Handler class should be static or leaks might occur

整理自：

http://stackoverflow.com/questions/11407943/this-handler-class-should-be-static-or-leaks-might-occur-incominghandler

问题：

下面的代码会有这样的告警：

This Handler class should be static or leaks might occur: IncomingHandler

代码如下：

public class UDPListenerService extends Service
{
    private static final String TAG = "UDPListenerService";
    //private ThreadGroup myThreads = new ThreadGroup("UDPListenerServiceWorker");
    private UDPListenerThread myThread;
    /**
     * Handler to communicate from WorkerThread to service.
     */
    private Handler mServiceHandler;
    // Used to receive messages from the Activity
    final Messenger inMessenger = new Messenger(new IncomingHandler());
    // Use to send message to the Activity
    private Messenger outMessenger;
    class IncomingHandler extends Handler
    {
        @Override
        public void handleMessage(Message msg)
        {
        }
    }
    /**
     * Target we publish for clients to send messages to Incoming Handler.
     */
    final Messenger mMessenger = new Messenger(new IncomingHandler());
    [ ... ]
}

原因分析：

If IncomingHandler class is not static, it will have a reference to your Service object.（因为非静态内部类的实例会拥有生成它的父类实例的引用）。

Handler objects for the same thread all share a common Looper object, which they post messages to and read from.

As messages contain target Handler, as long as there are messages with target handler in the message queue, the handler cannot be garbage collected. If handler is not static, your Service or Activity cannot be garbage collected, even after being destroyed.

This may lead to memory leaks, for some time at least - as long as the messages stay in the queue. This is not much of an issue unless you post long delayed messages.

You can make IncomingHandler static and have a WeakReference to your service:

static class IncomingHandler extends Handler {
    private final WeakReference<UDPListenerService> mService;
    IncomingHandler(UDPListenerService service) {
        mService = new WeakReference<UDPListenerService>(service);
    }
    @Override
    public void handleMessage(Message msg)
    {
         UDPListenerService service = mService.get();
         if (service != null) {
              service.handleMessage(msg);
         }
    }
}