Why can't load inner class? ClassNotFoundException

Question

Why does the following code cause ClassNotFoundException ?

public class App02 {

    public static class A {
    }

    public static void main(String[] args) throws ClassNotFoundException {

        try {
            System.out.println("A.class.getCanonicalName() = " + A.class.getCanonicalName());
            Class c = Class.forName("tests.App02.A"); //error on this line
            System.out.println(c.getName());
        }

        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Output:

A.class.getCanonicalName() = tests.App02.A
java.lang.ClassNotFoundException: tests.App02.A
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:190)
    at tests.App02.main(App02.java:15)

Answer 1

Try Class c = Class.forName("tests.App02$A") . It's not a top-level class, so use $ to locate it.

Answer 2

You need to use $ to access the nested class :

Class c = Class.forName("tests.App02$A");

When you compile your class, you will notice that the nested class is named as: App02$A.class , under package tests . It would make more sense then.

Answer 3

Because you are using a canonical name , but you should use name ( A.class.getName() ).

In your case you should use Class c = Class.forName("tests.App02$A");

Answer 4

在 commons-lang 中有一个有用的 util 支持这些类：

org.apache.commons.lang3.ClassUtils.get("tests.App02.A")