After AttachCurrentThread: Calling back to Java method crashes JVM

Question

Hi,
so I managed to get my multithreaded native code being invoked from a Java program.
However, as soon as I try to invoke a Java callback from native code not running in the main thread, the program segfaults. Here's my code:
// in the main thread, I store a pointer to the JVM that invokes this code
    if (env->GetJavaVM(&jvm) < 0) {
        std::cerr << "JNI ERROR: Failed getting reference to Java VM" << std::endl;
        return;
    assert (jvm != NULL);
// in some other concurrently running (native) thread:
    JNIEnv *env;
    if (jvm->AttachCurrentThread((void**)&env, NULL) != JNI_OK) {
        std::cerr << "ERROR: Could not attach event thread to JVM" << std::endl;
        return;
    assert (env != NULL);
    assert (connector != NULL);
    assert (focusEventCallback != NULL);
    // the following line segfaults
    env->CallVoidMethod(connector, focusEventCallback);"connector" is a POJO, focusEventCallback the jmethodID referencing the Java callback.
Am I missing something?
Thanks in advance,
Matthias

Answer 1

jschell wrote:
What do you mean by that? I get this reference in the main thread in JNI_OnLoad and I use it in another thread. So I use it across thread boundaries. Is that what you mean?Generally not a good idea.So what do you suggest?
Getting that reference inside the thread where I use it didn't work, because the class lookup failed (ClassNotFoundException).Which means your code was wrong, not that it wouldn't work.So, what in particular was wrong about my code? Here is the section of the code that did not work, because the class lookup failed:
    JNIEnv *env = NULL;
    // "import" the native user level thread as a Java thread in the JVM
    if (jvm->AttachCurrentThread((void**)&env, NULL) != JNI_OK) {
        std::cerr << "JNI ERROR: Could not attach X11 event thread to JVM" << std::endl;
        return;
    jclass connectorClass = env->FindClass("de/dfki/km/mydesk/nativeInterfaces/linux/NetWmConnector");
    if (env->ExceptionOccurred()) {
        env->ExceptionDescribe(); // will output ClassNotFoundException
    }The same code works inside JNI_OnLoad.
Well, an assertion is supposed to do that: Terminate the program if there is a condition that would stop the program from operating properly. The crashes I was talking about are segmentation faults. A failing assertion is not a crash, it will just terminate the program with an "assertion failed" message.You are not writing a C/C++ app. You are running some code in a shared library that is run from a VM.
Very different.
I haven't tried it but I am rather certain that if you cause an assertion failure that you are going to see a crash. So far as I know the only exit mechanism that doesn't cause the VM to crash explicitly is a system exit call. And that probably shouldn't be used either (not normally.)Well, as far as I know, assert() sends an abort signal and terminates the process (which is the VM in this case). I probably overused assertions in that I used them to probe for runtime errors, not for programming errors. I'll remove them from the retail code anyway, so for now, it's just fine that the program terminates if something goes wrong. It helps locating the offending LOC.