CODEWITHSUNDEEP
javajava-11java-bytecode-asmbyte-buddycglib
Barney
Barney

Reputation: 2836

Create dynamic proxy for existing Serializable object with no available constructor

I have an instance of an object for which I need to create proxy to intercept one of the methods:

So what I need to do is something like:

 TheObject obj = library.getObject();
 TheObject proxy = createProxyObject(obj);
 library.doSomethingWith(proxy);

It seems to me that theoretically this should be possible as the object is Serializable, but I can't find any way of using that.

Note on the following: I've been trying using cglib but I'm not tied to that at all. If it is possible in asm, javaassist, or any other library that would be fine.

What I have so far with cglib is I can proxy a simple object with a public constructor:

public class SimpleObject {
  private String name;
  public void setName(String name) {
    this.name = name;
  }
  public String getName() {
    return name;
  }
  // return a random number
  public int getRandom() {
    return (int)(Math.random() * 100);
  }
}

public void testCglibEnhancer() throws Exception {
  SimpleObject object = new SimpleObject();
  object.setName("object 1");
  System.out.println(object.getName() + " -> " + object.getRandom());

  Enhancer enhancer = new Enhancer();
  enhancer.setSuperclass(object.getClass());

  // intercept getRandom and always return 32
  ProxyRefDispatcher passthrough = proxy -> object;
  MethodInterceptor fixedRandom = (obj, method, args, proxy) -> 32;
  enhancer.setCallbacks(new Callback[]{passthrough, fixedRandom});
  enhancer.setCallbackFilter(method -> method.getName().equals("getRandom") ? 1 : 0);

  SimpleObject proxy = (SimpleObject)enhancer.create();
  System.out.println(proxy.getName() + " -> " + proxy.getRandom()); // always 32
}

But I've been unable to replicate this using an object with no public constructor:

public static class ComplexObject implements Serializable {
  public static ComplexObject create() {
    return new ComplexObject();
  }
  private String name;
  private ComplexObject() {
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getName() {
    return name;
  }
  public int getRandom() {
    return (int)(Math.random() * 100);
  }
}

ComplexObject proxy = (ComplexObject)enhancer.create();
// throws IllegalArgumentException: No visible constructors

As the object is Serializable, I can clone it:

public static <T extends Serializable> T cloneViaSerialization(T source) throws IOException, ClassNotFoundException {
  ByteArrayOutputStream bos = new ByteArrayOutputStream();
  ObjectOutputStream out = new ObjectOutputStream(bos);
  out.writeObject(source);

  ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
  ObjectInputStream in = new ObjectInputStream(bis);
  return (T)in.readObject();
}

public void testClone() throws Exception {
  ComplexObject object1 = ComplexObject.create();
  object1.setName("object 1");

  ComplexObject object2 = cloneViaSerialization(object1);
  object2.setName("object 2");

  System.out.println(object1.getName() + " -> " + object1.getRandom());
  System.out.println(object2.getName() + " -> " + object2.getRandom());
}

So is there any way I can get cglib (or any library) to use this approach?

ComplexObject object = library.getObject();
ObjectInputStream in = ... // serialised version of object

Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(object.getClass());
// add callbacks etc.

// note createFromSerialized is not a existing method of
// Enhancer - it is what I'm trying to synthesise somehow
ComplexObject proxy = (ComplexObject)enhancer.createFromSerialized(in);

Thanks

Upvotes: 2

Views: 846

Answers (1)

Barney
Barney

Reputation: 2836

Got it working:

  1. Created a derived class definition using ASM.
  2. Added this class definition to the same class loader as the target class.
  3. Serialised the existing object to a byte array.
  4. Injected the derived class name into the serialised byte array.
  5. Deserialised the resulting bytes using standard ObjectInputStream.readObject.

For (1), I couldn't get cglib or byte-buddy to create the class I needed, so I switched to ASM.

For (2), I used a custom class loader to load the whole jar file containing the target class.

This does mean I end up with a clone of the original object not a proxy as per the question, but that works fine for what I need.

For the serialisation hack:

I created a few example minimal classes, serialized them to disk and compared the resulting binary data.

To inject the class name:

  1. Serialised the full parent object to a byte array.
  2. Manually created a byte array for the derived class header (see below).

Both byte arrays contain the stream header data (magic, version and initial object type), so I took that from (2) for simplicity, although I did have to tweak a couple of bytes.

So then just created a stream of all of (2) followed by all of (1) except the first 6 bytes.

To create the byte array for the derived class, I just looked at the delivered Java source and came up with the following:

/*
 * Returned array contains:
 * - stream header
 * - class header
 * - end block / class desc markers for next class
 */
private byte[] derivedClass(Class<?> clss) throws IOException {
  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  ObjectOutputStream oos = new ObjectOutputStream(baos);

  ObjectStreamClass osc = ObjectStreamClass.lookup(clss);
  oos.writeUTF(osc.getName());
  oos.writeLong(osc.getSerialVersionUID());
  oos.write(SC_SERIALIZABLE);      // flags
  oos.writeShort(0);               // field count
  oos.writeByte(TC_ENDBLOCKDATA);
  oos.writeByte(TC_CLASSDESC);
  oos.flush();

  // header appears to write 0x77 (TC_BLOCKDATA) and 0x54 (???) for bytes 5 & 6
  // samples streamed from other files use 0x73 (TC_OBJECT) and 0x72 (TC_CLASSDESC)
  byte[] bytes = baos.toByteArray();
  bytes[4] = TC_OBJECT;
  bytes[5] = TC_CLASSDESC;

  return bytes;
}

Have not tested this as a general approach, but appears to work fine for my case.

Upvotes: 1

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