Editing a .class file directly, playing around with opcodes

Question

today I just tried to play a little bit around with the opcodes in compiled java class file. After inserting

iinc 1,1

the java virtual machine responds with:

Exception in thread "main" java.lang.ClassFormatError: Truncated class file
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: Test.  Program will exit.

This is my example source code:

public class Test {

    public static void main(String[] args) {
        int i = 5;
        i++;
        i++;
        i++;
        System.out.println("Number: " + i + "\n");
    }
}

The opcode for an increment is 0x84 + 2 bytes for operands. There's only one section in the resulting class file, which contains 0x84:

[..] 8401 0184 0101 8401 01[..]

So I would translate this as:

iinc 1,1
iinc 1,1
iinc 1,1

corresponding to my i++; i++; i++;

I then tried to append just 840101 to increment the variable once more, but that didn't work and resulted in the ClassFormatError.

Is there anything like a checksum for the class file? I looked up the format of a classfile in http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html but could not find anything which points out to some kind of bytes_of_classfile or something. I also don't understand why the error is "Truncated Class File", because I did append something :-)

I know its not a good idea to edit class files directly, but I'm just interested on the VM internals here.

Answer 1

(Disclaimer: I didn't disassemble your example.)

If you look at the structure of the class format, you'll see that method_info contains in its attributes Code_attributes (4.7.3), which in turn specify among others the code_length.

Because of your editing, you first violate the declared length, and second of course any subsequent data after the method you modified would now be at different offsets.