This Handler class should be static or leaks might occur (null)

错误的写法

我们通常在Activity中使用Handler一般都会采用内部类的形式，如下代码：

` public class TestActivity extends AppCompatActivity {

  TextView textView ;
  private Handler mHanlder = new Handler(){
    @Override public void handleMessage(Message msg) {
     super.handleMessage(msg);
     switch (msg.what){
        case 1:
         textView.setText("haha");
         break;
     }
    }
  };

  @Override protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textView = (TextView) findViewById(R.id.tv);
    Message msg = Message.obtain();
    msg.what = 1;
    mHanlder.sendMessage(msg);
    }
}

`

此时编译器会给出一大坨黄色警告：

This Handler class should be static or leaks might occur (null)

意在告诉我们这个Handler应该写成static类型，否则会导致内存溢出。

出现错误的场景

由于静态类只能静态的方法或者字段，导致很多怕麻烦的童鞋忽视这种警告，一般情况下看不出有什么bug，但是如果App上线之后，你会收到各种各样的闪退惊喜，出现的比较多的情形就是：Handler的周期大于了当前Activity的生命周期，如下：

 TextView textView ;
private Handler mHanlder = new Handler(){
@Override public void handleMessage(Message msg) {
  super.handleMessage(msg);
  switch (msg.what){
    case 1:
      textView.setText("haha");
      break;
  }
 }
};

 @Override protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textView = (TextView) findViewById(R.id.tv);
    Message msg = Message.obtain();
    msg.what = 1;
    mHanlder.sendMessageDelayed(msg, 3000);
    finish();

  }

错误日志：
 java.lang.NullPointerException
        at com.leaktest.myleaktest.TestActivity$1.handleMessage(TestActivity.java:19)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:146)
        at android.app.ActivityThread.main(ActivityThread.java:5752)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
        at dalvik.system.NativeStart.main(Native Method)

空指针，这简直比内存溢出还要可怕，因为程序直接崩了或者闪退了。

原因分析

• 代码中创建了一个内部类Handler
• 内部类会引用当前所在的类（比如Activity）
• 使用Handler向主线程Looper发送消息Message，这个Message会被保存起来直至执行，而且这个Message保存了对Handler的引用，所以当Activity销毁的时候，Message继续引用了Handler，Handler继续引用了Context，Context得不到释放，内存溢出了。

解决方案

• 使用静态内部类和弱引用（这个是从一个老外的网站上看到的，就是将弱引用包裹一下当前Activity给Handler引用，当然此时hanler只能调用Activity中的公共变量和方法了）

  public class SampleActivity extends Activity {
  
      /**
       * Instances of static inner classes do not hold an implicit
      * reference to their outer class.
      */
       private static class MyHandler extends Handler {
      private final WeakReference<SampleActivity> mActivity;
  
      public MyHandler(SampleActivity activity) {
        mActivity = new WeakReference<SampleActivity>(activity);
      }
  
      @Override
      public void handleMessage(Message msg) {
        SampleActivity activity = mActivity.get();
        if (activity != null) {
          // ...
        }
      }
      }
  
        private final MyHandler mHandler = new MyHandler(this);
  
        /**
         * Instances of anonymous classes do not hold an implicit
         * reference to their outer class when they are "static".
         */
        private static final Runnable sRunnable = new Runnable() {
            @Override
            public void run() { /* ... */ }
        };
  
        @Override
        protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
  
      // Post a message and delay its execution for 10 minutes.
      mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
  
      // Go back to the previous Activity.
      finish();
    }
  }
  

• 在onDestry中调用

      //null表示取消所有的callbak和Message，但是此种方式依然肯能会内存溢出，比如app直接crash掉，没有调用onDestroy
      mHandler.removeCallbacksAndMessages(null);