android handler的警告Handler Class Should be Static or Leaks Occur

泡在网上的日子 / 文 发表于2014-11-06 21:19 次阅读 handler

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.


他的建议写法是:

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


参考:

[1]https://groups.google.com/forum/?fromgroups=#!msg/android-developers/1aPZXZG6kWk/lIYDavGYn5UJ
[2]http://www.androiddesignpatterns.com/2013/01/inner-class-handler-memory-leak.html
[3]http://stackoverflow.com/questions/11407943/this-handler-class-should-be-static-or-leaks-might-occur-incominghandler



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