Enhanced Java util. Cglib will throw illegalargumentexception when using the date class

I tried to use cglib to enhance Java util. Date. It's useless. I have no experience with cglib, so I want to know what's wrong

For example, the following code enhances how ArrayList works:

@Test
public void enhance_ArrayList() {

    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(ArrayList.class);
    enhancer.setCallback(new FixedValue() {
                @Override
                public Object loadObject() throws Exception {
                  return "Hello cglib!";
                }
              });

    ArrayList enhanced = (ArrayList)enhancer.create();
}

The following codes:

@Test
public void enhance_Date() {
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(Date.class);
    enhancer.setCallback(new FixedValue() {
                @Override
                public Object loadObject() throws Exception {
                  return "Hello cglib!";
                }
              });

    Date enhanced = (Date)enhancer.create();
}

Cause this exception:

java.lang.IllegalArgumentException  
at org.objectweb.asm.ClassReader.<init>(UnkNown Source)
at org.objectweb.asm.ClassReader.<init>(UnkNown Source)
at org.objectweb.asm.ClassReader.<init>(UnkNown Source)
at net.sf.cglib.proxy.BridgeMethodResolver.resolveAll(BridgeMethodResolver.java:61)
at net.sf.cglib.proxy.Enhancer.emitMethods(Enhancer.java:911)
at net.sf.cglib.proxy.Enhancer.generateClass(Enhancer.java:498)
at net.sf.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:216)
at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)
at net.sf.cglib.proxy.Enhancer.create(Enhancer.java:285)

Solution

You seem to be using the JDK in version 8, which contains the class files in version 8 Cglib does not support these class files because this library relies on outdated ASM versions

To debug this, we must note that ASM does not contain any debugging information and does not provide all information in its stack trace All we know is that an illegalargumentexception was thrown from its classreader constructor (named < init >) Looking at the source code, you can see that there is only one possibility of this exception From the source code (used by the latest version of cglib) of ASM 4.2, we can see that if the class file is an unknown version of ASM, only such exceptions will be thrown:

// checks the class version
if (readShort(off + 6) > Opcodes.V1_7) {
  throw new IllegalArgumentException();
}

Unfortunately, there is no text message for this error. There is no real reason why this is not the case, but we must put up with it To fix this error, you need a cglib version that relies on ASM 5 that supports Java 8

As of today, there is no compatible version of cglib available because cglib is not really maintained anymore You may want to try alternative library such as byte buddy Enhancements can be:

new ByteBuddy().subclass(Date.class)
  .method(named("toString"))
  .intercept(FixedValue.value("Hello World!"))
  .make()
  .load(getClass().getClassLoader(),ClassLoadingStrategy.Default.WRAPPER)
  .getLoaded()
  .newInstance();

This overrides the toString method because byte buddy does not allow you to define classes with illegal return values