Context是如何泄漏的：Handlers和内部类（译）

本文翻译自：How to Leak a Context: Handlers & Inner Classes

在Android日常编程中，Handler在进行异步操作并处理返回结果时经常被使用。通常我会用这样的代码实现

    public class SampleActivity extends Activity {  
      
      private final Handler mLeakyHandler = new Handler() {  
        @Override  
        public void handleMessage(Message msg) {  
          // ...   
        }  
      }  
    }

其实不太明显的是，上面的代码可能导致严重的内存泄露，Android lint工具会给你如下警告：

In Android, Handler classes should be static or leaks might occur.

Issue: Ensures that Handler classes do not hold on to a reference to an outer class  
Id: HandlerLeak  
  
Since this Handler is declared as an inner class, it may prevent the outer class from being garbage collected.  
If the Handler is using a Looper or MessageQueue for a thread other than the main thread, then there is no issue.   
If the Handler is using the Looper or MessageQueue of the main thread, you need to fix your Handler declaration,   
as follows: Declare the Handler as a static class; In the outer class, instantiate a WeakReference to the outer   
class and pass this object to your Handler when you instantiate the Handler; Make all references to members   
of the outer class using the WeakReference object.

但是究竟是哪里泄漏了，它又是如何发生的呢？首先让我们通过已知的文档来探个究竟：

When an Android application first starts, the framework creates a Looper object for the application’s main thread. A Looper implements a simple message queue, processing Message objects in a loop one after another. All major application framework events (such as Activity lifecycle method calls, button clicks, etc.) are contained inside Message objects, which are added to the Looper’s message queue and are processed one-by-one. The main thread’s Looper exists throughout the application’s lifecycle.

When a Handler is instantiated on the main thread, it is associated with the Looper’s message queue. Messages posted to the message queue will hold a reference to the Handler so that the framework can call Handler#handleMessage(Message) when the Looper eventually processes the message.

In Java, non-static inner and anonymous classes hold an implicit reference to their outer class. Static inner classes, on the other hand, do not.

1.当一个Android应用启动的时候，会自动创建一个供应用主线程使用的Looper实例。Looper的主要工作就是一个一个处理消息队列中的消息对象。在Android中，所有Android框架的事件（比如Activity的生命周期方法调用和按钮点击等）都是放入到消息中，然后加入到Looper要处理的消息队列中，由Looper负责一条一条地进行处理。主线程中的Looper生命周期和当前应用一样长。

2.当一个Handler在主线程进行了初始化之后，我们发送一个target为这个Handler的消息到Looper处理的消息队列时，实际上已经发送的消息已经包含了一个Handler实例的引用，只有这样Looper在处理到这条消息时才可以调用Handler#handleMessage(Message)完成消息的正确处理。

3.在Java中，非静态的内部类和匿名内部类都会隐式地持有其外部类的引用。静态的内部类不会持有外部类的引用。

所以看出哪里内存泄漏了吗？它十分巧妙，让我们一起思考如下示例代码

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() { /* ... */ }
    }, 1000 * 60 * 10);
    
    // Go back to the previous Activity.
    finish();
  }
}

当我们执行了Activity的finish方法，被延迟的消息会在被处理之前存在于主线程消息队列中10分钟，而这个消息中又包含了Handler的引用，而Handler是一个匿名内部类的实例，其持有外面的SampleActivity的引用，所以这导致了SampleActivity无法回收，进行导致SampleActivity持有的很多资源都无法回收，这就是我们常说的内存泄露。

注意上面的new Runnable这里也是匿名内部类实现的，同样也会持有SampleActivity的引用，也会阻止SampleActivity被回收。

要解决这种问题，就不能用非静态内部类，继承Handler时，要么是放在单独的类文件中，要么就是使用静态内部类。因为静态的内部类不会持有外部类的引用，所以不会导致外部类实例的内存泄露。当你需要在静态内部类中调用外部的Activity时，我们可以使用弱引用来处理。另外关于同样也需要将Runnable设置为静态的成员属性。注意：一个静态的匿名内部类实例不会持有外部类的引用。修改后不会导致内存泄露的代码如下

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();
  }
}

其实在Android中很多的内存泄露都是由于在Activity中使用了非静态内部类导致的，就像本文提到的一样，所以当我们使用时要非静态内部类时要格外注意，如果其实例的持有对象的生命周期大于其外部类对象，那么就有可能导致内存泄露。个人倾向于使用文章的静态类和弱引用的方法解决这种问题。