When an Eclipse instance is launched in run mode, changes made to the source code aren't reflected in the running instance. However, debug mode allows changes made to the source to be reflected in the running test Eclipse instance.
-
Launch the test Eclipse in debug mode by clicking on the Debug
icon.
-
Click on the Hello World icon
in the test Eclipse to display the dialog, as before, and click on OK to dismiss it. It may be necessary to remove or resume the breakpoint in the host Eclipse instance to allow execution to continue.
- In the host Eclipse, open the
SampleHandlerclass and go to theexecute()method. - Change the title of the dialog to
Hello again, Eclipse worldand save the file. Provided that Project | Build Automatically is enabled, the change will be recompiled. - Click on the Hello World icon in the test Eclipse instance again. The new message should be shown.
By default, Eclipse ships with Project | Build Automatically enabled. Whenever changes are made to Java files, they are recompiled along with their dependencies if necessary.
When a Java program is launched in run mode, it will load classes in on-demand and then keep using that definition until the JVM shuts down. Even if the classes are changed, the JVM won't notice that they have been updated, and so no differences will be seen in the running application.
However, when a Java program is launched in debug mode, whenever changes to classes are made, it will update the running JVM with the new code if possible. The limits to what can be replaced are controlled by the JVM through the JVMTI and whether, for example, the virtual machine's canUnrestrictedlyRedefineClasses() call returns true. Generally, updating an existing method and adding a new method or field will work, but changes to interfaces and super classes may not be. (Refer to http://en.wikipedia.org/wiki/Java_Virtual_Machine_Tools_Interface for more information.)
Note
The ex-Sun Hotspot JVM cannot replace classes if methods are added or interfaces are updated. Some JVMs have additional capabilities which can substitute more code on demand. With the merging of JRockit and Hotspot over time, more may be replaceable at runtime than before; for everything else, there's JRebel.
Other JVMs, such as IBM's, can deal with a wider range of replacements.
Note that there are some changes which won't be picked up; for example, new extensions added to the plugin.xml file. In order to see these changes, it is possible to start and stop the plug-in through the command-line OSGi console, or restart Eclipse inside or outside the host Eclipse to see the change.