Understanding Extension Class Loading - Java Tutorial 5.0 英文版

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Understanding Extension Class Loading

The extension framework makes use of the new class-loading mechanism in the Java^TM 1.2 platform. When the runtime environment needs to load a new class for an application, it looks for the class in the following locations, in order:

1. Bootstrap classes: the runtime classes in rt.jar and internationalization classes in i18n.jar.
2. Installed extensions: classes in JAR files in the lib/ext directory of the JRE.
3. The class path: classes, including classes in JAR files, on paths specified by the system property java.class.path. If a JAR file on the class path has a manifest with the Class-Path attribute, JAR files specified by the Class-Path attribute will be searched also. By default, the java.class.path property's value is ., the current directory. You can change the value by setting the CLASSPATH environment variable or by using the -classpath or -cp command-line options. These command-line options override the setting of the CLASSPATH environment variable. Note that in the Java 1.2 software, java.class.path no longer includes the bootstrap classes in rt.jar and i18n.jar.

The precedence list tells you, for example, that the class path is searched only if a class to be loaded hasn't been found among the classes in rt.jar, i18n.jar or the installed extensions.

Unless your software instantiates its own class loaders for special purposes, you don't really need to know much more than to keep this precedence list in mind. In particular, you should be aware of any class name conflicts that might be present. For example, if you list a class on the class path, you'll get unexpected results if the runtime environment instead loads another class of the same name that it found in an installed extension.

The 1.2 Class Loading Mechanism

The Java 1.2 platform uses a new delegation model for loading classes. The basic idea is that every class loader has a "parent" class loader. When loading a class, a class loader first "delegates" the search for the class to its parent class loader before attempting to find the class itself.

Here are some highlights of the class-loading API:

• Constructors in java.lang.ClassLoader and its subclasses allow you to specify a parent when you instantiate a new class loader. If you don't explicitly specify a parent, the virtual machine's system class loader will be assigned as the default parent.
• The loadClass method in ClassLoader performs these tasks, in order, when called to load a class:
  1. If a class has already been loaded, it returns it.
  2. Otherwise, it delegates the search for the new class to the parent class loader.
  3. If the parent class loader doesn't find the class, loadClass calls the method findClass to find and load the class.
• The findClass method of ClassLoader searches for the class in the current class loader if the class wasn't found by the parent class loader. You will probably want to override this method when you instantiate a class loader subclass in your application.
• The class java.net.URLClassLoader has been added to the core API. This class serves as the basic class loader for extensions and other JAR files, overriding the findClass method of java.lang.ClassLoader to search one or more specified URLs for classes and resources.

To see a sample application that uses some of the new API as it relates to JAR files, see the JAR File Format lesson in this tutorial.

Changes to the java Command

The 1.2 platform's class-loading mechanism is reflected in some changes to the java command.

• The 1.2 JRE now includes the same Java interpreter, invoked with the java command, as is in the JDK 1.2 software. This tool replaces the old jre tool in JRE 1.1.
• In the 1.2 java tool, the -classpath option is a shorthand way to set the java.class.path property. Formerly the -classpath option was used to override the search path for system classes.
• The -cp option, formerly part of the jre utility, has been added as an option to the java command. The -cp and -classpath options are equivalent.
• The -jar option has been added for running applications that are packaged in JAR files. For a description and examples of this option, see the JAR File Format lesson in this tutorial.

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