Java Language Classloaders Implementing a custom classLoader
Example
Every custom loader must directly or indirectly extend the java.lang.ClassLoader class. The main extension points are the following methods:
findClass(String)- overload this method if your classloader follows the standard delegation model for class loading.loadClass(String, boolean)- overload this method to implement an alternative delegation model.findResourceandfindResources- overload these methods to customize resource loading.
The defineClass methods which are responsible for actually loading the class from a byte array are final to prevent overloading. Any custom behavior needs to be performed prior to calling defineClass.
Here is a simple that loads a specific class from a byte array:
public class ByteArrayClassLoader extends ClassLoader {
private String classname;
private byte[] classfile;
public ByteArrayClassLoader(String classname, byte[] classfile) {
this.classname = classname;
this.classfile = classfile.clone();
}
@Override
protected Class findClass(String classname) throws ClassNotFoundException {
if (classname.equals(this.classname)) {
return defineClass(classname, classfile, 0, classfile.length);
} else {
throw new ClassNotFoundException(classname);
}
}
}
Since we have only overridden the findClass method, this custom class loader is going to behave as follows when loadClass is called.
- The classloader's
loadClassmethod callsfindLoadedClassto see if a class with this name has already been loaded by this classloader. If that succeeds, the resultingClassobject is returned to the requestor. - The
loadClassmethod then delegates to the parent classloader by calling itsloadClasscall. If the parent can deal with the request, it will return aClassobject which is then returned to the requestor. - If the parent classloader cannot load the class,
findClassthen calls our overridefindClassmethod, passing the name of the class to be loaded. - If the requested name matches
this.classname, we calldefineClassto load the actual class from thethis.classfilebyte array. The resultingClassobject is then returned. - If the name did not match, we throw
ClassNotFoundException.
