JNI Native Threads

28 Oct 2016

Hey everybody!

In this post I will show you how to use native threads with Java. If you follow the previous posts you may have noted that each native method comes with a JNIEnv pointer and that pointer is used to access JNI’s vtable. The JNIEnv attaches the native call to the Java’s thread. The problem calling Java function from native threads is that you don’t have a JNIEnv to use, so how to proceed?

First you need a pointer to the JVM, of the JavaVM *. With this pointer in hands you can call AttachCurrentThread to initalize a JNIEnv variable. Okay, but now, how to obtain the JavaVM * pointer.

There are two functions that you can implement and that are called each time that the JVM is load or unload. They are JNI_OnLoad and JNI_OnUnload respectively. The last one will not be used at this example.

So now we have a full picture.

1. Implement JNI_OnLoad.
2. Save JavaVM * pointer.
3. Call AttachCurrentThread to initialized one JNIEnv **.
4. Call any JNI function using that JNIEnv ** pointer.
5. Call DetachCurrentThread to detach the previously attached thread.

The details can be grasped from the example code above. The comments provides further guidance. Take a look:

Cheers :+1: