java.lang
Class ClassLoader

java.lang.Object
  |
  +--java.lang.ClassLoader

Direct Known Subclasses:

SecureClassLoader

public abstract class ClassLoader

extends Object

The class ClassLoader is an abstract class. A class loader is an object that is responsible for loading classes. Given the name of a class, it 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 called upon 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 the newInstance method in class Class.

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 calls 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
              . . .
         }
     }
 

Since:

JDK1.0

See Also:

Class, Class.newInstance(), defineClass(byte[], int, int), loadClass(java.lang.String, boolean), resolveClass(java.lang.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

protected Class	defineClass(byte[] b, int off, int len) Deprecated. Replaced by defineClass(java.lang.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 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 specified class.
protected String	findLibrary(String libname) Returns the absolute path name of a native library.
protected Class	findLoadedClass(String name) Finds the class with the given name if it had been previously loaded through this class loader.
protected URL	findResource(String name) Finds the resource with the given name.
protected Enumeration	findResources(String name) Returns an Enumeration of URLs representing all the resources with the given name.
protected Class	findSystemClass(String name) Finds a class with the specified 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	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	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 name.
protected Class	loadClass(String name, boolean resolve) Loads the class with the specified 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 called. 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.

Since:

JDK1.2

See Also:

SecurityException, SecurityManager.checkCreateClassLoader()

ClassLoader

protected ClassLoader()

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

This constructor is invoked for every newly created class loader. Because the class ClassLoader is abstract, it is not possible to create a new instance of the class ClassLoader itself; however, every constructor for a subclass of ClassLoader necessarily invokes this constructor, explicitly or implicitly, directly or indirectly.

If there is a security manager, its checkCreateClassLoader method is called. 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.

See Also:

SecurityException, SecurityManager.checkCreateClassLoader()

Method Detail

loadClass

public Class loadClass(String name)
                throws ClassNotFoundException

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

Parameters:

name - the 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 name. The default implementation of this method searches for classes in the following order:

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

2. Call 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. Call 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 call the resolveClass(Class) method on the resulting class object.

From JDK1.2, subclasses of ClassLoader are encouraged to override findClass(String), rather than this method.

Parameters:

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

Parameters:

name - the name of the class

Returns:

the resulting Class object

Throws:

ClassNotFoundException - if the class could not be found

Since:

JDK1.2

defineClass

protected final Class defineClass(byte[] b,
                                  int off,
                                  int len)
                           throws ClassFormatError

Deprecated. Replaced by defineClass(java.lang.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 the class 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 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

See Also:

loadClass(java.lang.String, boolean), resolveClass(java.lang.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 contains the set of permissions granted when a call to Policy.getPolicy().getPermissions() is made with a Codesource of null,null. The default domain is created on the first invocation of defineClass, and re-used on subsequent calls.

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 name of the class, or null if not known, using '.' and not '/' as the separator and without a trailing ".class" suffix.

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 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 then this class, which is unsigned.

Since:

JDK1.1

See Also:

loadClass(java.lang.String, boolean), resolveClass(java.lang.Class), ProtectionDomain, Policy, CodeSource, SecureClassLoader

defineClass

protected final Class defineClass(String name,
                                  byte[] b,
                                  int off,
                                  int len,
                                  ProtectionDomain protectionDomain)
                           throws ClassFormatError

Converts an array of bytes into an instance of class Class, with an optional ProtectionDomain. If the domain is null, then a default domain will be assigned to the class as specified in the documentation for defineClass(String,byte[],int,int). Before the class can be used it must be resolved.

The first class defined in a package determines the exact set of certificates that all subsequent classes defined in that package must contain. The set of certificates for a class is obtained from the CodeSource within the ProtectionDomain of the class. Any classes added to that package must contain the same set of certificates or a SecurityException will be thrown. Note that if the name argument is null, this check is not performed. You should always pass in the name of the class you are defining as well as the bytes. This ensures that the class you are defining is indeed the class you think it is.

Parameters:

name - the name of the class

b - the class bytes

off - the start offset of the class bytes

len - the length of the class bytes

protectionDomain - the ProtectionDomain of the class

Returns:

the Class object created from the data, and optional ProtectionDomain.

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 then this class.

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(java.lang.String,byte[],int,int)

findSystemClass

protected final Class findSystemClass(String name)
                               throws ClassNotFoundException

Finds a class with the specified name, loading it if necessary.

Prior to JDK1.2, this method loads a class from the local file system in a platform-dependent manner, and returns a class object that has no associated class loader.

Since JDK1.2, this method loads the class through the system class loader(see getSystemClassLoader()). Class objects returned might have ClassLoaders associated with them. Subclasses of ClassLoader need not usually call this method, because most class loaders need to override just findClass(String).

Parameters:

name - the name of the class that is to be found

Returns:

a system class with the given name

Throws:

ClassNotFoundException - if the class could not be found

See Also:

ClassLoader(ClassLoader), getParent()

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 caller's class loader is not null and is not an ancestor of this class loader, then this method calls the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to ensure it's ok to access the parent class loader. If not, a SecurityException will be thrown.

Throws:

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

Since:

JDK1.2

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

setSigners

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

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

Parameters:

c - the Class object

signers - the signers for the class

Since:

JDK1.1

findLoadedClass

protected final Class findLoadedClass(String name)

Finds the class with the given name if it had been previously loaded through this class loader.

Parameters:

name - the class name

Returns:

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

Since:

JDK1.1

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 call findResource to find the resource.

Parameters:

name - resource name

Returns:

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

Since:

JDK1.1

See Also:

findResource(String)

getResources

public final Enumeration 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 - resource name

Returns:

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

Since:

JDK1.2

See Also:

getResource(java.lang.String), findResources(java.lang.String)

findResources

protected Enumeration findResources(String name)
                             throws IOException

Returns an Enumeration of URLs 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 URLs for the resources

Since:

JDK1.2

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

Since:

JDK1.2

getSystemResource

public static URL getSystemResource(String name)

Find a resource of the specified name from the search path used to load classes.

In JDK1.1, the search path used is that of the virtual machine's built-in class loader.

Since JDK1.2, this method locates the resource through the system class loader (see getSystemClassLoader()).

Parameters:

name - the resource name

Returns:

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

Since:

JDK1.1

getSystemResources

public static Enumeration 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 URLs

Since:

JDK1.2

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:

JDK1.1

getSystemResourceAsStream

public static InputStream getSystemResourceAsStream(String name)

Open for reading, a resource of the specified name from the search path used to load classes.

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

Parameters:

name - the resource name

Returns:

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

Since:

JDK1.1

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.

If a security manager is present, and the caller's class loader is not null and the caller's class loader is not the same as or an ancestor of the system class loader, then this method calls the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to ensure it's ok to access 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.

Since:

JDK1.2

See Also:

SecurityManager.checkPermission(java.security.Permission), RuntimePermission

definePackage

protected Package definePackage(String name,
                                String specTitle,
                                String specVersion,
                                String specVendor,
                                String implTitle,
                                String implVersion,
                                String implVendor,
                                URL sealBase)
                         throws IllegalArgumentException

Defines a package by name in this ClassLoader. This allows class loaders to define the packages for their classes. Packages must be created before the class is defined, and package names must be unique within a class loader and cannot be redefined or changed once created.

Parameters:

name - the package name

specTitle - the specification title

specVersion - the specification version

specVendor - the specification vendor

implTitle - the implementation title

implVersion - the implementation version

implVendor - the implementation vendor

sealBase - If not null, then this package is sealed with respect to the given code source URL. Otherwise, the package is not sealed.

Throws:

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

Since:

JDK1.2

getPackage

protected Package getPackage(String name)

Returns a Package that has been defined by this class loader or any of its ancestors.

Parameters:

name - the package name

Returns:

the Package corresponding to the given name, or null if not found

Since:

JDK1.2

getPackages

protected Package[] getPackages()

Returns all of the Packages defined by this class loader and its ancestors.

Since:

JDK1.2

findLibrary

protected String findLibrary(String libname)

Returns the absolute path name of a native library. The VM invokes this method to locate the native libraries that belong to classes loaded with this class loader. If this method returns null, the VM searches the library along the path specified as the java.library.path property.

Parameters:

libname - the library name

Returns:

the absolute path of the native library

Since:

JDK1.2

See Also:

System.loadLibrary(java.lang.String), System.mapLibraryName(java.lang.String)