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

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(); i++; }
        else if(t==8) dis.readShort();
        else dis.readInt();
    }
    dis.readShort(); // skip access flags
    return strings[classes[(dis.readShort()&0xffff)-1]-1].replace('/', '.');
}

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

Just for completeness, in cases where the use of ASM5 library is acceptable, the following call could be used to obtain the class name from its byte representation.

public String readClassName(final byte[] typeAsByte) {
    return new ClassReader(typeAsByte).getClassName().replace("/", ".");
}

Answer 4

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 5

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 6

Inspired by the solution provided by McDowell I created an updated version that works up to Java 19.

String getBinaryName(byte[] byteCode) {
        try (final var is = new DataInputStream(new ByteArrayInputStream(byteCode))) {
            final var magic = is.readInt();
            if (magic != 0xCAFEBABE) {
                throw new RuntimeException("Class file header is missing.");
            }
            
            final var minor = is.readUnsignedShort();
            final var major = is.readUnsignedShort();
            final int constantPoolCount = is.readShort();
            final var classes = new int[constantPoolCount - 1];
            final var strings = new String[constantPoolCount - 1];
            for (int i = 0; i < constantPoolCount - 1; i++) {
                int t = is.read();
                switch (t) {
                    case 1://utf-8
                        strings[i] = is.readUTF();
                        break;
                    case 3://Integer
                        is.readInt();
                        break;
                    case 4: // float
                        is.readFloat();
                        break;
                    case 5: // Long
                        is.readLong();
                        i++;
                        break;
                    case 6: // Double
                        is.readDouble();
                        i++;
                        break;
                    case 7: // Class index
                        classes[i] = is.readUnsignedShort();
                        break;
                    case 8: // string index
                        is.readShort();
                        break;
                    case 9: // field ref
                    case 10: // method ref
                    case 11: // interface method ref
                    case 12: // name and type
                    case 18: // invoke dynamic
                        is.readUnsignedShort();
                        is.readUnsignedShort();
                        break;
                    case 15: // method handle
                        is.read();
                        is.readUnsignedShort();
                        break;
                    case 16: // method type
                        is.readUnsignedShort();
                        break;
                    default:
                        throw new RuntimeException(format("Invalid constant pool tag %d at position %d", t,i));
                }
            }
            is.readShort(); // skip access flags
            final var classNameIndex = is.readShort();
            return strings[classes[(classNameIndex & 0xffff) - 1] - 1].replace('/', '.');
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }