Handler leaks

在使用Handler更新UI的时候，我是这样写的：

public class SampleActivity extends Activity {
                                               
  private final Handler mLeakyHandler = new Handler() {

    @Override

    public void handleMessage(Message msg) {

      // TODO

    }

  }

}

看起来很正常的，但是 Android Lint 却给出了警告：

This Handler class should be static or leaks might occur

意思是说：这个Handler 必须是static的，否则就会引发内存泄露。

其实，对于这个问题，Android Framework 的工程师 Romain Guy 早已经在Google论坛上做出过解释，并且给出了他的建议写法：

I wrote that debugging code because of a couple of memory leaks I 
found in the Android codebase. Like you said, a Message has a 
reference to the Handler which, when it's inner and non-static, has a 
reference to the outer this (an Activity for instance.) If the Message 
lives in the queue for a long time, which happens fairly easily when 
posting a delayed message for instance, you keep a reference to the 
Activity and "leak" all the views and resources. It gets even worse 
when you obtain a Message and don't post it right away but keep it 
somewhere (for instance in a static structure) for later use. 

class OuterClass {

                                  

  class InnerClass {

    private final WeakReference<OuterClass> mTarget;

                                  

    InnerClass(OuterClass target) {

           mTarget = new WeakReference<OuterClass>(target);

    }

                                  

    void doSomething() {

           OuterClass target = mTarget.get();

           if (target != null) {

                target.do();   

           }

     }

}

下面，我们进一步解释一下：

1.Android App启动的时候，Android Framework 为主线程创建一个Looper对象，这个Looper对象将贯穿这个App的整个生命周期，它实现了一个消息队列（Message  Queue），并且开启一个循环来处理Message对象。而Framework的主要事件都包含着内部Message对象，当这些事件被触发的时候，Message对象会被加到消息队列中执行。
2.当一个Handler被实例化时（如上面那样），它将和主线程Looper对象的消息队列相关联，被推到消息队列中的Message对象将持有一个Handler的引用以便于当Looper处理到这个Message的时候，Framework执行Handler的handleMessage(Message)方法。
3.在 Java 语言中，非静态匿名内部类将持有一个对外部类的隐式引用，而静态内部类则不会。

到底内存泄露是在哪里发生的呢？以下面代码为例：

public class SampleActivity extends Activity {

                  

  private final Handler mLeakyHandler = new Handler() {

    @Override

    public void handleMessage(Message msg) {

      // ...

    }

  }

                  

  @Override

  protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

                  

    // Post a message and delay its execution for 10 minutes.

    mLeakyHandler.postDelayed(new Runnable() {

      @Override

      public void run() { }

    }, 60 * 10 * 1000);

                  

    // Go back to the previous Activity.

    finish();

  }

}

当Activity被finish()掉，Message 将存在于消息队列中长达10分钟的时间才会被执行到。这个Message持有一个对Handler的引用，Handler也会持有一个对于外部类（SampleActivity）的隐式引用，这些引用在Message被执行前将一直保持，这样会保证Activity的上下文不被垃圾回收机制回收，同时也会泄露应用程序的资源（views and resources）。

为解决这个问题，下面这段代码中的Handler则是一个静态匿名内部类。静态匿名内部类不会持有一个对外部类的隐式引用，因此Activity将不会被泄露。如果你需要在Handler中调用外部Activity的方法，就让Handler持有一个对Activity的WeakReference，这样就不会泄露Activity的上下文了，如下所示：

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, 60 * 10 * 1000);

             

    // Go back to the previous Activity.

    finish();

  }

}

 

总结：

在实际开发中，如果内部类的生命周期和Activity的生命周期不一致（比如上面那种，Activity finish()之后要等10分钟，内部类的实例才会执行），则在Activity中要避免使用非静态的内部类，这种情况，就使用一个静态内部类，同时持有一个对Activity的WeakReference。