I'm trying to generate my Entity class using javassist. Everything went well until I added the GeneratedValue annotation to the Id field. The @Id annotation works fine but when I add @GeneeratedValue I get an exception. This is my code:

  ClassPool cp = ClassPool.getDefault();
  CtClass ctClass = cp.makeClass("test.Snake");
  ClassFile classFile = ctClass.getClassFile();
  classFile.setVersionToJava5();

  AnnotationsAttribute attribute = new AnnotationsAttribute(classFile.getConstPool(), AnnotationsAttribute.visibleTag);

  Annotation idAnnotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.Id"));
  attribute.addAnnotation(idAnnotation);

  Annotation gvAnnotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.GeneratedValue"));
  attribute.addAnnotation(gvAnnotation);

  CtField idField = new CtField(ClassPool.getDefault().get("java.lang.Long"), "id", ctClass);
  idField.getFieldInfo().addAttribute(attribute);
  ctClass.addField(idField);

  CtField nameField = new CtField(ClassPool.getDefault().get("java.lang.String"), "name", ctClass);
  ctClass.addField(nameField);

  AnnotationsAttribute attr = new AnnotationsAttribute(classFile.getConstPool(), AnnotationsAttribute.visibleTag);
  Annotation annotation = new Annotation(classFile.getConstPool(), ClassPool.getDefault().get("javax.persistence.Entity"));
  attr.addAnnotation(annotation);
  classFile.addAttribute(attr);

  snakeClass = ctClass.toClass();
  newInstance = snakeClass.newInstance();

And this is the exception I get:

java.lang.NullPointerException
 at javassist.bytecode.ConstPool.getUtf8Info(ConstPool.java:565)
 at javassist.bytecode.annotation.EnumMemberValue.getValue(EnumMemberValue.java:98)
 at javassist.bytecode.annotation.EnumMemberValue.write(EnumMemberValue.java:116)
 at javassist.bytecode.annotation.Annotation.write(Annotation.java:316)
 at javassist.bytecode.AnnotationsAttribute.setAnnotations(AnnotationsAttribute.java:246)
 at javassist.bytecode.AnnotationsAttribute.addAnnotation(AnnotationsAttribute.java:211)
 at ClassLoadingTest.javassitTest(ClassLoadingTest.java:56)

It seems to be a problem with @GeneratedValue. When I use it alone whithout id I get this exception either. When I use eclipse debugger to watch variable values, I get get this

com.sun.jdi.InvocationException occurred invoking method.

instead of the annotation value. but for Id annotation it shows a javassist annotation object.

I'm new to javassist. Can anyone help me?