JavaTM 2 Platform
Std. Ed. v1.3.1

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.

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(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.

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 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:
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)

JavaTM 2 Platform
Std. Ed. v1.3.1

Submit a bug or feature
For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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