HandlerThread
HandlerThread继承了Thread,它是一种可以使用Handler的Thread，它实现也很简单，就是在run中通过Looper.prepare()来创建消息队列，并且通过Looper.loop()来开启消息循环，这样再实际使用中就允许在HandlerThread中创建Handle了。

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;
    private @Nullable Handler mHandler;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }

    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }

    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }

    public Looper getLooper() {
        //Thread.IsAlive属性 ，表示该线程当前是否为可用状态
        //如果线程已经启动，并且当前没有任何异常的话，则是true，否则为false
        //Start()后，线程不一定能马上启动起来，也许CPU正在忙其他的事情，但迟早是会启动起来的！
        if (!isAlive()) {
            return null;
        }

        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    @NonNull
    public Handler getThreadHandler() {
        if (mHandler == null) {
            mHandler = new Handler(getLooper());
        }
        return mHandler;
    }

    public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }

    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

    public int getThreadId() {
        return mTid;
    }
}

IntentService
IntentService是一种特殊的Service，它继承了Service并且它是一个抽象类，因此必须创建它的子类才能使用IntentService
IntentService可用于执行后台耗时的任务，当任务执行后它会自动停止，同事由于IntentService是服务的原因，这导致它的优先级比单纯的线程要高很多，所以IntentService比较适合执行一些高级优先级的后台任务，因为它的优先级高不容易被系统杀死。
在实现上，IntentService封装了HandlerThread和Handler，这一点可以从它的onCreate方法中看出来。

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;

    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }

    public IntentService(String name) {
        super();
        mName = name;
    }

    public void setIntentRedelivery(boolean enabled) {
        mRedelivery = enabled;
    }

    @Override
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        mServiceLooper.quit();
    }

    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

使用：

class IntentServiceDemo : IntentService("IntentServiceDemo") {
    private val TAG = "zmm"

    /* (non-Javadoc)
     * @see android.app.IntentService#onCreate()
     */
    override fun onCreate() {
        Log.e(TAG, "=>onCreate")
        super.onCreate()
    }

    override fun onStartCommand(@Nullable intent: Intent?, flags: Int, startId: Int): Int {
        Log.e(TAG, "=>onStartCommand")
        return super.onStartCommand(intent, flags, startId)
    }

    /* (non-Javadoc)
         * @see android.app.IntentService#onDestroy()
         */
    override fun onDestroy() {
        Log.e(TAG, "=>onDestroy")
        super.onDestroy()
    }

    override fun onHandleIntent(arg0: Intent?) {
        try {
            Log.e(TAG, "IntentService 线程：" + Thread.currentThread().id)
            Thread.sleep(2000)
        } catch (e: InterruptedException) {
            e.printStackTrace()
        }

    }
}

var mServiceIntent = Intent(this@MainActivity, IntentServiceDemo::class.java)
        startService(mServiceIntent)