The Extension Mechanism - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

Support for Extensions and Applications
in the Version 1.2 of the Java^TM Platform

Introduction
The Extension Mechanism
Extension Sealing
Extension Security
API Details
Java Command Changes

Introduction

This document describes the Java platform's new mechanism for handling extensions. An extension is a group of packages housed in one or more JAR files that implement an API that extends the Java platform. Extension classes extend the platform in the sense that the virtual machine can find and load them without their being on the class path, much as if they were classes in the platform's core API.
An implementation of an extension may consist of code written in the Java programming language and, less commonly, platform-specific native code. In addition, it may include properties, localization catalogs, images, serialized data, and other resources specific to the extension.
Support for extensions in browsers such as Internet Explorer and Netscape Navigator is available through the Java Plug-in.
A standard extension is an implementation of an open, standard API (examples of standard extensions from Sun are JavaServlet, Java3D, JavaManagement). Most standard extension packages are rooted in the javax.* namespace, although there may be exceptions.

The Extension Mechanism

Architecture
The extension mechanism is designed to contain the following elements:

an extension or application packaged as a JAR file can declare dependencies on other JAR files, thus allowing an application to consist of multiple modules, and,

the class loading mechanism is augmented to search installed extensions (and other libraries) for classes, and, if that fails, to search along an application-specified path for classes.
Applications must therefore, in general, be prepared to specify and supply the extensions (and, more generally, libraries) that it needs. The system will prefer installed copies of extensions (and libraries) if they exist; otherwise, it will delegate to the class loader of the application to find and load the referenced extension (and library) classes.
This architecture, since it allows applications, applets and servlets to extend their own class path, also permits packaging and deploying these as multiple JAR files.
Each extension or application consists of at least one JAR file containing an optional manifest, code and assorted resources. As described below, this primary JAR file can also include additional information in its manifest to describe dependencies on other JAR files. The jar command line tool included with the JDK software provides a convenient means of packaging extensions, although any ZIP-compatible archiving tool should work as well. (See the reference pages for the jar tool: [Win32][Solaris])
An extension or application may refer to additional JAR files which will be referenced from the primary JAR, and these can optionally contain their own dependency information as well.
Packages comprising extensions should be named per the standard package naming conventions when implementing extensions. These conventions are outlined in The Java Language Specification, but the requirement that the domain prefix be specified in all upper case letters has been removed. For example, the package name com.sun.server is an accepted alternative to COM.sun.server. Unique package naming is recommended in order to avoid conflicts, because applications and extensions may share the same class loader.
Extension Deployment

An extension may either be bundled with an application or installed in the JRE for use by all applications. Bundled extensions are provided at the same code base as the application and will automatically be downloaded in the case of network applications (applets). For this reason, bundled extensions are often called download extensions. Installed extensions are loaded when first used and will be shared by all applications running in the JRE.
When packaging extensions, the JAR file manifest can be used to identify vendor and version information (see The Java Versioning Specification).
Classes for installed extensions are shared by all code in the same Java 1.2 virtual machine. Thus, installed extensions are similar to the platform's core classes (in rt.jar), but with an associated class loader and a pre-configured security policy as described below.
Classes for bundled extensions are private to the class loader of the application, applet or servlet. In the case of network applications such as applets, these extensions will be automatically downloaded as needed. Since class loaders are currently associated with a codebase, this permits multiple applets originating from the same codebase to share implementations (JARs).
Bundled Extensions
The manifest for an application or extension can specify one or more relative URLs referring to the JAR files and directories for the extensions (and other libraries) that it needs. These relative URLs will be treated relative to the code base that the containing application or extension JAR file was loaded from.
An application (or, more generally, JAR file) specifies the relative URLs of the extensions (and libraries) that it needs via the manifest attribute Class-Path. This attribute lists the URLs to search for implementations of extensions (or other libraries) if they cannot be found as extensions installed on the host Java virtual machine. These relative URLs may include JAR files and directories for any libraries or resources needed by the application or extension. Relative URLs not ending with '/' are assumed to refer to JAR files. For example,
Class-Path: servlet.jar infobus.jar acme/beans.jar images/
Multiple Class-Path headers may be specified, and are combined sequentially.
Currently, the URLs must be relative to the code base of the JAR file for security reasons. Thus, remote extensions will originate from the same code base as the application. A future enhancement will leverage the facilities of the Java 1.2 Security APIs to allow references to JAR files at other URLs.
Each relative URL is resolved against the code base that the containing application or extension was loaded from. If the resulting URL is invalid or refers to a resource that cannot be found then it is ignored.
The resulting URLs are used to extend the class path for the application, applet, or servlet by inserting the URLs in the class path immediately following the URL of the containing JAR file. Any duplicate URLs are omitted. For example, given the following class path:
	a.jar b.jar
If extension b.jar contained the following Class-Path manifest attribute:
	Class-Path: x.jar a.jar
Then the resulting application class path would be the following:
	a.jar x.jar b.jar
Of course, if x.jar had dependencies of its own then these would be added according to the same rules and so on for each subsequent URL. In the actual implementation, JAR file dependencies are processed lazily so that the JAR files are not actually opened until needed.
Installed Extensions
In Sun's implementation of the Java 1.2 virtual machine, the JAR files of a installed extension are placed in a standard local code source
<java-home>\lib\ext			[Win32]
<java-home>/lib/ext			[Solaris]
Its native code libraries, if any, are placed in
<java-home>\bin				[Win32]
<java-home>/lib/<arch>		        [Solaris]
Here <java-home> refers to the directory where the runtime software is installed (which is the top-level directory of the JRE or the jre directory in the JDK software), and <arch> refers to the Solaris processor architecture (sparc or i386).
A installed extension may additionally contain one or more shared libraries (such as .dll files) and executables. Native libraries may also be placed in jre/lib/ext/<arch> for both Win32 and Solaris, where <arch> will be i386 on Win32 systems. The jre/lib/ext/<arch> directory is searched after jre\bin (win32) or jre/lib/<arch> (Solaris).
Currently, an extension that contains native code cannot be downloaded by network code, whether such code is trusted or not, into the virtual machine at execution time. An extension that contains native code and is bundled with a network application must be installed in the JRE or JDK^TM software.
By default, installed extensions in this standard directory are trusted. That is, they are granted the same privileges as if they were core platform classes (those in rt.jar). This default privilege is specified in the system policy file, but can be overridden for a particular extension by adding the appropriate policy file entry.
Note also that if a installed extension JAR is signed by a trusted entity, then it will be granted the privileges associated with the trusted signer.
Other locations for installed extensions can be specified through the system property java.ext.dirs. This property specifies one or more directories to search for installed extensions, each separated by File.pathSeparatorChar. The default setting for this property is the standard directory for installed extensions indicated above.

Extension Sealing

JAR files and packages can be optionally sealed, so that an extension or package can enforce consistency within a version.
A package sealed within a JAR specifies that all classes defined in that package must originate from the same JAR. Otherwise, a SecurityException is thrown.
A sealed JAR specifies that all packages defined by that JAR are sealed unless overridden specifically for a package.
A sealed package is specified via the new manifest attribute, Sealed, whose value is true or false (case irrelevant). For example,
Name: javax/servlet/internal/
Sealed: true
specifies that the javax.servlet.internal package is sealed, and that all classes in that package must be loaded from the same JAR file.
If this attribute is missing, the package sealing attribute is that of the containing JAR file.
A sealed JAR is specified via the same new manifest header, Sealed, with the value again of either true or false. For example,
Sealed: true
specifies that all packages in this archive are sealed unless explicitly overridden for a particular package with the Sealed attribute in a manifest entry.
If this attribute is missing, the module is assumed to not be sealed, for backwards compatibility. The system then defaults to examining package headers for sealing information.
Package sealing is also important for security, because it restricts access to package-protected members to only those classes defined in the package that originated from the same JAR file. For example, in the runtime JAR rt.jar all of the standard core Java packages with the exception of packages sun.io and java.text.resources are sealed.
Package sealing is checked for installed as well as downloaded extensions, and will result in a SecurityException if violated. Also, the null package is not sealable, so classes that are to be sealed must be placed in their own packages.

Extension Security

The code source for a installed extension (namely <java-home>/lib/ext) has a pre-configured security policy associated with it. In Sun's implementation, the exact level of trust granted to JARs in this directory is specified by the standard security policy configuration file
<java-home>/lib/security/java.policy
The default policy is for a installed extension to behave the same way it would if were part of the core platform. This follows from the common need for a installed extension to load native code.
The Java 1.2 Security Model provides some safety when installed extension code is called from untrusted code. However extension code must be carefully reviewed for potential security breaches wherever it uses privileged blocks.
A remotely loaded extension that needs to use access-checked system services (such as file I/O) to function correctly must either be signed by a trusted entity or loaded from a trusted source.
Consult the Java 1.2 security documentation for further details regarding how to write extension and application code to use the new security features.

API Details

The following classes have been changed or added in the 1.2 platform to support the new extensions mechanism:
public class java.lang.ClassLoader (changed)
This class has been updated to support the new class loader delegation model for loading classes and resources. The class loader delegation model allows a "parent" class loader to be specified which will always be searched first when loading a class or resource before attempting to load it locally. New class loader implementations override the methods findClass and findResource in order to specify how classes and resources are loaded locally.
The following methods have been changed or added to support the new delegation model:

getSystemClassLoader
Returns the system class loader for delegation. This is the default delegation parent for new ClassLoader instances, and is typically the class loader used to start the application.

loadClass
If the class has already been loaded then just return it. Otherwise, try loading the class from the parent class loader (or virtual machine's built-in class loader, called the bootstrap class loader, if no parent was specified). If still not found, then call the method findClass to load the class locally.

findClass
Looks in this class loader for the specified class. This method should be overridden in new class loader implementations. Its default implementation throws ClassNotFoundException.

getResource
Try fetching the resource from the parent class loader (or bootstrap class loader if no parent was specified). If still not found, then call the method getLocalResource to load the resource locally.

findResource
Looks in this class loader for the specified resource. This method should be overridden by new class loader implementations. Its default implementation returns null.

getResources
Returns an enumeration of all the resource URLs matching the specified resource name. This enumeration includes all of the matched resources of the parent class loader followed by the enumeration returned by findResources.

findResources
Returns an enumeration of all the local resource URLs matching the given name. This method should be overridden by new class loader implementations. Its default implementation returns null.

The new delegation model provides a consistent and well defined search policy for loading classes and resources that simplifies class loader implementation. These changes are backward compatible and will not affect existing class loader implementations. However, applications that wish to create their own class loaders and use extensions must use the new delegation model.
The following example demonstrates a simple network class loader:
    public class NetClassLoader extends ClassLoader {
	URL url;

	public NetClassLoader(URL url, ClassLoader parent) {
	    super(parent);
	    this.url = url;
	}

	protected Class findClass(String name)
	    throws ClassNotFoundException
	{
	    .. load class from URL ...
	}

        protected URL findResource(String name) {
            try {
                URL u = new URL(url, name);
                if (u.openConnection() != null) {
                    return u;
                } else {
                    return null;
                }
            } catch (java.net.MalformedURLException mue) {
                   // handle exception
            } catch (java.io.IOException ioe) {
                   // handle exception
            }
        }
    }
This example will search the parent class loader (or bootstrap class loader, if no parent was specified) for classes and resources before checking the given URL. Classes and resources will be loaded according to the new delegation model as described above.
public class java.lang.Package
This class has been changed to include two new methods for accessing information about sealed packages. The method isSealed can be used to check if a package has been sealed, in which case it will return true. A second form of this method takes a URL and will return true if the package is sealed with respect to the specified URL. This can be used to in class loader implementations to keep track of currently sealed packages when loading new classes.

public class java.net.URLClassLoader
This class provides the basic class loader support for extensions and applications. It overrides both the findClass and findResource methods to search one or more base URLs for classes and resources. The search is lazy, such that a URL is not opened until needed.
URLClassLoader manages a search path of URLs that is used to load classes and resources. Initially, this is set to the URLs specified when the class loader was created, but can be extended by Class-Path manifest attribute as described above.
URLClassLoader supports all of the manifest attributes for versioning specified in The Java Versioning Specification.
In addition, the following main manifest attributes are supported:

Main-Class: <classname> (no default)
Specifies the main class of an application. This is used for invoking an application JAR file.

Class-Path: <urls..> (no default)
Local search path of relative JAR and directory URLs for loading classes and resources. URLs not ending in '/' are assumed to refer to JAR files.

Sealed: <true|false> (default false)
All of the packages defined in this JAR file, with the exception of the "null" package, are sealed. The "null" package can never be sealed. This header can also be used to seal individual packages.

Java Command Changes

Overview
The java command has been changed in version 1.2 of the JRE and JDK software in order to support stand-alone applications that use extensions. Previously, application classes had been loaded from the system class loader. In the new java command, an instance of URLClassLoader is created to load the application. The delegation parent is set to to the shared class loader maintained by the system for loading installed extensions. The resulting application class loader will then handle loading both installed and bundled extensions for the application.
Application Class Path
The property java.class.path is read to obtain the search path of URLs to use when creating the instance of URLClassLoader to load the application. This specifies the application's class path for loading application classes and resources. When loading a class or resource, the class loader will first search the system class path, then installed extensions, and finally the application class path.
A consequence of this change is that the property java.class.path no longer includes entries from the bootstrap class path. The system class path can now be obtained separately from the system property sun.boot.class.path.
The CLASSPATH environment variable specifies the default value of the property java.class.path. If this environment variable is not set, then the default value for java.class.path is set to the current directory.
The option -classpath is now shorthand for setting the java.class.path property. Formerly, this option was used to override the search path for system classes. In the new java command there is no longer any need to set the system class path.
The -cp Option
The option -cp which had previously been part of only the jre command has now been added to java as well. Since it is no longer necessary to prepend the system class path this option is now merely an alias for -classpath.
For example, the following command usages are all equivalent:
java -classpath foo.jar:bar Foo java -Djava.class.path=foo.jar:bar Foo java -cp foo.jar:bar Foo

Setting Security Properties
By default, applications are invoked without a security manager so that no policy checking is enabled. This is compatible with the previous command behavior where everything was loaded from the system class path.
To install a security manager when invoking an application, use the -Djava.security.manager command-line option:
java -Djava.security.manager Foo

The default system policy file resides at {java.home}/lib/security/java.policy. Individual users may augment this with policy files located at {user.home}/.java.policy. To specify a policy file to be used in addition to these default policies, use:
java -Djava.security.policy=somePolicy Foo

See the JDK 1.2 documentation for more information on policy files.
The -jar Option
For conveniently running applications packaged in JAR files the new -jar option has been added to the java application-launcher tool. This allows applications in JAR files to be executed directly using the java command.
When using the -jar option the java.class.path property is set to the JAR file being executed, which overrides any other definition specified via the -classpath or -cp command-line options. The JAR file's manifest is then consulted to find the name of the application's main class specified via the Main-Class manifest attribute. For example, if the manifest for server.jar contained the following attribute:
Main-Class: com.sun.server.Main
then server.jar could be executed using either of the following:
java -jar server.jar java -Djava.class.path=server.jar com.sun.server.Main

Executable JAR Files
On Win32 systems the Java 2 runtime environment's installation program will register a default association for JAR files so that double-clicking a JAR file on the desktop will automatically run it with javaw -jar. Dependent extensions bundled with the application will also be loaded automatically. This feature makes the end-user runtime environment easier to use on Win32 systems.
The Solaris 2.6 kernel has already been extended to recognize the special "magic" number that identifies a JAR file, and to invoke java -jar on such a JAR file as if it were a native Solaris executable. A application packaged in a JAR file can thus be executed directly from the command line or by clicking an icon on the CDE desktop.
The jre Command
The jre command is obsolete for JRE 1.2 and has been merged with the java tool. The only environment variable still used is CLASSPATH and this can now be overridden by using the java tool's -classpath or -cp option. The -cp option which used to prepend the system class path in the JRE is now identical to -classpath in the new java command.
Compatibility
Applications which install their own class loaders or security managers may not be able to run under the new java command until they are updated for version 1.2 of the platform. These applications must be updated to use the new security features and delegation class loader introduced in the 1.2 JRE and JDK software.
For compatibility we still provide the old-style launcher behavior in the command oldjava which can be used to run applications from the system class path as in version 1.1 of the Java platform. However, extensions are not supported with the oldjava command.

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The Extension Mechanism - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

Support for Extensions and Applications in the Version 1.2 of the JavaTM Platform

Contents

Introduction

The Extension Mechanism

Architecture

Extension Deployment

Bundled Extensions

Installed Extensions

Extension Sealing

Extension Security

API Details

Java Command Changes

Overview

Application Class Path

The -cp Option

Setting Security Properties

The -jar Option

Executable JAR Files

The jre Command

Compatibility

Support for Extensions and Applications
in the Version 1.2 of the Java^TM Platform