Is there a Java library that can "diff" two Objects?

Is there a Java utility library that is analogous to the Unix program diff, but for Objects? I'm looking for something that can compare two objects of the same type and generate a data structure that represents the differences between them (and can recursively compare differences in instance variables). I'm not looking for a Java implementation of a text diff. I'm also not looking for help with how to use reflection to do this.

The application I'm maintaining has a fragile implementation of this functionality that had some poor design choices and that needs to be rewritten, but it would be even better if we could use something off the shelf.

Here's an example of the kind of thing I'm looking for:

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

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

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

DiffDataStructure diff = OffTheShelfUtility.diff(a, b);  // magical recursive comparison happens here

After comparison, the utility would tell me that "prop1" is different between the two objects and "prop2" is the same. I think it's most natural for DiffDataStructure to be a tree, but I'm not going to be picky if the code is reliable.

Solution 1:

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;

Solution 2:

http://javers.org is library that does exacly what you need: has methods like compare(Object leftGraph, Object rightGraph) returning the Diff object. Diff contains a list of changes (ReferenceChange, ValueChange, PropertyChange) e.g.

given:
DummyUser user =  dummyUser("id").withSex(FEMALE).build();
DummyUser user2 = dummyUser("id").withSex(MALE).build();
Javers javers = JaversTestBuilder.newInstance()

when:
Diff diff = javers.compare(user, user2)

then:
diff.changes.size() == 1
ValueChange change = diff.changes[0]
change.leftValue == FEMALE
change.rightValue == MALE

It can handle cycles in graphs.

In addition you can get Snapshot of any graph object. Javers has JSON serializers and deserializers to snapshot, and changes so you can easily save them in database. With this library you can easily implement a module for auditing.

Solution 3:

Yes, the java-util library has a GraphComparator class that will compare two Java Object Graphs. It returns the difference as a List of deltas. The GraphComparator also permits you to merge (apply) the deltas as well. This code only has dependencies on the JDK, no other libraries.

Solution 4:

You could also take a look at the solution from Apache. Most projects already have it on their classpath since its part of commons-lang.

Check difference for specific field(s):
http://commons.apache.org/proper/commons-lang/javadocs/api-3.9/org/apache/commons/lang3/builder/DiffBuilder.html

Check difference by using reflection:
http://commons.apache.org/proper/commons-lang/javadocs/api-3.9/org/apache/commons/lang3/builder/ReflectionDiffBuilder.html