and create a new launch configuration.

For Location, click on Browse File System and browse to the path of javah tool (typically <java_home>\bin).

For Working Directory, click on Browse Workspace and select upto project output directory (directory where class files reside – typically ${workspace_loc:/your.jni.project/bin})

Various options of javah command may be added here which is then followed by ${java_type_name} workspace variable which returns the fully qualified Java type name of the primary type in the selected resource.

The argument list instructs the tool to keep the generated header file in a directory named jni under the project folder. If -d option is not specified, the header files will be stored in bin folder itself.

"${project_loc}${system_property:file.separator}jni" ${java_type_name}
External Tools Configuration

External Tools Configuration

// That’s all, folks!

September 22, 2012

Eclipse Task Tags Meaning

by Jay Jonas

TODO, along with FIXME and XXX, is known in Eclipse as a task tag, and is indexed by the IDE to let you find the spots marked with those tags easily. You can edit such tags in the Eclipse Preferences -> Java -> Compiler -> Task Tags.

This is the list for default Eclipse Task Tags and their meaning:


Comments that mark something for later work, later revision or at least later reconsideration. ODO comments should be considered a very useful technique, although like all good things on Earth, there’s certainly potential for abuse.


A standard put in comments near a piece of code that is broken and needs work.


A marker that attention is needed. Commonly used in program comments to indicate areas that are kluged up or need to be. Some hackers like `XXX’ to the notional heavy-porn movie rating. Use it to flag something that is bogus but works. Use FIXME to flag something that is bogus and broken.

ZZZ   /* This tag is mine!  ; ) */

Sometimes I like to use this marker for a stuff that was commented as deprecated just to remind me what happened there. However, when I’m sure, I delete it along with the code.

// TODO Need to optimize this before the end of the world.
// FIXME This won't work if the programmer is missing. 
// XXX This method badly needs refactoring: should be decaffeinated.
// ZZZ This was useful before the Y2K Millennium Bug.

From Sun/Oracle's Java code conventions, section 10.5.4:

October 4, 2011

JavaBeans PropertyChangeSupport Template

by Jay Jonas

Property-change events occur whenever the value of a bound property changes for a java bean. You can use a Eclipse template to help you to write all the standard setter code

Open Windows | Preferences. Browse to Java | Editor | Templates. Click on New and enter property_change_support as Name, choose Java Statements as Context and mark Automatically Insert option. Write a meaning description as “property change support for setter method” and copy & paste into Pattern the code as follows:

if (this.${enclosing_method_arguments} == null && ${enclosing_method_arguments} == null) {
propertyChangeSupport.firePropertyChange("${enclosing_method_arguments}", this.${enclosing_method_arguments}, ${word_selection})

Take care about that there is no new line at the end of the last line.
Take notes and will never forget it.

August 6, 2011

How to remove unwanted Preference Pages

by Jay Jonas

Remove them in the ApplicationWorkbenchWindowAdvisor.postWindowCreate():

PreferenceManager pm = PlatformUI.getWorkbench( ).getPreferenceManager();

The org.eclipse.ui.preferencePages.Workbench id removes the “General” preference page group. If you don’t know the id for the page you want to remove, so you can first try to print them like this:

PreferenceManager pm = PlatformUI.getWorkbench( ).getPreferenceManager();
IPreferenceNode[] arr = pm.getRootSubNodes();

for(IPreferenceNode pn:arr){
 System.out.println("Label:" + pn.getLabelText() + " ID:" + pn.getId());
January 30, 2011

ClassNotFoundException: org.objectweb.asm.ClassVisitor

by Jay Jonas

Yes! I’m a Jetty Server and Vaadin newbie. But that was a pain try to run Address Book Demo Vaadin tutorial with EclipseLink JPA 2.0 support. I tried with Jetty 7 and Jetty 8 and got all sort of errors using Eclipse WTP. I tested so many configurations and I restarted from ground zero many many times without any success.

Frustrated I installed Apache Tomcat 7 server and automagically all worked as expected on the first strike.

But I really like Jetty and I will give it a new try as soon as I solve this error:

2011-01-30 01:09:50.590:WARN::Unable to reach node goal: started
java.lang.NoClassDefFoundError: org/objectweb/asm/ClassVisitor
 at org.eclipse.jetty.annotations.AnnotationConfiguration.configure(
 at org.eclipse.jetty.webapp.WebAppContext.configure(
 at org.eclipse.jetty.webapp.WebAppContext.startContext(
 at org.eclipse.jetty.server.handler.ContextHandler.doStart(
 at org.eclipse.jetty.webapp.WebAppContext.doStart(
 at org.eclipse.jetty.util.component.AbstractLifeCycle.start(
 at org.eclipse.jetty.deploy.bindings.StandardStarter.processBinding(
 at org.eclipse.jetty.deploy.AppLifeCycle.runBindings(
 at org.eclipse.jetty.deploy.DeploymentManager.requestAppGoal(
 at org.eclipse.jetty.deploy.DeploymentManager.addApp(
 at org.eclipse.jetty.deploy.providers.ScanningAppProvider.fileAdded(
 at org.eclipse.jetty.deploy.providers.ScanningAppProvider$1.fileAdded(
 at org.eclipse.jetty.util.Scanner.reportAddition(
 at org.eclipse.jetty.util.Scanner.reportDifferences(
 at org.eclipse.jetty.util.Scanner.scan(
 at org.eclipse.jetty.util.Scanner.start(
 at org.eclipse.jetty.deploy.providers.ScanningAppProvider.doStart(
 at org.eclipse.jetty.util.component.AbstractLifeCycle.start(
 at org.eclipse.jetty.deploy.DeploymentManager.startAppProvider(
 at org.eclipse.jetty.deploy.DeploymentManager.doStart(
 at org.eclipse.jetty.util.component.AbstractLifeCycle.start(
 at org.eclipse.jetty.server.Server.doStart(
 at org.eclipse.jetty.util.component.AbstractLifeCycle.start(
 at org.eclipse.jetty.xml.XmlConfiguration$
 at Method)
 at org.eclipse.jetty.xml.XmlConfiguration.main(
 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 org.eclipse.jetty.start.Main.invokeMain(
 at org.eclipse.jetty.start.Main.start(
 at org.eclipse.jetty.start.Main.parseCommandLine(
 at org.eclipse.jetty.start.Main.main(

[Updated: 20111013 | from the Juuso Backman’s comment. Thanks, Juuso!]

The problem was that Eclipse hadn’t added all the necessary .jars into the Jetty classpath.

The jars can be added to the classpath for Jetty via Eclipse like so:

  • Open the Servers view
  • Open your server’s config (right-click -> Open, or F3)
  • On the Overview tab, click “Open launch configuration”
  • Classpath tab -> add external jars

// Adding JPA based persistence to the Address Book Demo

January 20, 2011

Splash Screen of the Day: Eclipse 1.0

by Jay Jonas
Eclipse 1.0

Eclipse 1.0 Splash Screen - version 1.0

Released at November 7, 2001.  (c) Copyright IBM Corp. 2000, 2001. All Rights Reserved. It can be downloaded in Eclipse Foundation following this link here.

January 19, 2011

PDE build: Bundle failed to resolve

by Jay Jonas

When exporting a feature, or a product based on features you get an error like:

Processing inclusion from feature <your feature here>: Bundle <version here> failed to resolve.:

This is caused by a missing window system/operating system/architecture filter on the fragment’s inclusion in the feature.xml. This can easily be set in the Plug-ins Tab of the feature editor by selecting the fragment on the left and specifying the Operating System, Window System and Architecture on the right.

Plugins and Fragments

Plugin details in Plugins and Fragments editor

Quoted from Eclipse PDE Build FAQ.

January 17, 2011

Eclipse: Import cannot be resolved

by Jay Jonas

Eclipse suddenly, and everyone is against you (or some) of its import are displaying the error message:

 import MyClassName cannot be resolved.

Do not panic yet. Before, you could try the following tips:

  • Clean Project command is your friend: go to Project > Clean menu.
  • If using Ant or Maven, use – clean option to clean your builds.
  • Refresh your project folder:  right click on your project folder  and click Refresh command.
  • Re-build your project.
  • Create a new Workspace and Import your project there.
  • ‘Switch’ Workspace – go to other Workspace the return back.
  • Double-check your JRE. Remove and re-add as follows:
  1. right click on your Project and click on  Properties from context menu;
  2. click on the Libraries tab;
  3. next, click on the JRE;
  4. click Remove, then OK;
  5. repeat 1-3 again, but this time add the JRE.
  • Finally, if your Target Plataform is divided into components in various folders, double check for duplicate plugins, ie plugins with same ID and version. If find one, remove it  mercilessly.

The tips can be followed in the order you prefer because not have a specific order and can work or not. Do not blame me if not. And make a backup before start.

January 13, 2011

Open a Eclipse View zoomed

by Jay Jonas
public class MyOpenViewZoomedCmdHandler extends AbstractHandler {

    public Object execute(ExecutionEvent event) throws ExecutionException {

        IWorkbenchWindow window = HandlerUtil.getActiveWorkbenchWindowChecked(event);

        IWorkbenchPage page = window.getActivePage();

        try {

            // any view in your application
            IViewPart view = page.showView(MyView.VIEW_ID);

            if (!page.isPageZoomed()) {
                IWorkbenchPartReference ref = page.getReference(view);
         } catch (PartInitException e) {
            // TODO Auto-generated catch block

        return null;