Artinsoft provides means to migrated Java Applications to .NET. However as with many medium size to large applications it could be a complex task.
Sometimes due to cost or time restrictions just a portion of the system is migrated. The migration team analyses the code and determines which are the "connection points" or the areas where both systems are connected and therefore, means must be develop to provide communications between the Java and the .NET world.
There are many alternatives.
If you're facing this problem I will recommend the following references:

 Microsoft .NET and J2EE Interoperability Toolkit  by Simon Guest
He also published an interesting article where he recommends a products like Javena, and JNBridge
You can also check out JIntegra