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java.lang.Object | +----java.lang.ClassLoader
ClassLoader is an abstract class. Applications implement subclasses of ClassLoader in order to extend the manner in which the Java Virtual Machine dynamically loads classes. 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. 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 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();
. . . The network class loader subclass must define the method loadClass 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 {
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;
}
}
ClassLoader() 
defineClass(byte[], int, int) Class. Deprecated. defineClass(String, byte[], int, int) findLoadedClass(String) findSystemClass(String) getResource(String) getResourceAsStream(String) 
getSystemResource(String) getSystemResourceAsStream(String) loadClass(String) loadClass(String, boolean) resolveClass(Class) setSigners(Class, Object[]) 
ClassLoader
protected ClassLoader()
If there is a security manager, its checkCreateClassLoader method is called. This may result in a security exception.
loadClass
public Class loadClass(String name) throws ClassNotFoundException
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. Class loaders should use a hashtable or other cache to avoid defining classes with the same name multiple times.
Class. Class, or null if it was not found. loadClass
protected abstract Class loadClass(String name,
boolean resolve) throws ClassNotFoundException
If the resolve flag is true, the method should call the resolveClass method on the resulting class object.
As an abstract method, loadClass() must be defined in a subclass of ClassLoader. By using a Hashtable, you can avoid loading the same Class more than once.
defineClass
protected final Class defineClass(byte data[],
int offset,
int length)
Class. Before the Class can be used it must be resolved. This method is deprecated in favor of the version that takes a "name" as a first argument, and is more secure. Class. Class data. Class data. Class object that was created from the data. defineClass
protected final Class defineClass(String name,
byte data[],
int offset,
int length)
Class. Class data. Class data. Class object that was created from the data. resolveClass
protected final void resolveClass(Class c)
loadClass if the resolve flag is true. Class instance to be resolved. findSystemClass
protected final Class findSystemClass(String name) throws ClassNotFoundException
A system class is a class loaded from the local file system in a platform- dependent way. It has no class loader.
class. setSigners
protected final void setSigners(Class cl, Object signers[])
findLoadedClass
protected final Class findLoadedClass(String name)
getSystemResourceAsStream
public static final InputStream getSystemResourceAsStream(String name)
The resource name may be any system resource (e.g. follows CLASSPATH order).
getSystemResource
public static final URL getSystemResource(String name)
The resource name may be any system resource (e.g. follows CLASSPATH order).
getResourceAsStream
public InputStream getResourceAsStream(String name)
The class loader can choose what to do to locate the resource.
getResource
public URL getResource(String name)
The class loader can choose what to do to locate the resource.
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