Java Serialization vs JSON vs XML

In general the important question is which client will receive the serialized objects - browsers/JavaScript engines like (node-js), Java client, unknown/multiple clients.

JSON - JSON syntax is basically JavaScript and therefore any component with a JS engine will handle its parsing very well - even complicated data-structures will be converted to "living" objects efficiently. JSON parsers exist for practically any language and it is easy to use even when not using a JS engine, (Take Google Gson for example that is able to convert JSON into corresponding objects with ease) which makes is a good candidate for cross-language communication - for example in a messaging architecture.

XML - Shares many of JSON's benefits - cross-language, lightweight, etc. Adobe Flex for example handles XML very well, even better than JSON. It's definitely an appropriate substitute for JSON. I personally prefer JSON for its JS like syntax, but XML is also good.

Java Serialization - Should be considered only for Java-to-Java communication. An important note is that the class definitions should be on the sending and the receiving ends and often you wouldn't gain much by passing the entire object. I wouldn't rule out RMI as a communication protocol, it does simplify development. However the resulting application components will be hard coupled which will make it very difficult to replace.

One more notes - Serialization in general has its overhead. However when the communication is performed over a network the bottleneck is often the network rather than the serialization/deserialization itself.

I think it depends. If you're sending an http request or something then JSON or XML is obviously a good choice. If you are just sending a java object over a tcp socket for some distributed algorithm or something I think the java serialization is easier/better