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<title>Using the Class Loader Properties File</title>
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<h1>Using the Class Loader Properties File</h1>
Eclipse 2.0.2 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. 
<p>

<ul>
<li><a href="#activation">Activation</a></li>
<li><a href="#location">Location</a></li>
<li><a href="#format">Format</a></li>
<li><a href="#future">Future</a></li>
<li><a href="#trouble">Trouble-Shooting</a></li>
</ul>
<p>

<h2><a name="activation"></a>Activation</h2>
To enable the classloader performance enhancements, the user must pass the <code>-classloaderProperties</code>
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. 
<p>

<h2><a name="location"></a>Location</h2>
The default location of the classloader properties file is as a sibling of the <code>boot.jar</code>
 in the <code>org.eclipse.core.boot</code> plug-in.
For instance:<br>
&nbsp;&nbsp;&nbsp;&nbsp;<code>/eclipse/plugins/org.eclipse.core.boot_2.0.2/classloader.properties</code> 
<p>

If the user specifies the location of the file, it can be either a path or a URL to a file. For instance:<br>
<code>
&nbsp;&nbsp;&nbsp;&nbsp;-classloaderProperties c:/temp/myfile.properties<br>
&nbsp;&nbsp;&nbsp;&nbsp;-classloaderProperties file:/c:/temp/myfile.properties
</code>
<p>

<h2><a name="format"></a>Format</h2>
The format of the classloader properties file is a <code>java.util.Properties</code> 
file. Each 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. 
<p>

It is quite common for jar files to contain code which reside in multiple packages. For instance, the 
<code>org.eclipse.core.runtime</code> plug-in contains code in the following packages:<br>
<code>
&nbsp;&nbsp;&nbsp;&nbsp;org.eclipse.core.runtime<br>
&nbsp;&nbsp;&nbsp;&nbsp;org.eclipse.core.internal.runtime,<br>
&nbsp;&nbsp;&nbsp;&nbsp;org.eclipse.core.internal.plugins<br>
&nbsp;&nbsp;&nbsp;&nbsp;org.eclipse.core.runtime.model</code>.
<p>

In this case, the classloader properties specifies:
<p>

&nbsp;&nbsp;&nbsp;&nbsp;<code>org.eclipse.core.runtime=org.eclipse.core</code>
<p> Notice that <code>org.eclipse.core</code> is a common prefix for all packages 
  in the plug-in. 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. <code>com.mycompany</code>) then you will not 
  be taking full advantage of all benefits if you list only the single prefix 
  &quot;com.mycompany&quot; in the file. 
<p>

When a plug-in contains multiple jar files with code, the entry in the the class loader 
properties file should account for all package prefixes from all jars.
<p> Note that missing a package prefix as a value entry in the file means that 
  your code will not work but 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. 
<p>See the file which is shipped with Eclipse for an example. (<code>/eclipse/plugins/org.eclipse.core.boot_2.0.2/classloader.properties</code>) 
<p>

<h2><a name="future"></a>Future</h2>
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 <code>plugin.xml</code> 
file to take advantage of this classloading performance behavior to make it easier 
for people who are extending the Eclipse platform. 
<p>

<h2><a name="trouble"></a>Trouble-shooting</h2>
If you get a <code>java.lang.ClassNotFoundException</code> 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. To verify this is the problem, use a number sign (#) to comment
out the line of the offending plug-in.
<p>

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