java代码

package com.example.test；
package android.util.Log;
public class Test {
    
// 加载jni
static {
    
	try {
    
		System.loadLibrary(“healthmonitor_jni”);
	} catch (SecurityException ex) {
    
		Log.e(TAG, “System.loadLibrary SecurityException “ + ex);
	} catch (NullPointerException ex) {
    
		Log.e(TAG, “System.loadLibrary NullPointerException “ + ex);
	} catch (UnsatisfiedLinkError ex) {
    
		Log.e(TAG, “System.loadLibrary UnsatisfiedLinkError “ + ex);
	}
}

public Test () {
    
    Log.d(TAG, "Test construct");
    // 调用jni接口
    native_register();
}

public static void TestCallBack(int a, int b) {
    
	// to do something ...
}

// 声明jni接口
private native void native_register();
private native void native_release();
}

jni代码

#include <jni.j>
#include <stdio.h>
#include <nativehelper/JNIHelp.h>
#include <Utils/Log.h>

#define JAVA_CLASSNAME_TEST "com.example.test.Test"

using namespace std;
using namespace android;

static JavaVM *mJvm;
static jclass mClazz;
static jmethodID mCallbackMethod;

// 通过JavaVM 获取JNIEnv 
// android 源码通过JNIEnv *env = AndroidRuntime::getJNIEnv(),自己写的jni由于ndk的制限，
// 使用该方式获取到的env为null
static JNIEnv *get_env() {
    
	if (mJvm == NULL) return NULL;
	JNIEnv *env = NULL;
	
	int status = mJvm->GetEnv((void**) &env, JNI_VERSION_1_4);
	if (status == JNI_EDETACHED || env == NULL) {
    
	    status = mJvm->AttachCurrentThread(&env, NULL);
	    if (status < 0) {
    
	        env = NULL;
	    }
	}
	return env;
}

static void com_example_callback(int a, int b) {
    
	JNIEnv *env = get_env();
	env->CallStaticVoidMethod(mClazz, mCallbackMethod , 
	a, b);
}

static void com_example_register(JNIEnv *env, jobject thiz) {
    
	// 调用c++ register() 函数, mTest为c++类对象
	mTest->register(com_example_callback);
}

static void com_example_release(JNIEnv *env, jobject thiz) {
    
	//不需要使用的时候，不要忘记释放全局引用
	(*mJvm)->DetachCurrentThread(*mJvm);
	(*mJvm)->DestroyJavaVM(mJvm);
	return 0;
}
static JNINativeMethod sMethods[] = {
    
{
    “native_register”, “()V”, (void )com_example_register}
{
    “native_release”, “()V”, (void )com_example_release}
};

// This function only registers the native methods
static int register_HealthMonitor_jni(JNIEnv *env) {
    
	return jniRegisterNativeMethods(env, JAVA_CLASSNAME_HEALTHMONITOR, 
	sMethods, NELEM(sMethods));
}

// 可以不重写该函数，会使用默认
jint JNI_OnLoad(JavaVM* vm, void* /* reserved */) {
    
	mJvm = vm;
	JNIEnv env = NULL;
	jint result = -1;
	jclass tmpClazz  = NULL;

	if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
    
	    goto bail;
	}
	assert(env != NULL);
	
	// 通过FindClass获取的jclass 并不是一个实例，只能回调静态方法
	// 回调非静态方法需要通过jclass clazz = env->GetObjectClass(obj)获取jclass，
	// 调用GetMethodID 获取 jmethodID 
	// 调用CallVoidMethod()回调给 java
	tmpClazz = env->FindClass(JAVA_CLASSNAME_HEALTHMONITOR);
	// 直接使用tmpClazz 会导致crash-->jni error (app bug): accessed stale local reference
	mClazz = (jclass)env->NewGlobalRef(tmpClazz);
	mCallbackMethod = env->GetStaticMethodID(mClazz, “TestCallBack”, “(II)V”);
	
	if (register_HealthMonitor_jni(env) < 0) {
    
	    goto bail;
	}

	/* success -- return valid version number */
	result = JNI_VERSION_1_4;
	
	bail:
	return result;
}