Wednesday, January 02, 2008

MyClassLoader - Java ClassLoader - Part 2

MyClassLoader will take one more entry for completion. Before writing our own custom ClassLoader, we have to devote sometime to see the methods of ClassLoader. Some of them need special attention while others we can ignore. Before starting with methods, we can see some type of ClassLoader available in jdk(openjdk) itself. AppletClassLoader, RMIClassLoader, SecureClassLoader, URLClassLoader are some of them. Remember all the custom ClassLoader need to extend ClassLoader except one :-). Any guesses ? Bootstrap Class Loader - Yes, this is responsible for loading runtime classes(rt.jar- very famous jar file in /jre/lib :-) ) . It has a native implementation and hence varies across JVM. So, when we write

java MyProgram

Bootstrap ClassLoader comes into the action first.

Alright, back to methods: we can see the whole list of methods of ClassLoader here. But we will see those of our interest:

- loadClass -> entry point for ClassLoader. In JDK 1.1 or earlier, this is the only method we need to override but after JDK 1.2 some dynamics get changed. Will discuss that later.

- defineClass -> As I mentioned in the last blog, this is one of the complex method which takes raw data and turn it into Class Object. Need not to worry, it is defined as final(so we can't change... who want to change).

- findSystemClass -> looks for the class file in local disk, if yes calls defineClass and convert the raw data into Class Object.

In JDK 1.2, new delegation model came into picture where if ClassLoader can't able to find a class, it asks (it's) parent ClassLoader to do it. JDK 1.2 came up with a new method called findClass which contains specialized code and help you when you are messed up with lot of ClassLoader. So, from JDK 1.2 and onwards, we just need to override findClass and everything will work fine, if not it will throw ClassNotFoundException. There are lot of other methods like getParent, getSystemClassLoader, but we can write our custom ClassLoader without touching these methods.

So, top skeleton looks like:

public class MyClassLoader extends ClassLoader {

public CustomClassLoader(){
//getClassLoader returns ClassLoader


//lot of thing after this

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