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Classworlds API Usage

The Java API can be used to create new realms and connect realms together through importation of specific packages.

The core of the Classworlds infrastructure is the ClassWorld class. An application must create a ClassWorld instance. It is advisable to store the instance as a singleton or some other handy location. 

ClassWorld world = new ClassWorld();

Once a ClassWorld is created, realms within it can be created. These realms effectively only allow loading of the core JVM classes initially. 

ClassWorld world = new ClassWorld();
ClassRealm containerRealm    = world.newRealm( "container" );
ClassRealm logComponentRealm = world.newRealm( "logComponent" );

In order to make each ClassRealm useful, constituents in form of URLs must be added to it where each can provide certain classes. The URL must return either a JAR or a directory on the default file system. 

containerRealm.addURL( containerJarUrl );
logComponentRealm.addURL( logComponentJarUrl );

ClassRealms can optionally be filtered to further restrict which classes/resources are exposed. The filter is provided as additional argument to world.newRealm( "filteredcontainer", myPredicate );

Now, links between the various realms need to be created to allow classes loaded from one to be available to classes loaded in another. 

logComponentRealm.importFrom( "container", 
                              "com.werken.projectz.component" );

The container implementation can then be loaded from its realm and used. 

Class containerClass = containerRealm.loadClass( CONTAINER_CLASSNAME );
MyContainer container = (MyContainer) containerClass.newInstance();
Thread.currentThread().setContextClassLoader( containerRealm.getClassLoader() );
container.run();

Ideally, the container itself would be responsible for creating a ClassRealm for each component that's loaded, and importing the component contract interfaces into the component's ClassRealm and using loadClass(..) to gain entry into the sandboxed component realm.