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)

What should `Class.forName` return if you had a package `tests.App02` with a class `A` in it? — tobias_k, Jan 27 '14 at 20:02
It's actually possible to call a class `$` in Java!? Why, oh the humanity! — tobias_k, Jan 27 '14 at 21:17
Yes it is possible :) For example `Jsoup` did this to emulate `JQuery` :) — Suzan Cioc, Jan 27 '14 at 21:30
And if you do this `class A$ {class B {}}; class A {class $B {}}` it says "duplicate class"... — tobias_k, Jan 28 '14 at 10:20

score 21 · Accepted Answer · answered Jan 27 '14 at 20:00

21

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

answered Jan 27 '14 at 20:00

Paul Hicks

13,289
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1

Or more exactly, App02 isn't a package, so you can't use `.` to descend into the next level. – Paul Hicks Jan 27 '14 at 20:01

score 7 · Answer 2 · answered Jan 27 '14 at 20:01

7

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.

answered Jan 27 '14 at 20:01

Rohit Jain

209,639
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@MarkoTopolnik Didn't knew that :) – Rohit Jain Jan 27 '14 at 20:03
The terminology is notoriously confusing. The high point is that everybody assumes *inner* means "nested class requiring an enclosing instance", but even that isn't true: an inner class may be declared in a static context. `class Toplevel { static Object x = new Object() {}; }` – Marko Topolnik Jan 27 '14 at 20:05
Umm. Anonymous inner class. That's where it starts to confuse more. On first look one might expect that instance to have the enclosing instance associated with it. Basically, that depends whether the reference is `static` or not. But whenever I feel confused, I stop there and look out for bytecode. That answers it :) – Rohit Jain Jan 27 '14 at 20:10
All anon classes are inner, and *inner* is about the restriction that it can't contain static members. Yes, if the anon class finds itself in a static context, then clearly there can't be any enclosing instance available. – Marko Topolnik Jan 27 '14 at 20:13

score 4 · Answer 3 · answered Jan 27 '14 at 20:02

4

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");

answered Jan 27 '14 at 20:02

Filipp Voronov

4,077
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score 2 · Answer 4 · answered Oct 19 '21 at 09:41

2

There is a helpful util in commons-lang which support these classes:

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

answered Oct 19 '21 at 09:41

Inshua

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Why can't load inner class? ClassNotFoundException

4 Answers4