| <?xml version="1.0"?> |
| <document> |
| <properties> |
| <title>Jackrabbit Extension Framework - Extension Classes</title> |
| </properties> |
| <body> |
| <section name="Extension Classes"> |
| <p> |
| The extension declaration may also contain the (fully |
| qualified) name of a class implementing the extension. |
| Besides finding extension descriptions the Jackrabbit Extension |
| Framework is also able to create instances of extensions |
| and prime them with the extension descriptor, which |
| defined how to instantiate the extension. |
| </p> |
| <p> |
| This page contains information on how class implementing |
| the extensions are loaded how the extensions are then |
| instantiated. |
| </p> |
| |
| <subsection name="Loading"> |
| <p> |
| All access to the Jackrabbit Extensions Framework goes |
| through an instance of the |
| <code>ExtensionManager</code> |
| class, which is created using a |
| <code>Session</code> |
| and a class loader. While the session is used to |
| access the repository and also confines access to a |
| certain workspace, the class loader provided is |
| actually the basis for loading extension classes. |
| </p> |
| <p> |
| As has been noted in the |
| <a href="index.html#Introduction">Introduction</a> |
| one of the advantages of the Jackrabbit Extension Framework |
| is its ability to load extension classes from the |
| repository, an extension may be packaged complete |
| with the descriptor, optional configuration and the |
| Java classes and/or archives implementing the |
| extensions. To be able to load the classes from the |
| repository, the Jackrabbit Extension Framework uses |
| instances of the <code>RepositoryClassLoader</code> |
| provided by the <code>classloader</code> project. |
| </p> |
| <p> |
| The extension manager creates a separate instance of |
| the |
| <code>RepositoryClassLoader</code> |
| for each extension type accessed. The class path |
| defined by an extension is added to that class |
| loader before trying to load the class through that |
| class loader. The class loader created by the |
| extension manager is linked into the system class |
| loader hierarchy by using the application provided |
| class loader given to the constructor of the |
| extension manager as its parent class loader. |
| </p> |
| <p> |
| Please be aware of an issue raising from the fact |
| that class may be loaded through a class loader |
| which is a child of the application's class loader: |
| While extensions have access to all the classes |
| provided by the repository class loader as well as |
| the application class loader and all its parents, |
| the application has no access to the classes loaded |
| through the repository class loader. This is easily |
| fixed by having the extensions implement interfaces |
| which are loaded through the application class |
| loader and to which the extension instances loaded |
| through the extension manager may be cast. |
| </p> |
| </subsection> |
| |
| <subsection name="Instantiation"> |
| <p> |
| The extension declaration may also contain the |
| (fully qualified) name of a class implementing the |
| extension. If so, the Jackrabbit Extension Framework |
| provides support functionality to load and |
| instantiate the respective class using either of two |
| constructors: |
| </p> |
| <ul> |
| <li> |
| If the class has a (public) constructor taking |
| an object of type |
| <code> |
| org.apache.jackrabbit.extension.ExtensionDescriptor |
| </code> |
| this constructor is used to instanti-ate the |
| extension instance. |
| </li> |
| <li> |
| Otherwise if the class has a (public) default |
| constructor this constructor is used to |
| in-stantiated the extension interface. |
| </li> |
| <li> |
| Otherwise the extension cannot be instantiated. |
| </li> |
| </ul> |
| <p> |
| If the extension cannot be instantiated through the |
| constructor taking the |
| <code>ExtensionDescriptor</code> |
| the application is responsible to provide the |
| relevant information to the extension instance if |
| required. |
| </p> |
| </subsection> |
| </section> |
| </body> |
| </document> |
