Service是在主线程中，因此不应该在Service中直接处理耗时的任务，一般我们要在Service中开启一个子线程去执行耗时操作。
IntentService是Service的子类，比普通的Service多了一个额外的功能，就是会创建独立的worker线程来处理所有的Intent请求。
首先来看onCreate()：
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); }这里创建了一个HandlerThread，HandlerThread就是一个自带Looper的线程，具体可以参考HandlerThread源码分析。
然后为这个HandlerThread创建了一个处理消息的ServiceHandler，ServiceHandler是Hanlder的子类：
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); } }它接到消息的时候就把消息放入一个任务队列，然后去调用onHandleIntent()，把所有的任务处理完，最后再调用stopSelf()关闭自己。
来看onHandleIntent()：
/** * This method is invoked on the worker thread with a request to process. * Only one Intent is processed at a time, but the processing happens on a * worker thread that runs independently from other application logic. * So, if this code takes a long time, it will hold up other requests to * the same IntentService, but it will not hold up anything else. * When all requests have been handled, the IntentService stops itself, * so you should not call {@link #stopSelf}. * * @param intent The value passed to {@link * android.content.Context#startService(Intent)}. * This may be null if the service is being restarted after * its process has gone away; see * {@link android.app.Service#onStartCommand} * for details. */ @WorkerThread protected abstract void onHandleIntent(@Nullable Intent intent);以上注释的意思是当某个请求需要处理时，这个方法会在工作线程被调用，一次仅仅会有一个请求被处理，但是处理过程会运行在工作者线程(独立于其他应用程序逻辑运行)。因此，如果某段代码需要执行很长时间，它会阻塞住其他提交到该IntentService的请求，但是不会阻塞住其他任何东西。当所有的请求被处理完成之后，IntentService会停止它自身，因此你不应该手动调用stopSelf()方法。
如果有多个任务的时候，stopSelf(int startId)中会去判断这个startId是否是最新的，如果是最新的就停止Service，如果不是，肯定是因为又新加入了任务，这个时候就不停止。
补充：stopSelf()其实就是stopSelf(-1)，这就直接停止了。我们还能看到在IntentService的onDestroy()中退出了工作线程：
@Override public void onDestroy() { mServiceLooper.quit(); }那消息是怎么传递给IntentService的呢，我们看到onStart()：
@Override public void onStart(@Nullable Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); }当startService()之后，onStart()会被调用，这个可以参考【Android】Service的生命周期。 我们看到onStart()中利用mServiceHandler以消息的形式向创建的子线程发送了传递进来的Intent。
到这里一切都明白了。
下面来看一个demo， TestIntentServiceActivity：
public class TestIntentServiceActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_test_intent_service); for (int i = 0; i < 5; i++) { startService(new Intent(TestIntentServiceActivity.this, MyIntentService.class)); } } }MyIntentService：
public class MyIntentService extends IntentService { public static final String TAG = "MyIntentService"; public MyIntentService() { super("MyIntentService"); } @Override protected void onHandleIntent(Intent intent) { for (int i = 0; i < 5; i++) { SystemClock.sleep(50); Log.d(TAG, Thread.currentThread().getName() + " " + i); } Log.d(TAG, Thread.currentThread().getName() + " " ); } }运行起来之后，Log的结果是这样的：
08-31 12:49:58.416 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 0 08-31 12:49:58.467 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 1 08-31 12:49:58.519 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 2 08-31 12:49:58.570 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 3 08-31 12:49:58.621 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 4 08-31 12:49:58.621 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 08-31 12:49:58.704 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 0 08-31 12:49:58.755 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 1 08-31 12:49:58.806 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 2 08-31 12:49:58.857 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 3 08-31 12:49:58.909 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 4 08-31 12:49:58.909 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 08-31 12:49:58.962 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 0 08-31 12:49:59.013 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 1 08-31 12:49:59.064 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 2 08-31 12:49:59.116 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 3 08-31 12:49:59.169 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 4 08-31 12:49:59.169 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 08-31 12:49:59.221 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 0 08-31 12:49:59.272 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 1 08-31 12:49:59.322 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 2 08-31 12:49:59.375 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 3 08-31 12:49:59.426 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 4 08-31 12:49:59.426 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 08-31 12:49:59.479 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 0 08-31 12:49:59.531 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 1 08-31 12:49:59.582 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 2 08-31 12:49:59.632 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 3 08-31 12:49:59.685 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService] 4 08-31 12:49:59.685 2839-2894/com.example.tsnt D/MyIntentService: IntentService[MyIntentService]完全符合我们以上的分析。
最后有一点注意，IntentService 源码中的 onBind() 默认返回 null；不适合 bindService() 启动服务，如果你执意要 bindService() 来启动 IntentService，可能因为你想通过 Binder 或 Messenger 使得 IntentService 和 Activity 可以通信，这样那么 onHandleIntent() 不会被回调，相当于在你使用 Service 而不是 IntentService。
参考： 1.IntentService 示例与详解 2.Android理解：IntentService 3.Android：IntentService的使用 4.IntentService使用
