IntentService
Service 的一个子类，该类会创建一个线程来处理所有的 start 请求，一次处 理一个。如果你不需要你的 service 同时处理多个请求，这个类是你的最佳 选择。你只需实现 onHandleIntent() 方法即可，该方法会收到每次 start 请求的 intent，你可以针对每个请求做处理。
IntentService实现的代码也很简单，在IntentService内部定义了一个内部类ServiceHandler，顾名思义是继承Handler的，
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); } } 在onStart()中每次都会把activity传过来的intent发送给handler处理。 private volatile ServiceHandler mServiceHandler; @Override public void onStart(Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); } 最后的处理在onHandleIntent(Intent intent)中，这个方法在IntentService里是一个抽象方法，由子类实现。 /** * 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)}. */ @WorkerThread protected abstract void onHandleIntent(Intent intent);这里有个workerThread，我理解的是所有的activity传过来的intent处理都是在一个工作线程处理的，就是子线程的意思，并不在主线程。
因为在IntentService中重写了onCreate()，在里面申请了一个工作线程给了handler
@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); }