CODEWITHSUNDEEP
javaandroidcjava-native-interfacejnienv
Nathan F.
Nathan F.

Reputation: 3489

Call a function in java from C outside of a JNI function (Android)?

I'm trying to call a Java function from my C code using the JNI in Android, but I'm in a somewhat awkward situation.

My C code is being executed outside of a JNI function in a callback that is passed to a library.

Here is an example of the java code

package com.my.java.package;

class MyClass {
    public function handleData(byte[] data) {
        doSomethingWithThisData(data);
    }
}

Here is an example of the C code

void handleData(uint8_t *data, size_t len) {
    // I need to call handleData in my java
    // class instance from here, but i have
    // no access to a JNIEnv here.

    // I don't think I can create one, since 
    // it has to be the same object that's 
    // sending JNI calls elsewhere.
}

. . . 

myCLibInstance.callback = handleData;

Now whenever the C Lib does what it needs to do, it will trigger the callback. But I have no way to send it back to the java class to handle the data.

Upvotes: 3

Views: 2030

Answers (2)

Nathan F.
Nathan F.

Reputation: 3489

I noticed some issues with Brandons Solution, specifically around the way that you handle status codes and the unnecessary getJavaVM function, so i made some alterations and added some notes. This is the only functional version of this I have managed to get working.

Note that for some reason, the JNIEnv* returned by getJNIEnv() does not work with the Java class loader when used from another thread. I'm unsure why. So in this example I store static instances to the classes, loading them directly in the JNI_OnLoad function and use them later when needed.

If anyone knows a work around to get the JNIEnv* returned by getJNIEnv() to support the Java class loader from other threads, let me know.

// JavaVM instance stored after JNI_OnLoad is called
JavaVM* javaVM = NULL;

// Since the class loader will not work with getJNIEnv(),
// you can store classes in GlobalRefs.
static jclass my_class_class;

/**
 * Load the JNIEnv and store the JavaVM instance for ater calls to getJNIEnv().
 *
 * @param jvm      The Java VM
 * @param reserved Reserved pointer
 *
 * @return         The supported version of JNI.
 */
JNIEXPORT jint JNICALL
JNI_OnLoad(
        JavaVM* jvm,
        void* reserved
) {
    javaVM = jvm;

    // Here we load the classes since getJNIEnv() does 
    // not work with the class loader from other threads
    JNIEnv* env = getJNIEnv();
    my_class_class = (*env)->NewGlobalRef(env, (*env)->FindClass(env, "com/my/java/package/MyClass"));

    // Return the supported JNI version.
    return JNI_VERSION_1_6;
}

/**
 * Retrieve an instance of JNIEnv to use across threads.
 *
 * Note that the class loader will not work with this instance (unsure why).
 *
 * @return a JNIEnv instance
 */
JNIEnv* getJNIEnv() {
    JNIEnv *env;

    // If the current thread is not attached to the VM, 
    // sets *env to NULL, and returns JNI_EDETACHED.
    //
    // If the specified version is not supported, sets *env to NULL, 
    // and returns JNI_EVERSION.
    //
    // Otherwise, sets *env to the appropriate interface, and returns JNI_OK.
    int status = (*javaVM)->GetEnv(javaVM, (void**)&env, JNI_VERSION_1_6);

    // Check if the JVM is not currently attached to the
    // calling thread, and if so attempt to attach it.
    if (status == JNI_EDETACHED) {
        // Attaches the current thread to a Java VM.
        // Returns a JNI interface pointer in the JNIEnv argument.
        status = (*javaVM)->AttachCurrentThread(javaVM, &env, NULL);
    }

    // If the result of GetEnv was JNI_EVERSION,
    // we want to abort.
    assert(status != JNI_EVERSION);

    // Return the ENV if we have one
    return env;
}

void handleData(uint8_t *data, size_t len) {
    JNIEnv* env = getJNIEnv();

    // ... call jni function using env and my_class_class ...
}

Upvotes: 0

Brandon
Brandon

Reputation: 23495

On some version of the Android NDK, JNI_GetCreatedJavaVMs could be used to get the current VM.. however, the better option is to override JNI_OnLoad and save the VM there. Using either method, once you have the VM, you can attach to the current thread and get call the function..

extern jint JNI_GetCreatedJavaVMs(JavaVM **vm, jsize size, jsize *size2);

static JavaVM *jvm = NULL;


static jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    jvm = vm;
    JNIEnv *env = NULL;

    if (jvm && (*jvm)->GetEnv(jvm, (void**)&env, JNI_VERSION_1_6) == JNI_OK)
    {
        return JNI_VERSION_1_6;
    }
    return -1;
}

JavaVM* getJavaVM() {
    if (jvm)
    {
        return jvm;
    }

    jint num_vms = 0;
    const jint max_vms = 5;
    JavaVM* vms[max_vms] = {0};
    if (JNI_GetCreatedJavaVMs(vms, max_vms, &num_vms) == JNI_OK)
    {
        for (int i = 0; i < num_vms; ++i)
        {
            if (vms[i] != NULL)
            {
                return vms[i];
            }
        }
    }
    return NULL;
}

 void handleData(uint8_t *data, size_t len) {
    JavaVM *jvm = getJavaVM();

    if (jvm)
    {
        JNIEnv *env = NULL;
        if ((*jvm)->AttachCurrentThread(jvm, (void **)&env, NULL)  == JNI_OK)
        {
            if (env)
            {
                //Call function with JNI..
            }

            (*jvm)->DetachCurrentThread(jvm);
        }
    }
}

Upvotes: 2

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