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 Class.newInstance.
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For example, an application could create a
network class loader to
download class files from a server. Sample code might look like:
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| 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:
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| 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
. . .
}
} |
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