重复运行runOnUiThread（）会导致异常

Question

（2016.3.15更新）

上周我遇到了一个奇怪的问题，我想和你讨论一下这个问题。

03-15 20:12:25.912/cn.example.app E/AndroidRuntime: FATAL EXCEPTION: Thread-10820  
   Process: cn.example.app, PID: 31532  
   java.lang.NullPointerException  
       at cn.example.app.homepage.GymFragment$searchThread.run(GymFragment.java:257)  
       at java.lang.Thread.run(Thread.java:841)  

但是，如果我使用sendMessage（SearchHandler.Object...）而不是runOnUiThread，一切都会很顺利！！

java：

@Override  
public void onActivityCreated(Bundle savedInstanceState){  
    super.onActivityCreated(savedInstanceState);
    new Thread(new SearchThread()).start();  
}  
class SearchThread implements Runnable{  
        @Override  
        public void run() {  
        String s = "";  
            try {  
                Thread.sleep(4000);  
            } catch (Exception e) {;}  
            //searchHandler.sendMessage(searchHandler.obtainMessage(0, s));//Correct   
            getActivity().runOnUiThread(new Runnable() { //Throw NullPointer Exception or pool-1-thread-1 (with ExecutorService) 
                @Override  
                public void run() {  
                    ;  
                }  
            });  
        }  
}  

SearchActivity.java：

class queryChangeListener implements SearchView.OnQueryTextListener{
    ...
    @Override
    public boolean onQueryTextChange(String newText) {
        currentSearchTip = newText;
        if (newText != null && newText.length() > 0) {
            searchDelayed(newText);
        }
        return true;
    }
}
private Handler searchHandler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        if (msg == null)
            return;
        String searchText = (String) msg.obj;
        if (currentSearchTip != null && currentSearchTip.isEmpty()==false) {
            GymFragment fragment = GymFragment.newInstance(searchText);
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_layout, fragment).commit();
        }
    }
};
private ScheduledExecutorService scheduledExecutor = Executors.newScheduledThreadPool(10);
private String currentSearchTip;
public void searchDelayed(String newText) {
    scheduledExecutor.schedule(new SearchThread(newText),500, TimeUnit.MILLISECONDS);
}
class SearchThread implements Runnable {
    String newText;
    public SearchThread(String newText){
        this.newText = newText;
    }
    public void run() {
        if (newText != null && newText.equals(currentSearchTip)) {
            searchHandler.sendMessage(searchHandler.obtainMessage(0, newText));
        }
    }
}

2）在运行getActivity().runonuithread()之前，首先判断getActivity()==null。

2016.3.14:

“一些有趣的事情：）”

null

public final void runOnUiThread(Runnable action) {  
        if (Thread.currentThread() != mUiThread) {  
            mHandler.post(action);  
        } else {  
            action.run();  
        }  
}

让我们看看post（Runnable r）中有什么内容：

public final boolean post(Runnable r)  {    
       return  sendMessageDelayed(getPostMessage(r), 0);    
}   

并且在getPostMessage（Runnable r）中：

private final Message getPostMessage(Runnable r) {    
        Message m = Message.obtain();    
        m.callback = r;    
        return m;    
}   

然后在message.auttain（）中：

public final Message obtainMessage()  {    
      return Message.obtain(this);    
}

public static Message obtain(Message orig) {    
       Message m = obtain();    
       m.what = orig.what;    
       m.arg1 = orig.arg1;    
       m.arg2 = orig.arg2;    
       m.obj = orig.obj;    
       m.replyTo = orig.replyTo;    
       if (orig.data != null) {    
           m.data = new Bundle(orig.data);    
       }    
       m.target = orig.target;    
       m.callback = orig.callback;    

       return m;    
}  

/**   
     * Return a new Message instance from the global pool. Allows us to   
     * avoid allocating new objects in many cases.   
     */    
public static Message obtain() {    
      synchronized (sPoolSync) {    
            if (sPool != null) {    
                Message m = sPool;    
                sPool = m.next;    
                m.next = null;    
                sPoolSize--;    
                return m;    
            }    
      }    
      return new Message();  
 }   

2、“sendmessage（消息msg）”的源代码：

public final boolean sendMessage(Message msg)  {    
       return sendMessageDelayed(msg, 0);    
}

在这里我们可以看到，sendMessage（Message msg）的实现几乎等同于post（Runnable r）的实现。差异可能与getPostMessage（Runnable r）相关联。

那么，简单地说，当我同时重复执行SearchHandler.SendMessage（SearchHandler.ObtainMessage(0,str))时，是什么实际导致正确执行SearchHandler.SendMessage（SearchHandler.ObtainMessage(0,str))，而导致getActivity（）.RunonUithRead（new Runnable{...}）的异常呢？谢谢！

Answer 1

我说不清为什么你的片段会脱离活动。但是如果您希望运行该代码而不考虑应用程序的状态，您可以简单地使用一个处理程序而不是RunonuithRead。只需确保在UI/Main线程中创建处理程序，所有对post（）的调用都将导致Runnable在UI/Main线程上运行。