platform-core-home/docs/package-prefixes/prefixes.html
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Thu May 19 21:19:33 2005 UTC (4 years, 6 months ago) by johna
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Changes since 1.1: +1 -1 lines
Thu May 19 21:19:33 2005 UTC (4 years, 6 months ago) by johna
Branch: MAIN
CVS Tags: HEAD
Changes since 1.1: +1 -1 lines
Bug 36171 Typo 'attrbute' in org.eclipse.platform.doc.isv/reference/misc/plugin_package_prefixes.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>Using the Class Loader Properties File</TITLE> <META http-equiv=Content-Type content="text/html; charset=windows-1252"> <META content="MSHTML 6.00.2800.1126" name=GENERATOR></HEAD> <BODY> <H1>Using the Class Loader Enhancements</H1> <EM>Last update: December 9, 2002</EM> <P>Eclipse 2.1 includes many performance enhancements, including some in the area of classloading. In order to help out with this, mark-up has been added to the plug-in manifest file and this file explains its use. <P> <UL> <LI><A href="#activation">Activation</A> <LI><A href="#format">Format</A> <LI><A href="#classloader.properties">Use with the Classloader Properties file</A> <LI><A href="#trouble">Trouble-Shooting</A> <LI><A href="#known issues">Known Issues</A> </UL> <P> <H2><A name=activation></A>Activation</H2> The classloader performance enhancements are automatically enabled by default. If the user wishes to override this behaviour and disable them, then they may pass the <CODE>-noPackagePrefixes</CODE> command-line argument to the Eclipse executable. <P> <H2><A name=format></A>Format</H2> <p>Declaration of the package prefixes in the plug-in manifest file is indicated by a <code>packages</code> element for each library which is declared in the file. The <code>packages</code> element has a <code>prefixes</code> attribute which lists the package prefixes for that library.</p> <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> org.eclipse.core.runtime<BR> org.eclipse.core.internal.runtime,<BR> org.eclipse.core.internal.plugins<BR> org.eclipse.core.runtime.model</CODE>. </p> <P>In this case, the <code>plugin.xml</code> specifies: <P> <code> <runtime><br> <library name="runtime.jar"><br> <export name="*"/><br> <packages prefixes="org.eclipse.core"/><br> </library><br> </runtime> </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 "com.mycompany" in the file. <P>When a plug-in contains multiple library declarations, each one should account for the package prefixes from its jar. <P>If you choose not to include any <code>packages</code> elements in your plug-in manifest your code will still work but you will not be taking advantage of the class loading optimization. Note that your list of package prefixes <b>must</b> be complete for all the packages in all the libraries in your plug-in. If this list is not complete then your code will not work. <H2><A name=classloader.properties>Use with the Class Loader Properties file</A></H2> <p>In Eclipse 2.0.2 and early Eclipse 2.1 Integration builds, a feature was added which allowed the user to specify a class loader properties file which contained the package prefixes to enable the classloader enhancements. Use of this file is specified <a href="http://dev.eclipse.org/viewcvs/index.cgi/%7Echeckout%7E/platform-core-home/docs/classloader-properties/classloader_properties.html">here</a>. </p> <p>Eclipse 2.1 is fully backwards compatible with the class loader properties file and, in fact, it can be used in conjunction with the package prefix declarations in the plug-in manifest. This section describes the behaviour of the interaction between these two mechanisms.</p> <p><em>Default behaviour: </em>(no command-line arguments) The package prefixes will be read from the <code>plugin.xml</code> file and applied to the class loading strategy. The <code>classloader.properties</code> file is not accessed or considered. <p><em>Using -classloaderProperties: </em>The package prefixes are read from the <code>plugin.xml</code> file and then the <code>classloader.properties</code> file is read. If there is an entry in the classloader properties file for a plug-in which had prefixes defined in the manifest file, the entry in the properties file will over-ride and take precedence. (the results are NOT merged) If the entry in the properties file does not contain a value (e.g. <code>org.eclipse.core.runtime=</code>) then any prefixes declared in the manifest file will be removed and the class loading strategy will default to not using the enhancements. <p><em>Using -noPackagePrefixes: </em>Any package prefixes declared in the plug-in manifest file are ignored and the class loader properties file is also not taken into consideration. The class loading strategy defaults to not using the prefix enhancements. <p><em>Using both -classloaderProperties and -noPackagePrefixes: </em>Any package declarations in the plug-in manifest file are ignored. The class loader properties file is read and any declarations defined within it are taken into consideration when in class loading. <P><h2><A name=trouble>Trouble-shooting</a></h2> <p>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 manifest or properties files. Try running Eclipse with the <code>-noPackagePrefixes</code> command-line argument and do NOT use the <code>-classloaderProperties</code> argument. This will disable all class loading enhancements with respect to the package prefixes. </p> <p>If your code runs fine after doing this, then the files 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, comment out the appropriate lines in the file. Use a number sign (#) to comment out the line of the offending plug-in in the properties file or use HTML comment characters (<!-- ... -->) to comment out the packages declaration in the offending <code>plugin.xml</code> file. </p> <H2><A name="known issues">Known Issues</A></H2> If a plug-in manifest file declares the list of package prefixes, then all fragments which contribute to it must also list their package prefixes. If they don't then it will result in a <code>NoClassDefFoundError</code> problem. This can be resolved either by using the <code>-noPackagePrefixes</code> command-line argument to turn off the classloader enhancements, or by adding the appropriate package prefix entries to the plug-in and all its fragments. </BODY></HTML>
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