FindClass from any thread in Android JNI

After much trying and crashing of my app, a colleague and I managed to cache and succesfully use the class loader in another, native, thread. The code we used is shown below (C++11, but easily converted to C++2003), posted here since we couldn't find any examples of the aforementioned "Cache a reference to the ClassLoader object somewhere handy, and issue loadClass calls directly. This requires some effort.". Calling findClass worked perfectly when called from a thread different from the one of JNI_OnLoad. I hope this helps.

JavaVM* gJvm = nullptr;static jobject gClassLoader;static jmethodID gFindClassMethod;JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *pjvm, void *reserved) {    gJvm = pjvm;  // cache the JavaVM pointer    auto env = getEnv();    //replace with one of your classes in the line below    auto randomClass = env->FindClass("com/example/RandomClass");    jclass classClass = env->GetObjectClass(randomClass);    auto classLoaderClass = env->FindClass("java/lang/ClassLoader");    auto getClassLoaderMethod = env->GetMethodID(classClass, "getClassLoader",                                             "()Ljava/lang/ClassLoader;");    gClassLoader = env->CallObjectMethod(randomClass, getClassLoaderMethod);    gFindClassMethod = env->GetMethodID(classLoaderClass, "findClass",                                    "(Ljava/lang/String;)Ljava/lang/Class;");    return JNI_VERSION_1_6;}jclass findClass(const char* name) {    return static_cast<jclass>(getEnv()->CallObjectMethod(gClassLoader, gFindClassMethod, getEnv()->NewStringUTF(name)));}JNIEnv* getEnv() {    JNIEnv *env;    int status = gJvm->GetEnv((void**)&env, JNI_VERSION_1_6);    if(status < 0) {            status = gJvm->AttachCurrentThread(&env, NULL);        if(status < 0) {                    return nullptr;        }    }    return env;}

Try attaching your native thread to the JVM first.

The pointer to jvm you can obtain first thing in JNI_OnLoad

env->GetJavaVM(&jvm);

Then from your native thread

JNIEnv *env;jvm->AttachCurrentThread((void **)&env, NULL);

Then use that env for FindClass