How to have 2 JVMs talk to one another

Multiple options for IPC:

Socket-Based (Bare-Bones) Networking

• not necessarily hard, but:
  • might be verbose for not much,
  • might offer more surface for bugs, as you write more code.
• you could rely on existing frameworks, like Netty

RMI

• Technically, that's also network communication, but that's transparent for you.

Fully-fledged Message Passing Architectures

• usually built on either RMI or network communications as well, but with support for complicated conversations and workflows
• might be too heavy-weight for something simple
• frameworks like ActiveMQ or JBoss Messaging

Java Management Extensions (JMX)

• more meant for JVM management and monitoring, but could help to implement what you want if you mostly want to have one process query another for data, or send it some request for an action, if they aren't too complex
• also works over RMI (amongst other possible protocols)
• not so simple to wrap your head around at first, but actually rather simple to use

File-sharing / File-locking

• that's what you're doing right now
• it's doable, but comes with a lot of problems to handle

Signals

• You can simply send signals to your other project
• However, it's fairly limited and requires you to implement a translation layer (it is doable, though, but a rather crazy idea to toy with than anything serious.

Without more details, a bare-bone network-based IPC approach seems the best, as it's the:

• most extensible (in terms of adding new features and workflows to your
• most lightweight (in terms of memory footprint for your app)
• most simple (in terms of design)
• most educative (in terms of learning how to implement IPC). ^{(as you mentioned "socket is hard" in a comment, and it really is not and should be something you work on)}

That being said, based on your example (simply requesting the other process to do an action), JMX could also be good enough for you.

I've added a library on github called Mappedbus (http://github.com/caplogic/mappedbus) which enable two (or many more) Java processes/JVMs to communicate by exchanging messages. The library uses a memory mapped file and makes use of fetch-and-add and volatile read/writes to synchronize the different readers and writers. I've measured the throughput between two processes using this library to 40 million messages/s with an average latency of 25 ns for reading/writing a single message.

What you are looking for is inter-process communication. Java provides a simple IPC framework in the form of Java RMI API. There are several other mechanisms for inter-process communication such as pipes, sockets, message queues (these are all concepts, obviously, so there are frameworks that implement these).

I think in your case Java RMI or a simple custom socket implementation should suffice.