jni starter question

Question

I started looking into JNI and from what I understand is that if a problem occurs with the loaded dll, the jvm is possible to terminate on the spot.
I.e. the process can not be protected e.g. like when catching an exception.
So if my understanding is correct, my question is if there is a standard approach/pattern for this situation when using jni.
Or to state it differently, are processes using jni designed in way to avoid these issues? Or such problems are not expected to occur?

Thank you.

Answer 1

Yes, the JVM will just terminate which is one of the reasons why JNI code is really hard to debug. If you are using C++ code you can use exceptions and then map them to a Java exception which at least gives you some level of security but doesn't help with things like bad memory access etc.

From an architecture point of view I suggest to decouple you code from JNI as much as possible. Create a class / procedure structure that is entirely testable from C++/ C and let the JNI code do only all the conversion stuff. If the JVM then crashes you at least know where you have to look.

Answer 2

You can have exactly the same spectrum of error handling in a JNI library as in anything else.

You can use try/catch. If you are on Windows, you can use SEH. If you are on Linux, you can call sigaction.

Still, if you mess up and there's a SIGSEGV, your JVM is probably toast whether you try to catch that signal or not.

Answer 3

The principles are no different from any multi-threaded C application:

1. Always check all your input thoroughly.
2. Always free up temporary memory you allocated.
3. Make sure your functions are re-entrant.
4. Don't rely on undefined behaviour.

The Java virtual machine offers you no extra protection for your native code, if it fails or is leaking, your VM will fail or leak.