How can I synchornize two Java processes?

It is not possible to do something like you want in Java. Different Java applications will use different JVM’s fully separating themselves into different ‘blackbox’es. However, you have 2 options:

• Use sockets (or channels). Basically one application will open the listening socket and start waiting until it receives some signal. The other application will connect there, and send signals when it had completed something. I’d say this is a preferred way used in 99.9% of applications.
• You can call winapi from Java (on windows).