org.apache.java.lang
Class AdaptiveClassLoader

java.lang.Object
  |
  +--java.lang.ClassLoader
        |
        +--org.apache.java.lang.AdaptiveClassLoader

public class AdaptiveClassLoader
extends java.lang.ClassLoader

A class loader that loads classes from directories and/or zip-format file such as JAR file. It tracks the modification time of the classes it loads to permit reloading through re-instantiation.

When the classloader reports its creator that one of the classes it has loaded has changed on disk, it should discard the classloader and create a new instance using reinstantiate. The classes are then reloaded into the new classloader as required.

The classloader can also load resources, which are a means for packaging application data such as images within a jar file or directory.

The classloader always first tries to load classes and resources from the system, and uses it's own path if that fails. This is also done if an empty repository is passed at construction.

How autoreload works:

The Java VM considers two classes the same if they have the same fully-qualified name and if they were loaded from the same ClassLoader.

There is no way for a classloader to 'undefine' a class once it has been loaded. However, the servlet engine can discard a classloader and the classes it contains, causing the

The JServServletManager creates a new instance of the classloader each time it detects that any of the loaded classes have changed.

Before terminating, all servlets are destroyed.

According to the Java Language Specification (JLS), classes may be garbage-collected when there are no longer any instances of that class and the java.lang.Class object is finalizable. It is intended that this be the case when a JServClassLoader is discarded.

Many VM releases did not implement class garbage collection properly. In such a VM, the memory usage will continue to grow if autoreloading is enable. Running the VM with -verbosegc (or the corresponding option for non-Javasoft VMs) may give some debugging information.

It is important that the destroy method be implemented properly, as servlets may be destroyed and reinitialized several times in the life of a VM.

Version:
$Revision: 1.1 $ $Date: 1999/02/04 02:38:18 $
Author:
Francis J. Lacoste, Martin Pool, Jim Heintz,
Constructor Summary
AdaptiveClassLoader(java.util.Vector classRepository)
          Creates a new class loader that will load classes from specified class repositories.
 
Method Summary
 java.net.URL getResource(java.lang.String name)
          Find a resource with a given name.
 java.io.InputStream getResourceAsStream(java.lang.String name)
          Get an InputStream on a given resource.
protected  java.lang.Class loadClass(java.lang.String name, boolean resolve)
          Resolves the specified name to a Class.
 AdaptiveClassLoader reinstantiate()
          Re-instantiate this class loader.
 boolean shouldReload()
          Check whether the classloader should be reinstantiated.
 boolean shouldReload(java.lang.String classname)
          Check to see if a given class should be reloaded because of a modification to the original class.
 
Methods inherited from class java.lang.ClassLoader
defineClass, defineClass, defineClass, definePackage, findClass, findLibrary, findLoadedClass, findResource, findResources, findSystemClass, getPackage, getPackages, getParent, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, loadClass, resolveClass, setSigners
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

AdaptiveClassLoader

public AdaptiveClassLoader(java.util.Vector classRepository)
                    throws java.lang.IllegalArgumentException
Creates a new class loader that will load classes from specified class repositories.
Parameters:
classRepository - An set of File classes indicating directories and/or zip/jar files. It may be empty when only system classes are loaded.
Method Detail

shouldReload

public boolean shouldReload(java.lang.String classname)
Check to see if a given class should be reloaded because of a modification to the original class.
Parameters:
className - The name of the class to check for modification.

shouldReload

public boolean shouldReload()
Check whether the classloader should be reinstantiated.

The classloader must be replaced if there is any class whose origin file has changed since it was last loaded.


reinstantiate

public AdaptiveClassLoader reinstantiate()
Re-instantiate this class loader.

This method creates a new instance of the class loader that will load classes form the same path as this one.


loadClass

protected java.lang.Class loadClass(java.lang.String name,
                                    boolean resolve)
                             throws java.lang.ClassNotFoundException
Resolves the specified name to a Class. The method loadClass() is called by the virtual machine. As an abstract method, loadClass() must be defined in a subclass of ClassLoader.
Overrides:
loadClass in class java.lang.ClassLoader
Parameters:
name - the name of the desired Class.
resolve - true if the Class needs to be resolved; false if the virtual machine just wants to determine whether the class exists or not
Returns:
the resulting Class.
Throws:
java.lang.ClassNotFoundException - if the class loader cannot find a the requested class.

getResourceAsStream

public java.io.InputStream getResourceAsStream(java.lang.String name)
Get an InputStream on a given resource. Will return null if no resource with this name is found.

The JServClassLoader translate the resource's name to a file or a zip entry. It looks for the resource in all its repository entry.

Overrides:
getResourceAsStream in class java.lang.ClassLoader
Parameters:
name - the name of the resource, to be used as is.
Returns:
an InputStream on the resource, or null if not found.
See Also:
Class.getResourceAsStream(String)

getResource

public java.net.URL getResource(java.lang.String name)
Find a resource with a given name. The return is a URL to the resource. Doing a getContent() on the URL may return an Image, an AudioClip,or an InputStream.

This classloader looks for the resource only in the directory repository for this resource.

Overrides:
getResource in class java.lang.ClassLoader
Parameters:
name - the name of the resource, to be used as is.
Returns:
an URL on the resource, or null if not found.