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

问题

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.

回答1:

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);
        }
    }
}

来源：https://stackoverflow.com/questions/44447454/call-a-function-in-java-from-c-outside-of-a-jni-function-android

标签

java

android

java-native-interface

jnienv