Hello. I have a Java app that takes pretty much time to be initialized (so I can't use command-line like interface) and I need to pass text and receive the output of a Java method from Python. Is it possible to load the Java application, have it opened all the time the Python script runs and use a method from that app?
I don't think the use of Python helps all that much over a command line (at least not a *nix command line), but the basic idea is to communicate over a socket or some similar mechanism. That means that the Java application will have to be wrapped in some code which opens a socket and waits for the python script to contact it. If you are most comfortable with python, you could look to implement that wrapper in Jython.
I've used JPype for something similar and it worked great.
JPype is an effort to allow python programs full access to java class libraries. This is achieved not through re-implementing Python, as Jython/JPython has done, but rather through interfacing at the native level in both Virtual Machines.
If the java app is running you should also consider xml-rpc as it also works well.