java.lang

Class ClassLoader

java.lang.Object

java.lang.ClassLoader

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.

Class loaders that support concurrent loading of classes are known as parallel capable class loaders and are required to register themselves at their class initialization time by invoking the ClassLoader.registerAsParallelCapable method. Note that the ClassLoader class is registered as parallel capable by default. However, its subclasses still need to register themselves if they are parallel capable.
In environments in which the delegation model is not strictly hierarchical, class loaders need to be parallel capable, otherwise class loading can lead to deadlocks because the loader lock is held for the duration of the class loading process (see loadClass methods).

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

Constructors

Modifier	Constructor and Description
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

Methods

Modifier and Type	Method and Description
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<?>	findClass(String name) Finds the class with the specified binary name.
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 Object	getClassLoadingLock(String className) Returns the lock object for class loading operations.
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.
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.

Unless overridden, this method synchronizes on the result of getClassLoadingLock method during the entire class loading process.

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

getClassLoadingLock

protected Object getClassLoadingLock(String className)

Returns the lock object for class loading operations. For backward compatibility, the default implementation of this method behaves as follows. If this ClassLoader object is registered as parallel capable, the method returns a dedicated object associated with the specified class name. Otherwise, the method returns this ClassLoader object.

Parameters:

className - The name of the to-be-loaded class

Returns:

the lock for class loading operations

Throws:

NullPointerException - If registered as parallel capable and className is null

Since:

1.7

See Also:

loadClass(String, boolean)

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.

SecurityException - If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or if an attempt is made to define a class in a package with a fully-qualified name that starts with "java.".

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 ProtectionDomain to the newly defined class. The ProtectionDomain is effectively granted the same set of permissions returned when Policy.getPolicy().getPermissions(new CodeSource(null, null)) is invoked. The default domain is created on the first invocation of defineClass, and re-used on subsequent invocations.

To assign a specific ProtectionDomain to the class, use the defineClass method that takes a ProtectionDomain as one of its arguments.

Parameters:

name - The expected binary name of the class, or null if not known

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.

SecurityException - If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class (which is unsigned), or if name begins with "java.".

Since:

1.1

See Also:

loadClass(String, boolean), resolveClass(Class), CodeSource, SecureClassLoader

resolveClass

protected final void resolveClass(Class<?> c)

Links the specified class. This (misleadingly named) method may be used by a class loader to link a class. If the class c has already been linked, then this method simply returns. Otherwise, the class is linked as described in the "Execution" chapter of The Java™ Language Specification.

Parameters:

c - The class to link

Throws:

NullPointerException - If c is null.

See Also:

defineClass(String, byte[], int, int)

findLoadedClass

protected final 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. Otherwise null is returned.

Parameters:

name - The binary name of the class

Returns:

The Class object, or null if the class has not been loaded

Since:

1.1

setSigners

protected final void setSigners(Class<?> c,
              Object[] signers)

Sets the signers of a class. This should be invoked after defining a class.

Parameters:

c - The Class object

signers - The signers for the class

Since:

1.1

getResource

public URL getResource(String name)

Finds the resource with the given name. A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.

The name of a resource is a '/'-separated path name that identifies the resource.

This method will first search the parent class loader for the resource; if the parent is null the path of the class loader built-in to the virtual machine is searched. That failing, this method will invoke findResource(String) to find the resource.

Parameters:

name - The resource name

Returns:

A URL object for reading the resource, or null if the resource could not be found or the invoker doesn't have adequate privileges to get the resource.

Since:

1.1

getResources

public Enumeration<URL> getResources(String name)
                              throws IOException

Finds all the resources with the given name. A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.

The name of a resource is a /-separated path name that identifies the resource.

The search order is described in the documentation for getResource(String).

Parameters:

name - The resource name

Returns:

An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources that the class loader doesn't have access to will not be in the enumeration.

Throws:

IOException - If I/O errors occur

Since:

1.2

See Also:

findResources(String)

findResource

protected URL findResource(String name)

Finds the resource with the given name. Class loader implementations should override this method to specify where to find resources.

Parameters:

name - The resource name

Returns:

A URL object for reading the resource, or null if the resource could not be found

Since:

1.2

findResources

protected Enumeration<URL> findResources(String name)
                                  throws IOException

Returns an enumeration of URL objects representing all the resources with the given name. Class loader implementations should override this method to specify where to load resources from.

Parameters:

name - The resource name

Returns:

An enumeration of URL objects for the resources

Throws:

IOException - If I/O errors occur

Since:

1.2

getSystemResource

public static URL getSystemResource(String name)

Find a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (see getSystemClassLoader()).

Parameters:

name - The resource name

Returns:

A URL object for reading the resource, or null if the resource could not be found

Since:

1.1

getSystemResources

public static Enumeration<URL> getSystemResources(String name)
                                           throws IOException

Finds all resources of the specified name from the search path used to load classes. The resources thus found are returned as an Enumeration of URL objects.

The search order is described in the documentation for getSystemResource(String).

Parameters:

name - The resource name

Returns:

An enumeration of resource URL objects

Throws:

IOException - If I/O errors occur

Since:

1.2

getResourceAsStream

public InputStream getResourceAsStream(String name)

Returns an input stream for reading the specified resource.

The search order is described in the documentation for getResource(String).

Parameters:

name - The resource name

Returns:

An input stream for reading the resource, or null if the resource could not be found

Since:

1.1

getSystemResourceAsStream

public static InputStream getSystemResourceAsStream(String name)

Open for reading, a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (see getSystemClassLoader()).

Parameters:

name - The resource name

Returns:

An input stream for reading the resource, or null if the resource could not be found

Since:

1.1

getParent

public final ClassLoader getParent()

Returns the parent class loader for delegation. Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class loader's parent is the bootstrap class loader.

If a security manager is present, and the invoker's class loader is not null and is not an ancestor of this class loader, then this method invokes the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to verify access to the parent class loader is permitted. If not, a SecurityException will be thrown.

Returns:

The parent ClassLoader

Throws:

SecurityException - If a security manager exists and its checkPermission method doesn't allow access to this class loader's parent class loader.

Since:

1.2

getSystemClassLoader

public static ClassLoader 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.

This method is first invoked early in the runtime's startup sequence, at which point it creates the system class loader and sets it as the context class loader of the invoking Thread.

The default system class loader is an implementation-dependent instance of this class.

If the system property "java.system.class.loader" is defined when this method is first invoked then the value of that property is taken to be the name of a class that will be returned as the system class loader. The class is loaded using the default system class loader and must define a public constructor that takes a single parameter of type ClassLoader which is used as the delegation parent. An instance is then created using this constructor with the default system class loader as the parameter. The resulting class loader is defined to be the system class loader.

If a security manager is present, and the invoker's class loader is not null and the invoker's class loader is not the same as or an ancestor of the system class loader, then this method invokes the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to verify access to the system class loader. If not, a SecurityException will be thrown.

Returns:

The system ClassLoader for delegation, or null if none

Throws:

SecurityException - If a security manager exists and its checkPermission method doesn't allow access to the system class loader.

IllegalStateException - If invoked recursively during the construction of the class loader specified by the "java.system.class.loader" property.

Error - If the system property "java.system.class.loader" is defined but the named class could not be loaded, the provider class does not define the required constructor, or an exception is thrown by that constructor when it is invoked. The underlying cause of the error can be retrieved via the Throwable.getCause() method.