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Java(TM) 2 Platform, Standard Edition, v1.2.2 API Specification: Class URLClassLoader - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

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java.net
Class URLClassLoader

java.lang.Object
  |
  +--java.lang.ClassLoader
        |
        +--java.security.SecureClassLoader
              |
              +--java.net.URLClassLoader

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public class URLClassLoader

extends SecureClassLoader

This class loader is used to load classes and resources from a search path of URLs referring to both JAR files and directories. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.

The AccessControlContext of the thread that created the instance of URLClassLoader will be used when subsequently loading classes and resources.

The classes that are loaded are by default granted permission only to access the URLs specified when the URLClassLoader was created.

Since:

JDK1.2

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Constructor Summary

URLClassLoader(URL[] urls)           Constructs a new URLClassLoader for the specified URLs using the default delegation parent ClassLoader.

URLClassLoader(URL[] urls, ClassLoader parent)           Constructs a new URLClassLoader for the given URLs.

URLClassLoader(URL[] urls, ClassLoader parent, URLStreamHandlerFactory factory)           Constructs a new URLClassLoader for the specified URLs, parent class loader, and URLStreamHandlerFactory.

 

Method Summary
protected  void	addURL(URL url)           Appends the specified URL to the list of URLs to search for classes and resources.
protected  Package	definePackage(String name, Manifest man, URL url)           Defines a new package by name in this ClassLoader.
protected  Class	findClass(String name)           Finds and loads the class with the specified name from the URL search path.
URL	findResource(String name)           Finds the resource with the specified name on the URL search path.
Enumeration	findResources(String name)           Returns an Enumeration of URLs representing all of the resources on the URL search path having the specified name.
protected  PermissionCollection	getPermissions(CodeSource codesource)           Returns the permissions for the given codesource object.
URL[]	getURLs()           Returns the search path of URLs for loading classes and resources.
static URLClassLoader	newInstance(URL[] urls)           Creates a new instance of URLClassLoader for the specified URLs and default parent class loader.
static URLClassLoader	newInstance(URL[] urls, ClassLoader parent)           Creates a new instance of URLClassLoader for the specified URLs and parent class loader.

 

Methods inherited from class java.security.SecureClassLoader

defineClass

 

Methods inherited from class java.lang.ClassLoader

defineClass, defineClass, defineClass, definePackage, findLibrary, findLoadedClass, findSystemClass, getPackage, getPackages, getParent, getResource, getResourceAsStream, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, loadClass, loadClass, resolveClass, setSigners

 

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

 

Constructor Detail

URLClassLoader

public URLClassLoader(URL[] urls,
                      ClassLoader parent)

Constructs a new URLClassLoader for the given URLs. The URLs will be searched in the order specified for classes and resources after first searching in the specified parent class loader. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:

urls - the URLs from which to load classes and resources

parent - the parent class loader for delegation

Throws:

SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.

See Also:

SecurityManager.checkCreateClassLoader()

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URLClassLoader

public URLClassLoader(URL[] urls)

Constructs a new URLClassLoader for the specified URLs using the default delegation parent ClassLoader. The URLs will be searched in the order specified for classes and resources after first searching in the parent class loader. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:

urls - the URLs from which to load classes and resources

Throws:

SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.

See Also:

SecurityManager.checkCreateClassLoader()

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URLClassLoader

public URLClassLoader(URL[] urls,
                      ClassLoader parent,
                      URLStreamHandlerFactory factory)

Constructs a new URLClassLoader for the specified URLs, parent class loader, and URLStreamHandlerFactory. The parent argument will be used as the parent class loader for delegation. The factory argument will be used as the stream handler factory to obtain protocol handlers when creating new URLs.

If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed.

Parameters:

urls - the URLs from which to load classes and resources

parent - the parent class loader for delegation

factory - the URLStreamHandlerFactory to use when creating URLs

Throws:

SecurityException - if a security manager exists and its checkCreateClassLoader method doesn't allow creation of a class loader.

See Also:

SecurityManager.checkCreateClassLoader()

Method Detail

addURL

protected void addURL(URL url)

Appends the specified URL to the list of URLs to search for classes and resources.

Parameters:

url - the URL to be added to the search path of URLs

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getURLs

public URL[] getURLs()

Returns the search path of URLs for loading classes and resources. This includes the original list of URLs specified to the constructor, along with any URLs subsequently appended by the addURL() method.

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findClass

protected Class findClass(String name)
                   throws ClassNotFoundException

Finds and loads the class with the specified name from the URL search path. Any URLs referring to JAR files are loaded and opened as needed until the class is found.

Overrides:

findClass in class ClassLoader

Parameters:

name - the name of the class

Returns:

the resulting class

Throws:

ClassNotFoundException - if the class could not be found

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definePackage

protected Package definePackage(String name,
                                Manifest man,
                                URL url)
                         throws IllegalArgumentException

Defines a new package by name in this ClassLoader. The attributes contained in the specified Manifest will be used to obtain package version and sealing information. For sealed packages, the additional URL specifies the code source URL from which the package was loaded.

Parameters:

name - the package name

man - the Manifest containing package version and sealing information

url - the code source url for the package, or null if none

Throws:

IllegalArgumentException - if the package name duplicates an existing package either in this class loader or one of its ancestors

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findResource

public URL findResource(String name)

Finds the resource with the specified name on the URL search path. Returns a URL for the resource, or null if the resource could not be found.

Overrides:

findResource in class ClassLoader

Parameters:

name - the name of the resource

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findResources

public Enumeration findResources(String name)
                          throws IOException

Returns an Enumeration of URLs representing all of the resources on the URL search path having the specified name.

Overrides:

findResources in class ClassLoader

Parameters:

name - the resource name

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getPermissions

protected PermissionCollection getPermissions(CodeSource codesource)

Returns the permissions for the given codesource object. The implementation of this method first calls super.getPermissions, to get the permissions granted by the policy, and then adds additional permissions based on the URL of the codesource.

If the protocol is "file" and the path specifies a file, then permission to read that file is granted. If protocol is "file" and the path is a directory, permission is granted to read all files and (recursively) all files and subdirectories contained in that directory.

If the protocol is not "file", then to connect to and accept connections from the URL's host is granted.

Overrides:

getPermissions in class SecureClassLoader

Parameters:

codesource - the codesource

Returns:

the permissions granted to the codesource

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newInstance

public static URLClassLoader newInstance(URL[] urls,
                                         ClassLoader parent)

Creates a new instance of URLClassLoader for the specified URLs and parent class loader. If a security manager is installed, the loadClass method of the URLClassLoader returned by this method will invoke the SecurityManager.checkPackageAccess method before loading the class.

Parameters:

urls - the URLs to search for classes and resources

parent - the parent class loader for delegation

Returns:

the resulting class loader

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newInstance

public static URLClassLoader newInstance(URL[] urls)

Creates a new instance of URLClassLoader for the specified URLs and default parent class loader. If a security manager is installed, the loadClass method of the URLClassLoader returned by this method will invoke the SecurityManager.checkPackageAccess before loading the class.

Parameters:

urls - the URLs to search for classes and resources

Returns:

the resulting class loader

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