为什么一个后台线程和主服务线程有相同的线程id？答案

【问题标题】：Why does one background thread have the same thread id as the main service thread?为什么一个后台线程和主服务线程有相同的线程id？
【发布时间】：2014-09-08 17:59:33
【问题描述】：

尝试创建 2 个后台线程并将消息从线程 0 发送到线程 1 由 MessageHandler 使用 android Looper 和与 thread1 关联的消息队列进行处理。

我希望看到线程 0,1 和主线程的单独线程 ID。然而，我惊讶地看到了一些意想不到的东西。

09-08 14:23:21.089: D/MQ(4514): Hi, the system is up! Today is: Sep 8, 2014 2:23:21 PM
09-08 14:23:21.099: V/MQ(4514): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 14:23:21.099: V/MQ(4514): BackgroundThread0. Thread id = 280 sent message 0
09-08 14:23:21.099: V/MQ(4514): MessageHandler on thread Thread id = 1 received message 0

为什么BackgroundThread1的线程id和主服务线程id=1一样？

我使用运行方式从 Eclipse 安装它，并在 Windows 控制台窗口中启动它：

D:\>adb shell am startservice --user 0 -a android.intent.action.MAIN -n "com.sandbox.mq/.MainService"
Starting service: Intent { act=android.intent.action.MAIN cmp=com.sandbox.mq/.MainService }

-- 包 com.sandbox.mq;

public class StartMainService extends Application {
    final static String TAG = "MQ";

    public void onCreate() {
        super.onCreate();
        Context context = getApplicationContext();
        Log.d(TAG, "Hi, the system is up! Today is: " + DateFormat.getDateTimeInstance().format(new Date()));
    }
}

public class MainService extends Service {
    final static String TAG = "MQ";
    BackgroundThread0 bthread0;
    BackgroundThread1 bthread1;

    public class MqBinder extends Binder {
        public MqBinder(Context ctxt) {
            Log.v(TAG, "MqBinder() " + "Thread id = "
                    + Thread.currentThread().getId());
        }
    }

    @Override
    public IBinder onBind(Intent arg0) {
        return new MqBinder(this);
    }

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.v(TAG, "onStartCommand(). I am INSIDE THE main sERVICE "
        + "Thread id = " + Thread.currentThread().getId());
bthread1 = new BackgroundThread1();
if (!bthread1.isAlive()) {
    bthread1.start();
} else {
    Log.v(TAG, "onStartCommand(). bthread1 was already started");
}
bthread0 = new BackgroundThread0();
if (!bthread0.isAlive()) {
    bthread0.start();
} else {
    Log.v(TAG, "onStartCommand(). bthread0 was already started");
}       
return START_STICKY;
}

    private class BackgroundThread0 extends Thread {
        Handler b1Handler;

        @Override
        public void run() {
            super.run();
            b1Handler = bthread1.b1Handler;
            Message msg = b1Handler.obtainMessage(MessageHandler.TYPE0);
            b1Handler.sendMessage(msg);
            Log.v(TAG, "BackgroundThread0. " + "Thread id = "
                    + Thread.currentThread().getId() + " sent message "
                    + msg.what);
        }

    }

    private class BackgroundThread1 extends Thread {
        public BackgroundThread1() {
            super();
            b1Handler = new MessageHandler();
        }

        Handler b1Handler;

        @Override
        public void run() {
            super.run();
            Looper.prepare();
            Looper.loop();
        }

    }

    private static class MessageHandler extends Handler {
        static final int TYPE0 = 0;
        static final int TYPE1 = 1;
        static final int TYPE2 = 2;

        public MessageHandler() {

        }

        @Override
        public void handleMessage(Message msg) {
            Log.v(TAG, "MessageHandler on thread " + "Thread id = "
                    + Thread.currentThread().getId() + " received message "
                    + msg.what);

            switch (msg.what) {
            case TYPE0:
                break;
            case TYPE1:
                break;
            case TYPE2:
                break;
            }
            super.handleMessage(msg);
        }
    }
}

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.sandbox.mq" >

    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />

    <uses-sdk
        android:minSdkVersion="19"
        android:targetSdkVersion="19" />

    <application
        android:name="com.sandbox.mq.StartMainService"
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:persistent="true"
        android:theme="@style/AppTheme" >
        <service android:name="com.sandbox.mq.MainService" android:exported="true"> 
             <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>          
        </service>        
    </application>

</manifest>

来自 Eclipse，DDMS 窗口

1   4514    Native  0   2   main    
*2  4518    VmWait  0   0   GC  
*3  4519    VmWait  0   0   Signal Catcher  
*4  4520    Runnable    0   2   JDWP    
*5  4521    VmWait  0   0   Compiler    
*6  4522    Wait    0   0   ReferenceQueueDaemon    
*7  4523    Wait    0   0   FinalizerDaemon 
*8  4524    Wait    0   0   FinalizerWatchdogDaemon 
9   4525    Native  0   0   Binder_1    
10  4526    Native  0   0   Binder_2    
11  4527    Native  0   0   Thread-279

按照下面的 TacBoss 建议后，我得到了预期的线程 id=450。线程 id 451 可能已完成。

1   11756   Native  1   1   main    
*2  11760   VmWait  0   0   GC  
*3  11761   VmWait  0   0   Signal Catcher  
*4  11762   Runnable    0   0   JDWP    
*5  11763   VmWait  0   0   Compiler    
*6  11764   Wait    0   0   ReferenceQueueDaemon    
*7  11765   Wait    0   0   FinalizerDaemon 
*8  11766   Wait    0   0   FinalizerWatchdogDaemon 
9   11767   Native  0   0   Binder_1    
10  11768   Native  0   0   Binder_2    
11  11770   Native  0   0   Thread-450  


09-08 15:06:05.089: D/MQ(7931): Hi, the system is up! Today is: Sep 8, 2014 3:06:05 PM
09-08 15:06:05.089: V/MQ(7931): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:06.149: D/MQ(7946): Hi, the system is up! Today is: Sep 8, 2014 3:06:06 PM
09-08 15:06:06.149: V/MQ(7946): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:10.269: D/MQ(7964): Hi, the system is up! Today is: Sep 8, 2014 3:06:10 PM
09-08 15:06:10.269: V/MQ(7964): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:26.339: D/MQ(8110): Hi, the system is up! Today is: Sep 8, 2014 3:06:26 PM
09-08 15:06:26.339: V/MQ(8110): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:36.319: D/MQ(8166): Hi, the system is up! Today is: Sep 8, 2014 3:06:36 PM
09-08 15:06:36.319: V/MQ(8166): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:37.389: D/MQ(8182): Hi, the system is up! Today is: Sep 8, 2014 3:06:37 PM
09-08 15:06:37.389: V/MQ(8182): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:06:41.469: D/MQ(8200): Hi, the system is up! Today is: Sep 8, 2014 3:06:41 PM
09-08 15:06:41.469: V/MQ(8200): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:08:27.439: D/MQ(8577): Hi, the system is up! Today is: Sep 8, 2014 3:08:27 PM
09-08 15:08:27.439: V/MQ(8577): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:08:28.479: D/MQ(8593): Hi, the system is up! Today is: Sep 8, 2014 3:08:28 PM
09-08 15:08:28.489: V/MQ(8593): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:08:32.579: D/MQ(8612): Hi, the system is up! Today is: Sep 8, 2014 3:08:32 PM
09-08 15:08:32.579: V/MQ(8612): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:08:48.679: D/MQ(8630): Hi, the system is up! Today is: Sep 8, 2014 3:08:48 PM
09-08 15:08:48.679: V/MQ(8630): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:29:00.949: D/MQ(11756): Hi, the system is up! Today is: Sep 8, 2014 3:29:00 PM
09-08 15:29:00.949: V/MQ(11756): onStartCommand(). I am INSIDE THE main sERVICE Thread id = 1
09-08 15:29:00.959: V/MQ(11756): BackgroundThread0. Thread id = 451 sent message 0
09-08 15:29:00.959: V/MQ(11756): MessageHandler on thread Thread id = 450 received message 0

--代码更改

    private class BackgroundThread1 extends Thread {
        public BackgroundThread1() {
            super();
            start();
        }

        Handler b1Handler;

        @Override
        public void run() {
            super.run();
            Looper.prepare();
            Looper looper = Looper.myLooper();
            b1Handler = new MessageHandler(looper);
            Looper.loop();
        }

    }

        public int onStartCommand(Intent intent, int flags, int startId) {
        Log.v(TAG, "onStartCommand(). I am INSIDE THE main sERVICE "
                + "Thread id = " + Thread.currentThread().getId());
        bthread1 = new BackgroundThread1();
//      if (!bthread1.isAlive()) {
//          bthread1.start();
//      } else {
//          Log.v(TAG, "onStartCommand(). bthread1 was already started");
//      }
        bthread0 = new BackgroundThread0();
        if (!bthread0.isAlive()) {
            bthread0.start();
        } else {
            Log.v(TAG, "onStartCommand(). bthread0 was already started");
        }       
        return START_STICKY;
    }

【问题讨论】：

尝试从调试视图粘贴线程列表
@TacB0sS 好主意！我在上面发布了新的输出。我也在实例化thread0，然后是1 - 一个错误。现在我正在创建线程 1，然后是 0。不过，原来的问题仍然存在
看起来你的线程没有相同的 ID 或名称
@TacB0sS 是真的。我运行控制台命令启动服务，然后单击 DDMS 窗口和更新线程按钮和包名称。

标签： java android multithreading android-service

【解决方案1】：

好的，这并没有直接弹出......

问题是你在 BGThread1 的构造函数中创建了处理程序，而这段代码是从主线程调用的！这就是为什么处理程序队列也附加到主线程的原因。

public BackgroundThread1() {
    super();
    b1Handler = new MessageHandler(); // this is the main thread here...
}

如果您希望此处理程序在另一个线程上运行，您首先必须执行以下操作：

private class BackgroundThread1 extends HandlerThread {
    public BackgroundThread1() {
        super();
         ...
          Some code
         ...
        start();
        b1Handler = new MessageHandler(getLooper()); 
            // this is the main thread here... but you create the handler with the looper of the new thread!
    }
    ...
}

这将在您期望的上下文中创建线程。

================================================ ====

扩展“常规”线程的示例：

class MyThread
        extends Thread {

    private Handler myHandler;

    public MyThread() {
        start();
    }

    @Override
    public void run() {
        Looper.prepare();
        Looper looper = Looper.myLooper();
        myHandler = new Handler(looper);
        Looper.loop();
    }
}

现在处理程序在新线程上运行。

【讨论】：

能否请您展示如何扩展 Thread 而不是 HandlerThread？
你写了BackgroundThread1 extends HandlerThread。我们如何使用BackgroundThread1 extends Thread 来做到这一点？
优秀 - 这就是解决方案。但是为什么现在调用了这么多次 onStartCommand() 呢？请参阅上面有问题的跟踪。
从看起来您的应用程序一遍又一遍地重新创建...您确定打印的内容与您发布的代码相符吗？
是的，我的代码匹配。我在 Windows 7、android 4.4、Nexus 5 上。你能在你的 android 设备上运行它，看看你是否得到相同的结果？

【解决方案2】：

这是因为 Service 生命周期在主线程上处理，即：Service 方法如 onCreate、onStartCommand、onDestruction 默认在主线程上调用，如在第二段android.app.Service's Class Overview

请注意，服务与其他应用程序对象一样，在其托管进程的主线程中运行。

【讨论】：

你读过这个问题吗？我说的是3个线程。 0,1,主
是的，我读过。你接受的答案和我一样，那就是：因为日志语句在Service.onCreate里面，打印的线程id是1是正常的。此外，1通常是主线程的id，所以说线程'0, 1, main' 令人困惑。
日志语句在handleMessage() 中，它原本是在thread1 中运行的。