Using the classloader.properties file

Eclipse v2.0.2 includes many performance enhancements, including some in the area of classloading. In order to help out with this a classloader properties file was created and this file explains its use.

Activation

To enable the classloader performance enhancements, the user must pass the -classloaderProperties command-line argument to the Eclipse executable. If the argument is used by itself the default location (see below) is used for the classloader properties file. Alternatively the argument can be followed by a path or URL to the appropriate file to use.

Location

The default location of the classloader properties file is as a sibling of the boot.jar in the org.eclipse.core.boot plug-in. For instance:
/eclipse/plugins/org.eclipse.core.boot/classloader.properties

If the user specifies the location of the file, it can be either a path or a URL to a file. For instance:
-classloaderProperties c:/temp/myfile.properties -classloaderProperties file:/c:/temp/myfile.properties

Format

The format of the classloader properties file is a java.util.Properties file. The key in the file is the name of the plug-in and the value is a comma-separated list of the package prefixes for the packages in the plug-in's jar.

It is quite common for jar files to contain code which reside in multiple packages. For instance, the runtime.jar file in the org.eclipse.core.runtime plug-in contains code in the following packages:
org.eclipse.core.runtime org.eclipse.core.internal.runtime, org.eclipse.core.internal.plugins org.eclipse.core.runtime.model.

In this case, in the classloader properties file we put specify the following line:

org.eclipse.core.runtime=org.eclipse.core

Notice that org.eclipse.core is a common prefix for all packages in the runtime.jar. The alternative is to declare all 4 prefixes in the file as a comma-separated list. In this case one must weigh the trade-off between the number of checks required against multiple entries and a prefix which may include false hits. Depending on the way that your code is structured, it might be best to list as many as 5-10 package prefixes rather than going with a more general prefix. For instance, if all your code across multiple plug-ins contains the same prefix (e.g. com.mycompany) then you will not be taking full advantage of all benefits if you list only the single prefix "com.mycompany" in the file.

It is also common for plug-ins to contain multiple jar files with code. In this case all package prefixes from all jars should appear in the classloader properties file.

Note that not having an entry for your plug-in as a key in the file doesn't mean that your code will not work, it just means that you will not be able to take advantage of classloading optimization. And note that missing a package prefix as a value entry in the file does mean that your code will not work.

See the file which is shipped with Eclipse for an example. (/eclipse/plugins/org.eclipse.core.boot/classloader.properties)

Future

As described above, currently one must add all package prefixes to the same properties file. Future work in this area includes the ability to add mark-up to your plugin.xml file to take advantage of this classloading performance behaviour to make it easier for people who are extending the Eclipse platform.

Trouble-shooting

If you get a java.lang.ClassNotFoundException then that is an indication that there might be a problem with your entries in the properties file. The file could have the correct syntax, but the package prefixes in the comma-separated list might be missing some entries. The workaround for this is to comment out the line of the offending plug-in.