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public  abstract class  java.lang.ClassLoader
    extends  java.lang.Object  (I-§1.12)
{
        // Constructors
    protected ClassLoader();	§1.4.1

        // Methods
    protected final Class 	§1.4.2
        defineClass(byte  data[], int  offset, int  length);
    protected final Class findSystemClass(String  name);	§1.4.3
    protected abstract Class 	§1.4.4
        loadClass(String  name, boolean  resolve);
    protected final void resolveClass(Class  c);	§1.4.5
}

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 (I-§1.4.2) converts an array of bytes into an instance of class Class (I-§1.3). Instances of this newly defined class can be created using the newInstance method in class Class (I-§1.3.7).

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 (I-§1.4.4) of the class loader that originally created the class. If the Java Virtual Machine only needs to determine if the class exists and if it does exist to know its superclass, the resolve flag is set to false. However if an instance of the class is being created or any of its methods are being called, the class must also be resolved. In this case the resolve flag is set to true, and the resolveClass method (I-§1.4.5) should be called.

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();
....

	class  NetworkClassLoader  {
	        String  host;
	        int  port;
	        Hashtable  cache  =  new  Hashtable();
	        private  byte  loadClassData(String  name)[]  {
        		//  load  the  class  data  from  the  connection
        		...
	        }


	        public  synchronized  Class  loadClass(String  name, 
                                boolean resolve)  {
	                Class  c  =  cache.get(name);
                		if  (c  ==  null)  {
                		        byte  data[]  =  loadClassData(name);
	                       c = defineClass(data, 0, data.length);
                		        cache.put(name,  c);
                		}
                if (resolve)
                        resolveClass(c);
                		return  c;


	        }
	}

Constructs a new class loader and initializes it.

If there is a security manager, its checkCreateClassLoader method (I-§1.15.8) is called. This may result in a security exception (I-§1.43).

Throws

SecurityException (I-§1.43)

If the current thread does not have permission to create a new class loader.

Converts an array of bytes into an instance of class Class.

Parameters:

data - the bytes that make up the Class

offset - the start offset of the Class data

length - the length of the Class data

Returns:

the class object which was created from the data.

Throws

ClassFormatError (I-§1.47)

If the data does not contain a valid Class.

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

A system class is a class loaded from the local file system in a platform- dependent way. It has no class loader.

Parameters:

name - the name of the system Class

Returns:

a system class with the given name.

Throws

NoClassDefFoundError (I-§1.54)

If the Class is not found.

Throws

ClassNotFoundException (I-§1.28)

If it could not find a definition for the class

Requests the class loader to load a class with the specified name. The loadClass method is called by the Java Virtual Machine when a class loaded by a class loader first references another class. Every subclass of class ClassLoader must define this method.

If the resolve flag is true, the method should call the resolveClass method on the resulting class object.

Class loaders should use a hash table or other cache to avoid defining classes with the same name multiple times.

Parameters:

name - the name of the desired Class

resolve - true if the Class must be resolved

Returns:

the resulting Class, or null if it was not found.

Throws

ClassNotFoundException (I-§1.28)

If the classloader cannot find a definition for the class

Resolves the class so that an instance of the class can be created, or so that one of its methods can be called. This method should be called by loadClass if the resolve flag is true.

Parameters:

c - the Class instance to be resolved

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