It is well known a thread not generated from Java itself, like pthread_create on android, or a native activity, can’t find a JVM method by calling Java env’s FindClass. These method calls will return class not found (nullptr), even for prelude classes, like java.lang.String.
Turns out findClass method relies on a ClassLoader to find an specific jclass object. The solution, is simple, but subtle though.
Our native thread is not attached to any VM/Java Env and thus env->FindClass calls will fail. We need another way to find classes. And what class has a FindClass method ? Well, no other class does. But a ClassLoader exposes a loadClass method, which indirectly calls findClass and many other methods. Simply enough, we need to obtain a reference ClassLoader, being the most obvious java code to be: object.getClass().getClassLoader().
In my case, I get such class loader from the call to v8 initialisation. This method is public native void InitializeV8();. In native:
JNIEXPORT void JNICALL
Java_com_spellington_task_TaskRunnerInstance_InitializeV8(JNIEnv *env, jobject obj) {
...
// obj if implicit this on java call to the native method.
auto rc = env->GetObjectClass(obj);
// obj.getClass()
jclass clazz = env->GetObjectClass(rc);
// find getClassLoader method
jmethodID getClassLoaderMethod = env->GetMethodID(clazz,
"getClassLoader",
"()Ljava/lang/ClassLoader;");
// obtain class' ClassLoader reference.
jobject objClassLoader = env->CallObjectMethod(rc, getClassLoaderMethod);
// protect this ref from GC ! This is mandatory.
// objClassLoader must be stored somewhere else for later usage
objClassLoader = env->NewGlobalRef(objClassLoader);
// ClassLoader class
jclass objClassLoaderClass = env->FindClass("java/lang/ClassLoader");
// loadClass reference.
// Must be stored somewhere else for later usage.
jmethodID loadClassMethod = env->GetMethodID(
objClassLoaderClass,
"loadClass",
"(Ljava/lang/String;)Ljava/lang/Class;");
}
This code just got a reference to a valid ClassLoader and a methodID to its loadClass method. Now we have an alternative to just calling findClass:
const char* const className = "com/spellington/HCSurfaceView";
auto clazz = env->FindClass(className);
// if findClass failed, try the ClassLoader alternative.
// This will be true in two different scenarios:
// + className, effectively does not exist.
// + a native thread calls into JNI
if (clazz==nullptr) {
// findClass alternative based on our ClassLoader
clazz = static_cast<jclass>(
env->CallObjectMethod(
objClassLoader,
loadClassMethod,
env->NewStringUTF(className)));
}
// if clazz is nullptr, there another two different scenarios:
// + most likely, className is not a valid fully qualified class name.
// + a native thread is trying to find a class which does not exist in the ClassLoader,
// but might exist on another ClassLoader. This is likely if and only if your app
// has customs ClassLoader instances.
When using mixed Java/native thread, or just calling JNI in a multithread environment, you might want to pay attention to Java VM’s AttachCurrentThread method… But that’s another story.
