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);