New plug-ins can be installed and existing plug-ins can be updated without restarting the web server. If the plugins.autounload setting in base.config has been enabled all you have to do to update a plug-in is to replace the JAR file with a new version. BASE will automatically load the new classes the next time the plug-in is used. If the option isn't enabled, the server admin has to manually unload the old code from the web interface first.

Plug-ins may use it's own 3-rd party libraries without interfering with other plug-ins. This may be important because a plug-in may depend on a certain version of a library while another plug-in may depend on a different version. Since BASE is using different class-loaders for different plug-ins this is not a problem.

A plug-in can only access/use classes from it's own JAR file, BASE core classes, Java system classes and from JAR files listed in the plug-in's MANIFEST.MF file. See Section 26.1, “How to organize your plug-in project”.

A plug-in can also access other plug-ins, but only via the methods and interfaces defined in BASE. In the following example we assume that there are two plug-ins, ex.MyPlugin and ex.MyOtherPlugin, located in two different JAR files. The code below is executing in the ex.MyPlugin:

// Prepare to load MyOtherPlugin
SessionControl sc = ...
DbControl dc = ...
PluginDefinition def = PluginDefinition.getByClassName(dc, "ex.MyOtherPlugin");

// Ok
Plugin other = def.newInstance(Plugin.class, null, sc, null, null);

// Not ok; fails with ClassCastException
MyOtherPlugin other = def.newInstance(MyOtherPlugin.class, null, sc, null, null);

The reason that the second call fails is that BASE uses a different classloader to load the ex.MyOtherPlugin class. This class is not (in Java terms) the same as the ex.MyOtherPlugin class loaded by the classloader that loaded the ex.MyPlugin class. If, on the other hand, both plug-ins are located in the same JAR file BASE uses the same classloader and the second call will succeed.

The first call always succeeds because it uses the Plugin interface which is defined by BASE. This class is loaded by the web servers class loader and is the same for all plug-ins.

A third option is that the ex.MyPlugin lists the JAR file where ex.MyOtherPlugin is located in it's MANIFEST.MF file. Then, the following code can be used: MyOtherPlugin other = new MyOtherPlugin();