NDK开发第三章 JNI-Callback

本章所学到的知识

• jni创建java对象
• jni调用java静态方法
• 创建linux线程
• jni线程锁
• jni回调java对象方法

    /*
     * A function calling from JNI to update current timer
     * 此处注解，防止编译器删除
     */
    @Keep
    private void updateTimer() {
        ++second;
        if(second >= 60) {
            ++minute;
            second -= 60;
            if(minute >= 60) {
                ++hour;
                minute -= 60;
            }
        }
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                String ticks = "" + MainActivity.this.hour + ":" +
                        MainActivity.this.minute + ":" +
                        MainActivity.this.second;
                MainActivity.this.tickView.setText(ticks);
            }
        });
    }
    static {
        System.loadLibrary("hello-jnicallback");
    }
    public native  String stringFromJNI();
    public native void startTicks();
    public native void StopTicks();

 

2、用的是C语言写的 hello-jnicallback.c

/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
#include <string.h>
#include <inttypes.h>
#include <pthread.h>
#include <jni.h>
#include <android/log.h>
#include <assert.h>


// Android log function wrappers
static const char* kTAG = "hello-jniCallback";
#define LOGI(...) \
  ((void)__android_log_print(ANDROID_LOG_INFO, kTAG, __VA_ARGS__))
#define LOGW(...) \
  ((void)__android_log_print(ANDROID_LOG_WARN, kTAG, __VA_ARGS__))
#define LOGE(...) \
  ((void)__android_log_print(ANDROID_LOG_ERROR, kTAG, __VA_ARGS__))

// processing callback to handler class
typedef struct tick_context {
    JavaVM  *javaVM;
    jclass   jniHelperClz;
    jobject  jniHelperObj;
    jclass   mainActivityClz;
    jobject  mainActivityObj;
    pthread_mutex_t  lock;
    int      done;
} TickContext;
TickContext g_ctx;

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   hello-jniCallback/app/src/main/java/com/example/hellojnicallback/MainActivity.java
 */
JNIEXPORT jstring JNICALL
Java_com_example_hellojnicallback_MainActivity_stringFromJNI( JNIEnv* env, jobject thiz )
{
#if defined(__arm__)
    #if defined(__ARM_ARCH_7A__)
    #if defined(__ARM_NEON__)
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a/NEON (hard-float)"
      #else
        #define ABI "armeabi-v7a/NEON"
      #endif
    #else
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a (hard-float)"
      #else
        #define ABI "armeabi-v7a"
      #endif
    #endif
  #else
   #define ABI "armeabi"
  #endif
#elif defined(__i386__)
#define ABI "x86"
#elif defined(__x86_64__)
#define ABI "x86_64"
#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
#define ABI "mips64"
#elif defined(__mips__)
#define ABI "mips"
#elif defined(__aarch64__)
#define ABI "arm64-v8a"
#else
#define ABI "unknown"
#endif
    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");
}

/*
 *  A helper function to show how to call
 *     java static functions JniHelper::getBuildVersion()
 *     java non-static function JniHelper::getRuntimeMemorySize()
 *  The trivial implementation for these functions are inside file
 *     JniHelper.java
 */
void queryRuntimeInfo(JNIEnv *env, jobject instance) {
    // Find out which OS we are running on. It does not matter for this app
    // just to demo how to call static functions.
    // Our java JniHelper class id and instance are initialized when this
    // shared lib got loaded, we just directly use them
    //    static function does not need instance, so we just need to feed
    //    class and method id to JNI
    jmethodID versionFunc = (*env)->GetStaticMethodID(env, g_ctx.jniHelperClz,"getBuildVersion", "()Ljava/lang/String;");
    if (!versionFunc) {
        LOGE("Failed to retrieve getBuildVersion() methodID @ line %d",
             __LINE__);
        return;
    }
    jstring buildVersion = (*env)->CallStaticObjectMethod(env,g_ctx.jniHelperClz, versionFunc);//调用静态函数

    const char *version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
    if (!version) {
        LOGE("Unable to get version string @ line %d", __LINE__);
        return;
    }
    LOGI("Android Version - %s", version);
    (*env)->ReleaseStringUTFChars(env, buildVersion, version);

    // we are called from JNI_OnLoad, so got to release LocalRef to avoid leaking
    (*env)->DeleteLocalRef(env, buildVersion);

    // Query available memory size from a non-static public function
    // we need use an instance of JniHelper class to call JNI
    jmethodID memFunc = (*env)->GetMethodID(env, g_ctx.jniHelperClz,"getRuntimeMemorySize", "()J");
    if (!memFunc) {
        LOGE("Failed to retrieve getRuntimeMemorySize() methodID @ line %d",
             __LINE__);
        return;
    }
    jlong result = (*env)->CallLongMethod(env, instance, memFunc);
    LOGI("Runtime free memory size: %" PRId64, result);
    (void)result;  // silence the compiler warning
}

/*
 * processing one time initialization:
 *     Cache the javaVM into our context
 *     Find class ID for JniHelper
 *     Create an instance of JniHelper
 *     Make global reference since we are using them from a native thread
 * Note:
 *     All resources allocated here are never released by application
 *     we rely on system to free all global refs when it goes away;
 *     the pairing function JNI_OnUnload() never gets called at all.
 */
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved) {
    JNIEnv* env;
    memset(&g_ctx, 0, sizeof(g_ctx));
    LOGI("Android JNI_OnLoad");
    g_ctx.javaVM = vm;//为结构体进行赋值
    if ((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6) != JNI_OK) {
        return JNI_ERR; // JNI version not supported.
    }
    // 找到该类
    jclass  clz = (*env)->FindClass(env,"com/example/hellojnicallback/JniHandler");
    g_ctx.jniHelperClz = (*env)->NewGlobalRef(env, clz);

    jmethodID  jniHelperCtor = (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");//构造函数
    jobject    handler = (*env)->NewObject(env, g_ctx.jniHelperClz,jniHelperCtor);//new 出一个java对象
    g_ctx.jniHelperObj = (*env)->NewGlobalRef(env, handler);//创建 obj 参数所引用对象的新全局引用
    queryRuntimeInfo(env, g_ctx.jniHelperObj);//

    g_ctx.done = 0;
    g_ctx.mainActivityObj = NULL;
    return  JNI_VERSION_1_6;
}

/*
 * A helper function to wrap java JniHelper::updateStatus(String msg)
 * JNI allow us to call this function via an instance even it is
 * private function.
 */
void   sendJavaMsg(JNIEnv *env, jobject instance,
                   jmethodID func,const char* msg) {
    jstring javaMsg = (*env)->NewStringUTF(env, msg);
    (*env)->CallVoidMethod(env, instance, func, javaMsg);
    (*env)->DeleteLocalRef(env, javaMsg);
}

/*
 * Main working thread function. From a pthread,
 *     calling back to MainActivity::updateTimer() to display ticks on UI
 *     calling back to JniHelper::updateStatus(String msg) for msg
 void *可以接受任何类型的赋值：
 */
void*  UpdateTicks(void* context) {
    TickContext *pctx = (TickContext*) context;
    JavaVM *javaVM = pctx->javaVM;
    JNIEnv *env;
    jint res = (*javaVM)->GetEnv(javaVM, (void**)&env, JNI_VERSION_1_6);
    if (res != JNI_OK) {
        res = (*javaVM)->AttachCurrentThread(javaVM, &env, NULL);
        if (JNI_OK != res) {
            LOGE("Failed to AttachCurrentThread, ErrorCode = %d", res);
            return NULL;
        }
    }

    jmethodID statusId = (*env)->GetMethodID(env, pctx->jniHelperClz,"updateStatus","(Ljava/lang/String;)V");
    sendJavaMsg(env, pctx->jniHelperObj, statusId,
                "TickerThread status: initializing...");

    // get mainActivity updateTimer function
    jmethodID timerId = (*env)->GetMethodID(env, pctx->mainActivityClz, "updateTimer", "()V");//回调JAVA 方法

    struct timeval beginTime, curTime, usedTime, leftTime;
    const struct timeval kOneSecond = {
            (__kernel_time_t)1,
            (__kernel_suseconds_t) 0
    };

    sendJavaMsg(env, pctx->jniHelperObj, statusId, "TickerThread status: start ticking ...");
    while(1) {
        gettimeofday(&beginTime, NULL);
        pthread_mutex_lock(&pctx->lock);//加锁
        int done = pctx->done;
        if (pctx->done) {
            pctx->done = 0;
        }
        pthread_mutex_unlock(&pctx->lock);//释放锁
        if (done) {//activity onPause的时候，退出循环
            break;
        }
        (*env)->CallVoidMethod(env, pctx->mainActivityObj, timerId);

        gettimeofday(&curTime, NULL);
        timersub(&curTime, &beginTime, &usedTime);
        timersub(&kOneSecond, &usedTime, &leftTime);
        struct timespec sleepTime;
        sleepTime.tv_sec = leftTime.tv_sec;
        sleepTime.tv_nsec = leftTime.tv_usec * 1000;

        if (sleepTime.tv_sec <= 1) {
            nanosleep(&sleepTime, NULL);
        } else {
            sendJavaMsg(env, pctx->jniHelperObj, statusId,
                        "TickerThread error: processing too long!");
        }
    }

    sendJavaMsg(env, pctx->jniHelperObj, statusId,
                "TickerThread status: ticking stopped");
    (*javaVM)->DetachCurrentThread(javaVM);
    return context;
}

/*
 * Interface to Java side to start ticks, caller is from onResume()
 */
JNIEXPORT void JNICALL
Java_com_example_hellojnicallback_MainActivity_startTicks(JNIEnv *env, jobject instance) {
    pthread_t       threadInfo_;
    pthread_attr_t  threadAttr_;//线程属性

    pthread_attr_init(&threadAttr_);//创建属性
    pthread_attr_setdetachstate(&threadAttr_, PTHREAD_CREATE_DETACHED);
    //g_ctx 结构体。互斥锁的初始化
    pthread_mutex_init(&g_ctx.lock, NULL);

    jclass clz = (*env)->GetObjectClass(env, instance);
    g_ctx.mainActivityClz = (*env)->NewGlobalRef(env, clz);
    g_ctx.mainActivityObj = (*env)->NewGlobalRef(env, instance);
    //线程创建
    int result  = pthread_create( &threadInfo_, &threadAttr_, UpdateTicks, &g_ctx);
    assert(result == 0);
    // 销毁属性
    pthread_attr_destroy(&threadAttr_);

    (void)result;
}

/*
 * Interface to Java side to stop ticks:
 *    we need to hold and make sure our native thread has finished before return
 *    for a clean shutdown. The caller is from onPause
 */
JNIEXPORT void JNICALL
Java_com_example_hellojnicallback_MainActivity_StopTicks(JNIEnv *env, jobject instance) {
    pthread_mutex_lock(&g_ctx.lock);
    g_ctx.done = 1;
    pthread_mutex_unlock(&g_ctx.lock);

    // waiting for ticking thread to flip the done flag
    struct timespec sleepTime;
    memset(&sleepTime, 0, sizeof(sleepTime));
    sleepTime.tv_nsec = 100000000;
    while (g_ctx.done) {
        nanosleep(&sleepTime, NULL);
    }

    // release object we allocated from StartTicks() function
    (*env)->DeleteGlobalRef(env, g_ctx.mainActivityClz);
    (*env)->DeleteGlobalRef(env, g_ctx.mainActivityObj);
    g_ctx.mainActivityObj = NULL;
    g_ctx.mainActivityClz = NULL;

    pthread_mutex_destroy(&g_ctx.lock);
}

 

参考文献：

void类型和void* 的用法 - 个渣渣 - 博客园
https://www.cnblogs.com/yuanyongbin/p/8058755.html

Linux中pthread线程使用详解 - JoysonQin的博客 - 优快云博客
https://blog.youkuaiyun.com/joysonqin/article/details/70237422

pthread_mutex_init等多线程函数的使用总结 - hubinbin595959的专栏 - 优快云博客
https://blog.youkuaiyun.com/hubinbin595959/article/details/79031443