java.lang
Class ClassLoader

java.lang.Object   
  java.lang.ClassLoader

Direct Known Subclasses:

SecureClassLoader

public abstract class ClassLoader
extends Object   

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the binary name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a "class file" of that name from a file system.

Every Class object contains a reference to the ClassLoader that defined it.

Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader.

Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes.

Class loaders may typically be used by security managers to indicate security domains.

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a ClassLoader instance.

Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. For example, on UNIX systems, the virtual machine loads classes from the directory defined by the CLASSPATH environment variable.

However, some classes may not originate from a file; they may originate from other sources, such as the network, or they could be constructed by an application. The method defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using Class.newInstance .

The methods and constructors of objects created by a class loader may reference other classes. To determine the class(es) referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class.

For example, an application could create a network class loader to download class files from a server. Sample code might look like:

   ClassLoader loader = new NetworkClassLoader(host, port);
   Object main = loader.loadClass("Main", true).newInstance();
          . . .
 

The network class loader subclass must define the methods findClass and loadClassData to load a class from the network. Once it has downloaded the bytes that make up the class, it should use the method defineClass to create a class instance. A sample implementation is:

     class NetworkClassLoader extends ClassLoader {
         String host;
         int port;

         public Class findClass(String name) {
             byte[] b = loadClassData(name);
             return defineClass(name, b, 0, b.length);
         }

         private byte[] loadClassData(String name) {
             // load the class data from the connection
              . . .
         }
     }
 

Binary names

Any class name provided as a String parameter to methods in ClassLoader must be a binary name as defined by the Java Language Specification.

Examples of valid class names include:

   "java.lang.String"
   "javax.swing.JSpinner$DefaultEditor"
   "java.security.KeyStore$Builder$FileBuilder$1"
   "java.net.URLClassLoader$3$1"
 

Since:

1.0

See Also:

resolveClass(Class)

Constructor Summary

protected	ClassLoader () Creates a new class loader using the ClassLoader returned by the method getSystemClassLoader() as the parent class loader.
protected	ClassLoader (ClassLoader parent) Creates a new class loader using the specified parent class loader for delegation.

Method Summary

void	clearAssertionStatus () Sets the default assertion status for this class loader to false and discards any package defaults or class assertion status settings associated with the class loader.
protected Class <?>	defineClass (byte[] b, int off, int len) Deprecated. Replaced by defineClass(String, byte[], int, int)
protected Class <?>	defineClass (String name, byte[] b, int off, int len) Converts an array of bytes into an instance of class Class.
protected Class <?>	defineClass (String name, byte[] b, int off, int len, ProtectionDomain protectionDomain) Converts an array of bytes into an instance of class Class, with an optional ProtectionDomain.
protected Class <?>	defineClass (String name, ByteBuffer b, ProtectionDomain protectionDomain) Converts a ByteBuffer into an instance of class Class, with an optional ProtectionDomain.
protected Package	definePackage (String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase) Defines a package by name in this ClassLoader.
protected Class <?>	findClass (String name) Finds the class with the specified binary name.
protected String	findLibrary (String libname) Returns the absolute path name of a native library.
protected Class <?>	findLoadedClass (String name) Returns the class with the given binary name if this loader has been recorded by the Java virtual machine as an initiating loader of a class with that binary name.
protected URL	findResource (String name) Finds the resource with the given name.
protected Enumeration <URL >	findResources (String name) Returns an enumeration of URL objects representing all the resources with the given name.
protected Class <?>	findSystemClass (String name) Finds a class with the specified binary name, loading it if necessary.
protected Package	getPackage (String name) Returns a Package that has been defined by this class loader or any of its ancestors.
protected Package []	getPackages () Returns all of the Packages defined by this class loader and its ancestors.
ClassLoader	getParent () Returns the parent class loader for delegation.
URL	getResource (String name) Finds the resource with the given name.
InputStream	getResourceAsStream (String name) Returns an input stream for reading the specified resource.
Enumeration <URL >	getResources (String name) Finds all the resources with the given name.
static ClassLoader	getSystemClassLoader () Returns the system class loader for delegation.
static URL	getSystemResource (String name) Find a resource of the specified name from the search path used to load classes.
static InputStream	getSystemResourceAsStream (String name) Open for reading, a resource of the specified name from the search path used to load classes.
static Enumeration <URL >	getSystemResources (String name) Finds all resources of the specified name from the search path used to load classes.
Class <?>	loadClass (String name) Loads the class with the specified binary name.
protected Class <?>	loadClass (String name, boolean resolve) Loads the class with the specified binary name.
protected void	resolveClass (Class <?> c) Links the specified class.
void	setClassAssertionStatus (String className, boolean enabled) Sets the desired assertion status for the named top-level class in this class loader and any nested classes contained therein.
void	setDefaultAssertionStatus (boolean enabled) Sets the default assertion status for this class loader.
void	setPackageAssertionStatus (String packageName, boolean enabled) Sets the package default assertion status for the named package.
protected void	setSigners (Class <?> c, Object [] signers) Sets the signers of a class.

Methods inherited from class java.lang.Object

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

Constructor Detail

ClassLoader

protected ClassLoader(ClassLoader    parent)

Creates a new class loader using the specified parent class loader for delegation.

If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception.

Parameters:

parent - The parent class loader

Throws:

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

Since:

1.2

ClassLoader

protected ClassLoader()

Creates a new class loader using the ClassLoader returned by the method getSystemClassLoader() as the parent class loader.

If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception.

Throws:

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

Method Detail

loadClass

public Class   <?> loadClass(String    name)
                   throws ClassNotFoundException   

Loads the class with the specified binary name. This method searches for classes in the same manner as the loadClass(String, boolean) method. It is invoked by the Java virtual machine to resolve class references. Invoking this method is equivalent to invoking loadClass(name, false) .

Parameters:

name - The binary name of the class

Returns:

The resulting Class object

Throws:

ClassNotFoundException - If the class was not found

loadClass

protected Class   <?> loadClass(String    name,
                             boolean resolve)
                      throws ClassNotFoundException   

Loads the class with the specified binary name. The default implementation of this method searches for classes in the following order:

1. Invoke findLoadedClass(String) to check if the class has already been loaded.

2. Invoke the loadClass method on the parent class loader. If the parent is null the class loader built-in to the virtual machine is used, instead.

3. Invoke the findClass(String) method to find the class.

If the class was found using the above steps, and the resolve flag is true, this method will then invoke the resolveClass(Class) method on the resulting Class object.

Subclasses of ClassLoader are encouraged to override findClass(String) , rather than this method.

Parameters:

name - The binary name of the class

resolve - If true then resolve the class

Returns:

The resulting Class object

Throws:

ClassNotFoundException - If the class could not be found

findClass

protected Class   <?> findClass(String    name)
                      throws ClassNotFoundException   

Finds the class with the specified binary name. This method should be overridden by class loader implementations that follow the delegation model for loading classes, and will be invoked by the loadClass method after checking the parent class loader for the requested class. The default implementation throws a ClassNotFoundException.

Parameters:

name - The binary name of the class

Returns:

The resulting Class object

Throws:

ClassNotFoundException - If the class could not be found

Since:

1.2

defineClass

@Deprecated
protected final Class   <?> defineClass(byte[] b,
                                                int off,
                                                int len)
                              throws ClassFormatError   

Deprecated. Replaced by defineClass(String, byte[], int, int)

Converts an array of bytes into an instance of class Class. Before the Class can be used it must be resolved. This method is deprecated in favor of the version that takes a binary name as its first argument, and is more secure.

Parameters:

b - The bytes that make up the class data. The bytes in positions off through off+len-1 should have the format of a valid class file as defined by the Java Virtual Machine Specification.

off - The start offset in b of the class data

len - The length of the class data

Returns:

The Class object that was created from the specified class data

Throws:

ClassFormatError - If the data did not contain a valid class

IndexOutOfBoundsException - If either off or len is negative, or if off+len is greater than b.length.

See Also:

loadClass(String, boolean) , resolveClass(Class)

defineClass

protected final Class   <?> defineClass(String    name,
                                     byte[] b,
                                     int off,
                                     int len)
                              throws ClassFormatError   

Converts an array of bytes into an instance of class Class. Before the Class can be used it must be resolved.

This method assigns a default Protecti