Java and VoIP revisited
The Java subject has been discussed before on this blog, so I’m not going to repeat why it’s not a good idea to use pure Java for building a VoIP application; let’s just say it’s going to be a challenge for those who try. But if someone has got a Java based architecture and wants to enable or improve VoIP, will they have to throw their existing code away? Of course not – Java can utilize native, capable C++ libraries through the Java Native Interface (JNI) as long as this is supported by the JVM in the target environment. I wanted to ballpark the effort this would require, so I started hacking with the goal to make a VoIP call using a Java application.
I started out by downloading and installing the latest Java SDK for Windows from Sun. I then used a C++ VoIP library that I happen to have access to and chose the API functions I thought I was going to need to reach my goal (information hiding comes as a bonus – you don’t bring stuff into your Java class until you need and fully understand it). After copy/pasting these API functions into a new Java class, making some small adjustments to parameter types and adding the “public native” keywords in front of function declarations, I could use the “javah” tool to generate a C++ header for the glue code.
Some design decisions needed to be made, for example deciding where object specific information should be stored – I for one wanted to keep the data stored in the C++ glue code to a minimum and chose to retrieve member variables from the Java object when needed (JNI provides special C++ functions to do this). Callbacks, i.e. letting the C++ library call into the Java object, also required some thought (I won’t go any deeper here, but I tried it and it worked great). Basically, when all functions in the header file had been implemented by just making the corresponding call into the C++ library we started out with, the C++ glue code could be compiled/linked into a dll and used from the Java class we just wrote through the System.loadLibrary() call. And we’re done!
To conclude, it was only a matter of hours after downloading the JDK before I had made a (very) basic Java based integration of a VoIP library. This is not rocket science and a successful result was expected, but I still got a kick out of being able to type
java MyVoipApp 192.168.200.41
from a command line to get a high quality VoIP call going on the local network. And now that we are in a Java environment, adding a nice GUI should take like 5 minutes, right?
Tags: Java
