Overview | Package | Class | Use | Tree | Deprecated | Index | Help
| J2ME Foundation
PREV CLASS NEXT CLASS | FRAMES NO FRAMES
SUMMARY: INNER | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD

## 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()`, `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(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.

  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
  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 the Java 2 SDK, v1.2, 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:
    1.2

### 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.

  The specified class name cannot begin with "java.", since all classes in the java.* packages can only be defined by the bootstrap class loader.

  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 than this class, or if the class name begins with "java.".

### 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 the Java 2 SDK, 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 the Java 2 SDK v1.2, this method loads the class through the system class loader(see `getSystemClassLoader()`). Class objects returned might have `ClassLoader`s 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:
    the `Class` object for the specified `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.

  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
  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.
  Throws:
    `IOException` - if I/O errors occur
  Since:
    1.2
  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
  Throws:
    `IOException` - if I/O errors occur
  Since:
    1.2

### 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:
    1.2

### 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 the Java 2 SDK v1.2, 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
  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:
    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:
    1.2
  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.
  Returns:
    the newly defined `Package` object
  Throws:
    `IllegalArgumentException` - if package name duplicates an existing package either in this class loader or one of its ancestors
  Since:
    1.2

### 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:
    1.2

### getPackages

    protected Package[] getPackages()

  Returns all of the Packages defined by this class loader and its ancestors.
  Returns:
    the array of `Package` objects defined by this `ClassLoader`
  Since:
    1.2

### 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:
    1.2
  See Also:
    `System.loadLibrary(java.lang.String)`, `System.mapLibraryName(java.lang.String)`

Overview | Package | Class | Use | Tree | Deprecated | Index | Help
| J2ME Foundation
PREV CLASS NEXT CLASS | FRAMES NO FRAMES
SUMMARY: INNER | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD

Java is a trademark or registred trademark of Sun Microsystems, Inc. in the US and other countries.
Copyright 1993-2000 Sun Microsystems, Inc. 901 San Antonio Road,
Palo, Alto, California, 94303, U.S.A All Rights Reserved. < /font >