FindClass from any thread in Android JNI

Question

Android's JNI tips page mentions this FAQ: Why didn't FindClass find my class? They mention multiple solutions and the last option there is this one:

Cache a reference to the ClassLoader object somewhere handy, and issue loadClass calls directly. This requires some effort.

So, I tried to get it working and it seems that no matter what, this method simply does not work for me. Eventually, I figured how to use ClassLoader but it won't work if from a native thread I try to loadClass that hasn't been touched/loaded yet. Essentially, it's the identical to env->FindClass in behavior when called from a native thread, with the exception that it won't return 0 for classes that were already use in the app. Any idea if I didn't get it right, or it's impossible to access classes from a native thread that weren't used/loaded yet.

EDIT: I'll give more info to explain what exactly I mean. There is regular JNI env->FindClass(className), and another one that I wrote myFindClass(env, className) that uses cached ClassLoader->loadClass.

The class that I'm trying to access from native c/c++ is "com/noname/TestClient". Inside myFindClass I also use env->FindClass and log value that it returns:

jclass myFindClass(JNIEnv * env, const char* name)
{
    ...
    jclass c0 = env->FindClass(name);
    jclass c1 = (jclass)env->CallObjectMethod(ClassLoader,
        MID_loadClass, envNewStringUTF(name));
    dlog("myFindClass(\"%s\") => c0:%p, c1:%p, c0 and c1 are same: %d",
        name, c0, c1, env->IsSameObject(c0, c1));
    ...
}

Then, I have these 3 combinations to explain the issue.

1)

//inside JNI_OnLoad thread
myFindClass(env, "com/noname/TestClient");
...

//inside native thread created by pthread_create
myFindClass(env, "com/noname/TestClient");

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0x41b64558, c1:0x41b64558, c0 and c1 are same: 1
...
myFindClass("com/noname/TestClent") => c0:0, c1:0x41b64558, c0 and c1 are same: 0

2)

//inside JNI_OnLoad thread
env->FindClass("com/noname/TestClient");
...

//inside native thread created by pthread_create
myFindClass("com/noname/TestClient");

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0, c1:0x41b64558, c0 and c1 are same: 0

3)

//inside JNI_OnLoad thread
//"com/noname/TestClient" isn't touched from JNI_OnLoad.
...

//inside native thread created by pthread_create
myFindClass(env, "com/noname/TestClient");

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0, c1:0, c0 and c1 are same: 1

Basically, my issue is that ClassLoader doesn't find my class in the 3rd case. Is it a bug? What can be done to fix the problem?

EDIT2: On top of that, it seems that ClassLoader::loadClass is plainly buggy. If I ask myFindClass("noname/TestClent") then it returns some garbage, and when I use that returned jclass in any way the app crashes.

Answer 1

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;
}

Answer 2

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