內部Handler類引起內存泄露

如果您在Activity中定義了一個內部Handler類，如下代碼：

public class MainActivity extends Activity {
 
    private  Handler mHandler = newHandler() {
        @Override
        public void handleMessage(Message msg) {
            //TODO handle message...
        }
 
    };
 
    @TargetApi(11)
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mHandler.sendMessageDelayed(Message.obtain(), 60000);
 
        //just finish this activity
        finish();
    }
}

然後運行Android Lint工具會有一個內存泄露警告:

This Handler class should be static or leaks might occur (com.example.ta.MainActivity.1)

Issue: Ensures that Handler classes do not hold on to a reference to an outer class
Id: HandlerLeak

In Android, Handler classes should be static or leaks might occur. Messages enqueued on the application thread’s MessageQueue also retain their target Handler. If the Handler is an inner class, its outer class will be retained as well. To avoid leaking the outer class, declare the Handler as a static nested class with a WeakReference to its outer class.

原因是：

1. 當Android應用啓動的時候，會先創建一個應用主線程的Looper對象，Looper實現了一個簡單的消息隊列，一個一個的處理裏面的Message對象。主線程Looper對象在整個應用生命週期中存在。

2. 當在主線程中初始化Handler時，該Handler和Looper的消息隊列關聯。發送到消息隊列的Message會引用發送該消息的Handler對象，這樣系統可以調用 Handler#handleMessage(Message) 來分發處理該消息。

3. 在Java中，非靜態(匿名)內部類會引用外部類對象。而靜態內部類不會引用外部類對象。

4. 如果外部類是Activity，則會引起Activity泄露 。

當Activity finish後，延時消息會繼續存在主線程消息隊列中1分鐘，然後處理消息。而該消息引用了Activity的Handler對象，然後這個Handler又引用了這個Activity。這些引用對象會保持到該消息被處理完，這樣就導致該Activity對象無法被回收，從而導致了上面說的 Activity泄露。

要修改該問題，只需要按照Lint提示的那樣，把Handler類定義爲靜態即可，然後通過WeakReference 來保持外部的Activity對象。

private Handler mHandler = new MyHandler(this);
private static class MyHandler extendsHandler{
    private final WeakReference<Activity> mActivity;
    public MyHandler(Activity activity) {
        mActivity = newWeakReference<Activity>(activity);
    }
    @Override
    public void handleMessage(Message msg) {
        System.out.println(msg);
        if(mActivity.get() == null) {
            return;
        }
    }
}