This library seems to do the trick: google-diff-match-patch. It can create a patch string from differences and allow to reapply the patch.

edit: Another solution might be to https://code.google.com/p/java-diff-utils/

Might be a little late, but I was in the same situation like you and ended up creating my own library for exactly your use-case. Since I was forced to come up with a solution myself, I decided to release it on Github, to spare others the hard work. You can find it here: https://github.com/SQiShER/java-object-diff

--- Edit ---

Here is a little usage example based on the OPs code:

SomeClass a = new SomeClass();
SomeClass b = new SomeClass();

a.setProp1("A");
a.setProp2("X");

b.setProp1("B");
b.setProp2("X");

DiffNode diff = ObjectDifferBuilder.buildDefault().compare(a, b);

assert diff.hasChanges();
assert diff.childCount() == 1;
assert diff.getChild('prop1').getState() == DiffNode.State.CHANGED;

This library seems to be what you're after: google-diff-match-patch.

It has the following main features:

1. Diff: Compare two blocks of plain text and efficiently return a list of differences.
2. Match: Given a search string, find its best fuzzy match in a block of plain text. Weighted for both accuracy and location.
3. Patch: Apply a list of patches onto plain text. Use best-effort to apply patch even when the underlying text doesn't match.

In case you want an alternative, you could also try this: java-diff-utils