Java Serialization Problems, while using guava Lists.transform

Question

I had to serialize a complex object, but one of its component was non-serializable ( a third party graph object), so I created a custom serializable version of this Graph class and used Guava List transform to convert the non-serializable object to the custom objects. The serialization writeObject still failed. I will be interested to know why? My assumption is that the Lists.transform performs its operation Lazily (Holding an hidden reference to the orginal object.)

Also is there an workaround for this problem ?

Answer 1

Lists.transform() does perform lazily as you suspected. You could do one of

Lists.newArrayList(Lists.transform(...))

or, if you want an immutable version,

ImmutableList.copyOf(Lists.transform(...))

and then serialize the resulting list.

Answer 2

Lists.transform() returns a transformed view of the original list. From the Lists.transform() javadoc:

The returned list always implements Serializable, but serialization will succeed only when fromList and function are serializable.

When serializing the transformed view, you are actually serializing the original list, plus the function. In your case, it fails because your original list is not serializable (because it contains non-serializable graph elements). But it could also fail because the function does not implement serializable.

BTW, there is a little trick to create serializable functions without the verbosity. Instead of doing:

  private static final class MyFunction extends Function<String, String> implements Serializable {
    private static final MyFunction INSTANCE = new MyFunction();

    @Override
    public String apply(String input) {
      return "[" + input + "]";
    }

    private Object readResolve() {
      return INSTANCE;
    }
    private static final long serialVersionUID = 1;
  }

You can use the enum singleton pattern, which is a lot less verbose, and gets you serialization for free (since enums are serializable). It also makes sure your Function is a singleton:

  // enum singleton pattern
  private enum MyFunction implements Function<String, String> {
    INSTANCE;

    @Override
    public String apply(String input) {
      return "[" + input + "]";
    }
  }