Resolve class name from bytecode

Question

Is it possible to dig up a classes name from bytecode which is formed from the class' source code?

The situation is this: I get a classes bytecode remotely from somewhere, it doesn't matter where it comes from. To effectively load that class with a classloader i would need to have the class name as well... right?

Answer 1

You should be able to use javap to disassemble the byte code, if that just happens once in a while.

For doing it at runtime: use a byte-code manipulation library like Apache's BCEL (http://jakarta.apache.org/bcel) to analyse the byte code.

Answer 2

The easiest way is probably using something like ASM:

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.commons.EmptyVisitor;

public class PrintClassName {
  public static void main(String[] args) throws IOException {
    class ClassNamePrinter extends EmptyVisitor {
      @Override
      public void visit(int version, int access, String name, String signature,
          String superName, String[] interfaces) {
        System.out.println("Class name: " + name);
      }
    }

    InputStream binary = new FileInputStream(args[0]);
    try {
      ClassReader reader = new ClassReader(binary);
      reader.accept(new ClassNamePrinter(), 0);
    } finally {
      binary.close();
    }
  }
}

If you can't use a 3rd party library, you could read the class file format yourself.

Answer 3

I think you can use the ClassLoader.defineClass method, in a subclass of ClassLoader, to get the Class object for a given bytecode. (not tested)

Answer 4

If you just need the class name, it's probably easier to parse the beginning of the class file yourself instead of adding a 3rd party library for class code manipulation just for this purpose. You just need the classes and strings from the constant pool, skip the access flags and then replace / with . in the class name. If you have a byte array, you can call this method with new ByteArrayInputStream(byteArray):

public static String getClassName(InputStream is) throws Exception {
    DataInputStream dis = new DataInputStream(is);
    dis.readLong(); // skip header and class version
    int cpcnt = (dis.readShort()&0xffff)-1;
    int[] classes = new int[cpcnt];
    String[] strings = new String[cpcnt];
    for(int i=0; i<cpcnt; i++) {
     int t = dis.read();
     if(t==7) classes[i] = dis.readShort()&0xffff;
     else if(t==1) strings[i] = dis.readUTF();
     else if(t==5 || t==6) dis.readLong();
     else if(t==8) dis.readShort();
     else dis.readInt();
    }
    dis.readShort(); // skip access flags
    return strings[classes[(dis.readShort()&0xffff)-1]-1].replace('/', '.');
}