Java类签名和动态加载

Question

When will two classes considered same? Like, is there something acting as the signature of a class? If so, what counts in the signature, package info, class name, etc? I'm asking this because I need to dynamically load a class and I always got a ClassNotFoundException

A bit more detail: I'm using Eclipse. I have an abstract class Panel in my package com.example.project.sub1 . And a class Test in package com.example.project.sub2 , which will call

ClassLoader loader = new URLClassLoader(
        new URL[]{new URL("file://" + path)});
                /*the path is specified runtime and can be in a different
                  directory other than working directory. It's the path 
                  to the parent directory of the class file I need to load.
                */
Class&ltPanel> panelClass = (Class&ltPanel>)loader.loadClass(className);
                //class name is runtime specified.

That compiles fine. Then I copied all the stuff in Panel.java in a new directory and created a class MyPanel extends Panel along with Panel.java . That compiles fine too, but when I specify the path to my new MyPanel.class , I always got a ClassNotFoundException . Any idea where I'm wrong? Thanks.

EDIT: The stack trace:

java.lang.ClassNotFoundException: MyPanel
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at com.example.project.sub2 (Test.java:111)
    at java.lang.Thread.run(Thread.java:680)

Answer 1

java.lang.ClassNotFoundException: MyPanel

The class is not called "MyPanel", it is "com.example.project.sub1.MyPanel".

It's the path to the parent directory of the class file I need to load.

Yes, that won't work. It needs to be the directory at the root of the package hierarchy, so not "/some/path/classes/com/example/project/sub1/", but "/some/path/classes"

Answer 2

If you want to dynamically load the class com.example.project.sub1.Panel from a file URL, then this URL should refer to the directory containing the directory com . From there, the ClassLoader will look for the class in a subdirectory path matching the package path. And the class name to pass is the fully qualified name: com.example.project.sub1.Panel .

Also, loding the class MyPanel will return a Class<MyPanel> , not a Class<Panel> . You should use Class<? extends Panel> Class<? extends Panel> as the type of the variable.

I'm not sure why you're dynamically loading classes though. The actual problem you're trying to solve is probably solvable in another way.