Calling java method from different thread in ndk

Question

I'm trying to call a java static method from a detached thread in c++ with android's NDK.

So far I've:

    JNIEnv *env =AttachJava();

    jclass cls2 = env->FindClass("com/actvt/showdown/pluggin/Utils");  // try to find the class
    //jmethodID mid2 = env->GetStaticMethodID(cls2, "AddSound", "(Landroid/app/Activity;Ljava/lang/String;I)V");  // find method
    jmethodID mid = env->GetStaticMethodID(cls2, "addFrame", "(Landroid/app/Activity;JIIII)V");  // find method
    jclass dataClass = env->FindClass("android/app/Activity");
    jobject javaClassRef =  env->AllocObject(dataClass);
    env->CallStaticVoidMethod(cls2, mid,javaClassRef,(long)frame,0,footageIndex,frameNumber,width,height);

And I'm attaching this thread to the jvm with the called AttachJava which is:

 JNIEnv* Wrapper::AttachJava()
{
    JNIEnv* myNewEnv;
    JavaVMAttachArgs args;
    args.version = JNI_VERSION_1_6; // choose your JNI version
    args.name = nullptr; // you might want to give the java thread a name
    args.group = nullptr; // you might want to assign the java thread to a ThreadGroup
    Wrapper::jvm->AttachCurrentThread(&myNewEnv, &args);
    return myNewEnv;
}

And I'm having the following error at the runtime:

06-14 10:04:31.548 32423-484/com.learnopengles.airhockey A/art: art/runtime/java_vm_ext.cc:470] JNI DETECTED ERROR IN APPLICATION: JNI GetStaticMethodID called with pending exception java.lang.ClassNotFoundException: Didn't find class "com.actvt.showdown.pluggin.Utils" on path: DexPathList[[directory "."],nativeLibraryDirectories=[/system/lib, /vendor/lib, /system/lib, /vendor/lib]]
06-14 10:04:31.548 32423-484/com.learnopengles.airhockey A/art: art/runtime/java_vm_ext.cc:470]   at java.lang.Class dalvik.system.BaseDexClassLoader.findClass(java.lang.String) (BaseDexClassLoader.java:56)
06-14 10:04:31.548 32423-484/com.learnopengles.airhockey A/art: art/runtime/java_vm_ext.cc:470]   at java.lang.Class java.lang.ClassLoader.loadClass(java.lang.String, boolean) (ClassLoader.java:380)
06-14 10:04:31.548 32423-484/com.learnopengles.airhockey A/art: art/runtime/java_vm_ext.cc:470]   at java.lang.Class java.lang.ClassLoader.loadClass(java.lang.String) (ClassLoader.java:312)

Hi I'm creating the new thread this way : std::thread recordThread(&Recording::WaitForExport, this, &recorder, capture); recordThread.detach(); — user1796260, Jun 14 '17 at 08:33
sorry, i only worked with [pthreads](https://en.wikipedia.org/wiki/POSIX_Threads) — pskink, Jun 14 '17 at 08:40
Do you think it should works with pthread? I'm gonna give it a try :) — user1796260, Jun 14 '17 at 08:42
i dont know, this is what i once wrote just for test: https://pastebin.com/z96r3BMG — pskink, Jun 14 '17 at 08:45

score 3 · Accepted Answer · answered Jun 14 '17 at 14:09

3

The solution to my problem was to keep a reference to all the classes I will need in my different threads at the begginning of the ndk call before starting my threads and to use them inside those threads. I could also gave them to the thread as parameters.

Found it out thans to pskink and according to this documentation : https://developer.android.com/training/articles/perf-jni.html#faq_FindClass

answered Jun 14 '17 at 14:09

user1796260

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It is strongly recommended to prepare all method IDs only once, too, to avoid (significant) overhead of `GetXXXMethodID()`. Note that method IDs are simple numbers, don't depend on threads, and (unlike classes) don't require global references. – Alex Cohn Jun 14 '17 at 15:17

Calling java method from different thread in ndk

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