Question

I have a JNI wrapper for Java functions that are called from C… I’m trying to call some methods from different threads and I get an error when trying to get a new copy of the JNIEnv pointer… the code I’m using is below and is called in each method:

        JNIEnv* GetJniEnvHandle(){
        ThreadInfo();
        JNIEnv *envLoc; 
        //if(Thread::CurrentThread->IsBackground || Thread::CurrentThread->IsThreadPoolThread)  
        jint envRes = vm->GetEnv((void**)&envLoc, JNI_VERSION_1_4);
        if(envRes == JNI_OK){
            if(ThreadId != Thread::CurrentThread->ManagedThreadId)
                jint res = vm->AttachCurrentThread((void**)&envLoc, NULL);
        }else{          
            Log("Error obtaining JNIEnv* handle");  
        }
        return envLoc;
    }

The JVM has already been instantiated and this (and other methods) run when being called from the main/initial thread. When I get a value for envRes it holds a -2 when in a sub-thread.

Answer 1

Please refer to the documentation to the chapter Attaching to the VM.

You need to call AttachCurrentThread() for each native thread at least once before you can use any of the JNI functions.
Thread created in Java are already attached.
So I your example whenever the GetEnv call fails call AttachCurrentThread() and you should be fine. Or make sure that whenver you create a sub thread you attach it to the VM.