[問題] Handler & Thread 的問題

[givemepass (〆)]

Android內一執行Activity的時候,
就會開一支main thread在後面運行,

我想要測試Handler的兩種方法
sendMessage 以及 post的差異

第一種

private Handler mHandler = new Handler(){
    public void handleMessage(Message msg){
        switch(msg.what){
           case 0:
               Log.e("HandlerThreadID",
                   Long.toString(Thread.currentThread().getId()));

               Log.e("HandlerThreadName--->",Thread.currentThread().getName());
           break;
        }
    }
};

private Thread mThread = new Thread(new Runnable() {
    public void run() {

        Log.e("ThreadID",
                Long.toString(Thread.currentThread().getId()));
        Log.e("ThreadName", Thread.currentThread().getName());
        Message msg = new Message();
        msg.what = 0;
        handler.sendMessage(msg);
    }

});


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mThread.start();
    Log.e("ActivityThreadID",
    Long.toString(Thread.currentThread().getId()));

    Log.e("ActivityThreadName",Thread.currentThread().getName());
}

測出來的結果是

ThreadID: 78
ThreadName: Thread-78
ActivityThreadID: 1
ActivityThreadName: main
HandlerThreadID: 1
HandlerThreadName: main


由此可知 new 一個Thread以後, 就會開啟另外一支thread來執行,
但是透過handler傳訊息以後, 就會跑回main thread。

第二種寫法

private class MyHandler extends Handler{
    public MyHandler(){

       Log.e("HandlerThreadID",
          Long.toString(Thread.currentThread().getId()));

       Log.e("HandlerThreadName",Thread.currentThread().getName());
    }
}
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MyHandler mHandler2 = new MyHandler();
    mHandler2.post(new Runnable(){

       @Override
       public void run() {
          // TODO Auto-generated method stub

          Log.e("RunnableThreadID",
                Long.toString(Thread.currentThread().getId()));

          Log.e("RunnableThreadName",Thread.currentThread().getName());
       }

   });
   //mThread.start();

   Log.e("ActivityThreadID",
       Long.toString(Thread.currentThread().getId()));

   Log.e("ActivityThreadName",Thread.currentThread().getName());
}

結果是這樣

HandlerThreadID: 1
HandlerThreadName: main
ActivityThreadID: 1
ActivityThreadName: main
RunnableThreadID: 1
RunnableThreadName: main

並沒有產生新的thread

我的原本解讀是Runnable並不會產生新的thread,
而是會去找尋原本的main thread幫忙執行run()這個方法
這樣是否表示執行大量運算就會被卡住?


然後為了證實前面的想法是對的,
因此我又做了第三個範例
private class MyThread extends Thread{
    public MyThread(Runnable r){
        super(r);
        Log.e("ThreadID",Long.toString(Thread.currentThread().getId()));
        Log.e("ThreadName",Thread.currentThread().getName());
    }
}

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MyHandler mHandler2 = new MyHandler();
    mHandler2.post(new MyThread(new Runnable(){

       @Override
       public void run() {
          // TODO Auto-generated method stub

          Log.e("RunnableThreadID",
                Long.toString(Thread.currentThread().getId()));

          Log.e("RunnableThreadName",Thread.currentThread().getName());
       }

   }));
   //mThread.start();

   Log.e("ActivityThreadID",
       Long.toString(Thread.currentThread().getId()));

   Log.e("ActivityThreadName",Thread.currentThread().getName());
}

結果竟然

HandlerThreadID: 1
HandlerThreadName: main
ThreadID: 1
ThreadName: main
ActivityThreadID: 1
ActivityThreadName: main
RunnableThreadID: 1
RunnableThreadName: main

因此我得到結論

post的方法並不會產生新的thread
而sendMessage的方式會產生新的thread


上面的結論是錯的
應該說兩個方法都不會自動產生新的thread
而sendMessage所產生的thread
是由我們自己所啟動的




城市馬
http://uploadingit.com/file/fpctqb7fdbvjnwao/HandlerThreadDemo.zip

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◆ From: 114.46.162.97
→ elba:透過Handler送到佇列的工作是在建立Handler的執行緒做處理
→ elba:sendMessage跟post的差異已經在程式中展示了
→ elba:sendMessag收Message物件,要透過handleMessage處理
→ qweqweqweqwe:其實有點不懂為什麼得到第三個結論會很驚訝 @@?
→ elba:post收Runnable物件
→ givemepass:第三個我開了一支thread可是卻還是main thread所以我
→ givemepass:覺得驚訝
→ elba:Thread.start才會啟動新的執行緒
真的欸 我居然忘記要start才會產生thread

我後來去爬Handler的source code發現post最終傳進去的Runnable

也只是被指派為Message的Runnable物件而已

但是sendMessage是由我們去啟動一支thread

所以會產生另外一支thread

感謝解惑 不過這樣一來代表我的結論是錯的XD
※ 編輯: givemepass      來自: 114.46.162.97        (09/24 23:45)
→ qweqweqweqwe:XD 所以才說為什麼第三個結論很驚訝
推 tac0wu:sendMessage為什麼會產生一隻Thread
→ tac0wu:整篇唯一會產生thread只有mThread.start() 這個吧