NDK開發（九） ：Hello jniCallback

轉載請以連接形式標明出處： 本文出自:103style的博客

本文操做以 Android Studio 3.5 版本爲例

本文爲參考官方示例 hello-jniCallback 動手寫的 Demo.

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NDK開發文章彙總

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功能介紹

• 經過 JNI 獲取並調用靜態內部類 JniHandler 的 靜態方法getBuildVersion() 和 公開方法getRuntimeMemorySize()獲取 當前的系統版本 以及 當前可用內存。
• 經過 JNI 線程 實現 開始和中止每秒打印距離開始計時的秒數。

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編寫測試代碼

• 編寫測試代碼 JniCallbackDemo.

  public class JniCallbackDemo {
      private static final String TAG = "JniCallbackDemo";
      static {
          System.loadLibrary("jni_callback");
      }
      private int timeCount;
      public native void startTiming();
      public native void stopTiming();
  
      private void printTime() {
          Log.e(TAG, "timeCount = " + timeCount);
          timeCount++;
      }
      public static class JniHandler {
          public static String getBuildVersion() {
              return Build.VERSION.RELEASE;
          }
          public long getRuntimeMemorySize() {
              return Runtime.getRuntime().freeMemory();
          }
          private void updateStatus(String msg) {
              if (msg.toLowerCase().contains("error")) {
                  Log.e("JniHandler", "Native Err: " + msg);
              } else {
                  Log.i("JniHandler", "Native Msg: " + msg);
              }
          }
      }
  }
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• 建立 jni_callback.cpp.

• 添加如下代碼到CMakeLists.txt.

  add_library(
          jni_callback
          SHARED
          jni_callback.cpp)
  target_link_libraries(
          jni_callback
          ${log-lib})
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經過JNI實現功能邏輯

• 獲取當前的系統版本和可用內存。

  • 由於這個只須要執行一次就行了，咱們能夠放到JNI_OnLoad方法中去實現(在應用層調用.so庫首先會執行 JNI_OnLoad 方法)。
  • 獲取內部類用 $ 而不是 .，例如： env->FindClass("com/lxk/ndkdemo/JniCallbackDemo$JniHandler");.
  • 這裏構建了一個結構體jniCallback保存獲取的 類 和 實例。

  void queryRuntimeInfo(JNIEnv *env) {
      //獲取getBuildVersion的方法id
      jmethodID staticMethodId = env->GetStaticMethodID(jniCallback.jniHandlerClz, "getBuildVersion",
                                                        "()Ljava/lang/String;");
      if (!staticMethodId) {
          LOGE("Failed to retrieve getBuildVersion() methodID");
          return;
      }
      //執行靜態方法獲取getBuildVersion的方法id
      jstring releaseVersion = static_cast<jstring>(env->CallStaticObjectMethod(
              jniCallback.jniHandlerClz, staticMethodId));
      //獲取字符串的地址
      const char *version = env->GetStringUTFChars(releaseVersion, nullptr);
      if (!version) {
          LOGE("Unable to get version string");
          return;
      }
      LOGD("releaseVersion = %s", version);
      //釋放字符串的內存
      env->ReleaseStringUTFChars(releaseVersion, version);
      //刪除引用
      env->DeleteLocalRef(releaseVersion);
  
      //獲取非靜態方法 getRuntimeMemorySize 的id
      jmethodID methodId = env->GetMethodID(jniCallback.jniHandlerClz, "getRuntimeMemorySize", "()J");
      if (!methodId) {
          LOGE("Failed to retrieve getRuntimeMemorySize() methodID");
          return;
      }
      //調用 getRuntimeMemorySize
      jlong freeMemorySize = env->CallLongMethod(jniCallback.jniHandlerObj, methodId);
  
      //打印可用內存
      LOGD("Runtime free memory size: %"
                   PRId64, freeMemorySize);
  }
  
  JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
      LOGD("JNI_OnLoad");
      JNIEnv *env;
      //給jniCallback初始化地址
      memset(&jniCallback, 0, sizeof(jniCallback));
  
      jniCallback.javaVM = vm;
  
      if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_6) != JNI_OK) {
          return JNI_ERR;
      }
      //獲取JniCallbackDemo.JniHandler 類
      jclass clz = env->FindClass("com/lxk/ndkdemo/JniCallbackDemo$JniHandler");
      //賦值到結構體
      jniCallback.jniHandlerClz = static_cast<jclass>(env->NewGlobalRef(clz));
      if (!clz) {
          LOGE("FindClass JniCallbackDemo$JniHandler error");
          return JNI_ERR;
      }
      //獲取構造函數
      jmethodID initMethodId = env->GetMethodID(jniCallback.jniHandlerClz, "<init>", "()V");
      //構建類
      jobject instance = env->NewObject(jniCallback.jniHandlerClz, initMethodId);
      if (!instance) {
          LOGE("NewObject jniHandler error")
          return JNI_ERR;
      }
      //賦值到結構體
      jniCallback.jniHandlerObj = env->NewGlobalRef(instance);
  
      //調用 JniHandler 的相關方法
      queryRuntimeInfo(env);
  
      jniCallback.done = 0;
      jniCallback.jniCallbackDemoObj = nullptr;
      return JNI_VERSION_1_6;
  }
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• 建立線程執實現開始計時的邏輯：

  • 經過pthread_create建立線程的時候，第一個參數：線程id的指針；第二個參數：線程屬性的指針；第一個參數：在線程中運行的函數；第四個參數：運行函數的參數

  extern "C"
  JNIEXPORT void JNICALL
  Java_com_lxk_ndkdemo_JniCallbackDemo_startTiming(JNIEnv *env, jobject instance) {
      LOGD("jni startTiming");
  
      //線程ID
      pthread_t threadInfo;
      //線程屬性
      pthread_attr_t threadAttr;
      //初始化線程屬性
      pthread_attr_init(&threadAttr);
      //設置脫離狀態的屬性
      pthread_attr_setdetachstate(&threadAttr, PTHREAD_CREATE_DETACHED);
  
      //互斥鎖
      pthread_mutex_t lock;
      //初始化互斥鎖
      pthread_mutex_init(&lock, nullptr);
  
      //獲取當前類
      jclass clz = env->GetObjectClass(instance);
      //保存類和 實例 到 結構體中
      jniCallback.jniCallbackDemoClz = static_cast<jclass>(env->NewGlobalRef(clz));
      jniCallback.jniCallbackDemoObj = env->NewGlobalRef(instance);
  
      // StartTiming ：在線程中運行的函數  jniCallback 運行函數的參數
      int result = pthread_create(&threadInfo, &threadAttr, StartTiming, &jniCallback);
      assert(result == 0);
      //刪除線程屬性
      pthread_attr_destroy(&threadAttr);
  }
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• 線程的執行函數：

  /**
   * 調用 類 instance 的 void方法 methodId
   */
  void sendJavaMsg(JNIEnv *env, jobject instance, jmethodID methodId, const char *msg) {
      LOGD("jni sendJavaMsg");
      //獲取字符串
      jstring javaMsg = env->NewStringUTF(msg);
      //調用對應方法
      env->CallVoidMethod(instance, methodId, javaMsg);
      //刪除本地引用
      env->DeleteLocalRef(javaMsg);
  }
  
  void *StartTiming(void *context) {
      //獲取參數
      JniCallback *JniCallback = static_cast<jni_callback *>(context);
      JavaVM *javaVm = JniCallback->javaVM;
      JNIEnv *env;
  
      jint res = javaVm->GetEnv((void **) (&env), JNI_VERSION_1_6);
      LOGD("javaVm->GetEnv() res = %d", res);
      if (res != JNI_OK) {
          //連接到虛擬機
          res = javaVm->AttachCurrentThread(&env, nullptr);
          if (JNI_OK != res) {
              LOGE("Failed to AttachCurrentThread, ErrorCode = %d", res);
              return nullptr;
          }
      } else {
          LOGE("javaVm GetEnv JNI_OK");
      }
  
      //獲取 JniHandler 的 updateStatus 函數
      jmethodID statusId = env->GetMethodID(JniCallback->jniHandlerClz, "updateStatus",
                                            "(Ljava/lang/String;)V");
  
      sendJavaMsg(env, JniCallback->jniHandlerObj, statusId, "TimeThread status: initializing...");
      //獲取 JniCallbackDemo 的 printTime 函數
      jmethodID timerId = env->GetMethodID(JniCallback->jniCallbackDemoClz, "printTime", "()V");
      //聲明時間變量
      struct timeval beginTime, curTime, usedTime, leftTime;
      const struct timeval kOneSecond = {
              (__kernel_time_t) 1,
              (__kernel_suseconds_t) 0
      };
  
      sendJavaMsg(env, JniCallback->jniHandlerObj, statusId,
                  "TimeThread status: prepare startTiming ...");
  
      while (true) {
          //獲取當前的時間 賦值給 beginTime
          gettimeofday(&beginTime, nullptr);
          //佔有互斥鎖
          pthread_mutex_lock(&JniCallback->lock);
          //獲取當前的狀態
          int done = JniCallback->done;
          if (JniCallback->done) {
              JniCallback->done = 0;
          }
          //釋放互斥鎖
          pthread_mutex_unlock(&JniCallback->lock);
  
          if (done) {
              LOGD("JniCallback done");
              break;
          }
  
          //調用 printTime 函數
          env->CallVoidMethod(JniCallback->jniCallbackDemoObj, timerId);
  
          //獲取當前的時間 賦值給 curTime
          gettimeofday(&curTime, nullptr);
  
          //計算函數運行消耗的時間
          //usedTime = curTime - beginTime
          timersub(&curTime, &beginTime, &usedTime);
  
          //計算須要等待的時間
          //leftTime = kOneSecond - usedTime
          timersub(&kOneSecond, &usedTime, &leftTime);
  
          //構建等待的時間
          struct timespec sleepTime;
          sleepTime.tv_sec = leftTime.tv_sec;
          sleepTime.tv_nsec = leftTime.tv_usec * 1000;
  
          if (sleepTime.tv_sec <= 1) {
              //睡眠對應納秒的時間
              nanosleep(&sleepTime, nullptr);
          } else {
              sendJavaMsg(env, JniCallback->jniHandlerObj, statusId,
                          "TimeThread error: processing too long!");
          }
      }
      sendJavaMsg(env, JniCallback->jniHandlerObj, statusId,
                  "TimeThread status: ticking stopped");
      //釋放線程
      javaVm->DetachCurrentThread();
      return context;
  }
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• 中止計時的邏輯：

  extern "C"
  JNIEXPORT void JNICALL
  Java_com_lxk_ndkdemo_JniCallbackDemo_stopTiming(JNIEnv *env, jobject instance) {
      LOGD("jni stopTiming");
  
      //佔用互斥鎖
      pthread_mutex_lock(&jniCallback.lock);
      //修改線程運行的條件
      jniCallback.done = 1;
      //釋放互斥鎖
      pthread_mutex_unlock(&jniCallback.lock);
  
      //初始化等待時間
      struct timespec sleepTime;
      memset(&sleepTime, 0, sizeof(sleepTime));
      sleepTime.tv_nsec = 100000000;
  
      while (jniCallback.done) {
          nanosleep(&sleepTime, nullptr);
      }
  
      //刪除引用
      env->DeleteGlobalRef(jniCallback.jniCallbackDemoClz);
      env->DeleteGlobalRef(jniCallback.jniCallbackDemoObj);
      jniCallback.jniCallbackDemoObj = nullptr;
      jniCallback.jniCallbackDemoClz = nullptr;
  
      //刪除互斥鎖
      pthread_mutex_destroy(&jniCallback.lock);
  }
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執行測試代碼

private void testJniCallback() {
    jniCallbackRun(true);
}

private void jniCallbackRun(boolean run) {
    if (jniCallbackDemo == null) {
        jniCallbackDemo = new JniCallbackDemo();
    }
    if (run) {
        Toast.makeText(this, "開始計時，請查看控制檯日誌輸出", Toast.LENGTH_SHORT).show();
        jniCallbackDemo.startTiming();
    } else {
        jniCallbackDemo.stopTiming();
    }
}
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點擊按鈕jni callback test，在控制檯即會輸出如下信息。

08-22 17:00:34.726 18711-18711/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][63]: JNI_OnLoad
08-22 17:00:34.726 18711-18711/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][42]: releaseVersion = 6.0.1
08-22 17:00:34.726 18711-18711/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][59]: Runtime free memory size: 2439224
08-22 17:00:34.726 18711-18711/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][207]: jni startTiming
08-22 17:00:34.726 18711-18758/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][120]: javaVm->GetEnv() res = -2
08-22 17:00:34.726 18711-18758/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][104]: jni sendJavaMsg
08-22 17:00:34.736 18711-18758/com.lxk.ndkdemo I/JniHandler: Native Msg: TimeThread status: initializing...
08-22 17:00:34.736 18711-18758/com.lxk.ndkdemo D/JNI: [jni_callback.cpp][104]: jni sendJavaMsg
08-22 17:00:34.736 18711-18758/com.lxk.ndkdemo I/JniHandler: Native Msg: TimeThread status: prepare startTiming ...
08-22 17:00:34.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 0
08-22 17:00:35.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 1
08-22 17:00:36.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 2
08-22 17:00:37.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 3
08-22 17:00:38.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 4
08-22 17:00:39.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 5
08-22 17:00:40.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 6
08-22 17:00:41.736 18711-18758/com.lxk.ndkdemo E/JniCallbackDemo: timeCount = 7
...
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相關文檔

• jni 線程介紹
• 官方hello-jniCallback

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源碼

• jni_callback.cpp 完整代碼
• Demo地址：https://github.com/103style/NDKDoc/tree/master/NDKDemo

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