Editing a .class file directly, playing around with opcodes
- by echox
Hi, 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.
