We have a distributed system with components (including the ActiveMQ broker) running on jdk 1.6.
A potential customer would like to integrate a component that was built on jdk 1.4 with our system. While this customer is willing to write code to integrate with our system, they are not comfortable moving from jdk 1.4.
Would there be any problems with a system where one client is running off of an older jdk?
ActiveMQ 5.x works on Java 1.5 or later - any JVM 1.5 or later should work fine as ActiveMQ uses its own marshalling layer and does not rely on serialisation etc.
If you want to work with Java 1.4 you'll need to either install the Retrotranslator JIT or transform the jars to 1.4 complianct bytecode with Retrotranslator. There is a Maven retrotranslator plugin to help. See the ActiveMQ FAQ entry for more help
Another option is to write a simple STOMP client which is a good solution for applets etc.