Android NDK Call Java From C++

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原文链接：http://www.cnblogs.com/open-coder/archive/2013/01/26/2878364.html

文章标签：

#java #移动开发 #c/c++

本文介绍了一种在Android NDK环境下使用C++调用Java代码播放声音的方法。通过实现Java静态方法并在C++中调用，解决了C++直接播放声音的难题。文章详细展示了如何在C++中加载Java虚拟机，获取Java类和方法ID，以及如何调用Java静态方法。

I search the Android NDK document and Internet a bit, and could not find anything useful to help me play sound with C++ under the Android NDK. Most of topics are about how to play sound with Java, so I grape an idea that why not call Java from C++. I tried along this way, and finally all those stuff works as I expected before.

Implement Java static function

A Sound player class implemented in Java class. In addition to play sound basic feature, I also make it expose a static function that will be called from C++. The reason why I used a static function here is that static function could allow me correctly find the right Java object instance, the one with sound pool initialized well and file loaded ok will be the correct one.

public class SoundManager {
    ......
    
    public static void sPlaySound(int sound)
    {
        if ( null != sInstance )
            sInstance.PlaySound(sound);
    }
    
}

Call Java function from C/C++

static JavaVM* g_JavaVM = NULL;
static const char *g_JavaClassName = "com/easygame/SoundManager";

JNIEXPORT jint JNI_OnLoad(JavaVM* jvm, void* reserved)
{
    g_JavaVM = jvm;

    JNIEnv *env = NULL;
    if (jvm->GetEnv((void**) &env, JNI_VERSION_1_6) != JNI_OK)
        return -1;

    return JNI_VERSION_1_6;
}

JNIEXPORT void JNI_OnUnload(JavaVM* jvm, void* reserved)
{
    g_JavaVM = NULL;
    JNIEnv *env = NULL;
    if (jvm->GetEnv((void**) &env, JNI_VERSION_1_6) != JNI_OK)
        return;
}

void SysPlaySound(int soundId)
{
    if (g_JavaVM == NULL)
        return;

    int status = -1;
    JNIEnv *env = NULL;
    bool isAttached = false;

    status = g_JavaVM->GetEnv((void**) &env, JNI_VERSION_1_6);
    if (status < 0)  
    {
        status = g_JavaVM->AttachCurrentThread(&env, NULL);
        if (status < 0)
            return;
        isAttached = true;
    }

    if ( env != NULL )
    {
        jclass cls = env->FindClass(g_JavaClassName);
        if ( cls != 0 )
        {
            jmethodID mid = env->GetStaticMethodID(cls, "sPlaySound", "(I)V");
            if ( mid != 0 )
                env->CallStaticVoidMethod(cls, mid, soundId);
        }
    }

    if ( isAttached )
        g_JavaVM->DetachCurrentThread();    
}

Function JNI_OnLoad and JNI_OnUnload are Android NDK system call back functions, that will allow us to get the current Java virtual machine. We need to get the class static method or object member function from the Java virtual machine. Now we could call function SysPlaySound as a C/C++ native function.