GWT 2.2 and java.lang.IncompatibleClassChangeError

I’ve been updating to the latest versions of the various libraries we use and ran into some trouble while attempting to update GWT to 2.2. There were some incompatible bytecode changes made: the classes in the package were converted to interfaces. Java has a special invoke opcode for interface types, so the classes would fail to load with this obscure error when the verifier tried to load the classes:

java.lang.IncompatibleClassChangeError: Found interface, but class was expected

There are updated libraries available for some third-party GWT libraries, but others (like the gwt-google-apis) haven’t been updated yet.

The solution suggested by others was to manually recompile these against the latest GWT libraries to fix the problem. I thought I’d try a different approach: bytecode rewriting. The theory is pretty simple. Scan the opcodes of every method in a jar and replace the opcode used to invoke a method on a class, INVOKEVIRTUAL, with the opcode for invoking a method on interfaces, INVOKEINTERFACE. More info on Java opcodes is available here.

The ASM library makes this sort of transformation trivial to write. Compile the following code along with the asm-all-3.3.1.jar and plug in the file you want to transform in the main method. It’ll spit out a version of the library that should be compatible with GWT 2.2. You might need to add extra classes to the method rewriter, depending on the library you are trying to rewrite:

package com.gripeapp.bytecoderewrite;


import org.objectweb.asm.ClassAdapter;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodAdapter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class BytecodeRewrite {
  static class ClassVisitorImplementation extends ClassAdapter {
    public ClassVisitorImplementation(ClassVisitor cv) {

    public MethodVisitor visitMethod(int access, String name, String desc,
              String signature, String[] exceptions) {
        MethodVisitor mv;
        mv = cv.visitMethod(access, name, desc, signature, exceptions);
        if (mv != null) {
          mv = new MethodVisitorImplementation(mv);
        return mv;

  static class MethodVisitorImplementation extends MethodAdapter {

    public MethodVisitorImplementation(MethodVisitor mv) {

    public void visitMethodInsn(int opcode, String owner, String name,
        String desc) {
      if (owner.equals("com/google/gwt/core/ext/typeinfo/JClassType")
          || owner.equals("com/google/gwt/core/ext/typeinfo/JPackage")
          || owner.equals("com/google/gwt/core/ext/typeinfo/JMethod")
          || owner.equals("com/google/gwt/core/ext/typeinfo/JType")
          || owner.equals("com/google/gwt/core/ext/typeinfo/JParameterizedType")
          || owner.equals("com/google/gwt/core/ext/typeinfo/JParameter")) {
        super.visitMethodInsn(Opcodes.INVOKEINTERFACE, owner, name, desc);
      } else
        super.visitMethodInsn(opcode, owner, name, desc);

  public static void main(String[] args) throws IOException {
    ZipInputStream file = new ZipInputStream(new FileInputStream("/tmp/gwt-maps-1.1.0/gwt-maps.jar"));
    ZipEntry ze;

    ZipOutputStream output = new ZipOutputStream(new FileOutputStream(("/tmp/gwt-maps.jar")));

    while ((ze = file.getNextEntry()) != null) {
      output.putNextEntry(new ZipEntry(ze.getName()));
      if (ze.getName().endsWith(".class")) {
        ClassReader reader = new ClassReader(file);
        ClassWriter cw = new ClassWriter(0);
        reader.accept(new ClassVisitorImplementation(cw), ClassReader.SKIP_FRAMES);
      } else {
        byte[] data = new byte[16*1024];
        while (true) {
          int r =;
          if (r == -1)
          output.write(data, 0, r);


One Response to “GWT 2.2 and java.lang.IncompatibleClassChangeError”

  1. Incredible! Worked great for me, just had to add “JField” for gwt-maps.jar. This saved me a lot of time. We have a lot of smaller libraries as well, which are not really well maintained. Meaning the version we are using is not tagged, so it would be a massive headache to check out trunk everywhere and test.

    Thanks a lot!