Java object references with cache layer

Question

We've created a caching layer to our J2EE-application. In this instance we use Ehcache. This has created a few challenges.

Let's take this example.

OrderItem orderitem = cache.getOrderItemByID("id");
OrderItem old_orderitem = cache.getOrderItemID("id");

orderitem.setStatus(1);
old_orderitem.setStatus(2);

If we're not carefull, any changes made to either of those objects will affect the other (they reference the same object). Saving orderitem back to the dabase would make it have status=2

How would we solve this the best way?

We've tried making a .copyObject() method to each object. Which just creates a new object and sets all the values. But this doesn't seem like a good solution.

The example was just to illustrate. The code is far more complex than that, but the result is the same.

**********************Update 15.07.2010 **************************************************

In EHCache 2 there are some options to to turn on copyRead() and copyWrite(). And this solves all my problems :)

Answer 1

This is a problem with mutable state. It isn't only caching, it's any time you could have multiple references to the same object and that object is mutable. For example:

HashMap map = new HashMap();
map.put("one", new OrderItem());
OrderItem one = map.get("one");
OrderItem two = map.get("one");
one.setStatus(1);
two.setStatus(2);

Will have exactly the same problem. This becomes even more complicated when you have a concurrent environment. One way to solve this is to only have immutable objects. That way, if you want an object with different state, you're going to have to make a new one. It also makes concurrent programming easier.

You're right to consider copying the object. Your options are:

• Cloning
• Deep copying via serialization
• Deep copying via copy constructors

Each has their strengths and weaknesses and which one works best will depend on your environment.

Answer 2

It sounds like you want to cache the data representing an object (for example, the results of a database query) rather than an object itself (for example, session data for a user that you may want to retrieve elsewhere).

If this is the case, your custom cacheing layer (around ehcache) needs to create an object from the cache data when a user makes a request - this will give you a unique object every time and you won't get object interference.