Android中IntentService与Service的区别

Android中的Service是用于后台服务的，当应用程序被挂到后台的时候，问了保证应用某些组件仍然可以工作而引入了Service这个概念，那么这里面要强调的是Service不是独立的进程，也不是独立的线程，它是依赖于应用程序的主线程的，也就是说，在更多时候不建议在Service中编写耗时的逻辑和操作，否则会引起ANR。

那么我们当我们编写的耗时逻辑，不得不被service来管理的时候，就需要引入IntentService，IntentService是继承Service的，那么它包含了Service的全部特性，当然也包含service的生命周期，那么与service不同的是，IntentService在执行onCreate操作的时候，内部开了一个线程，去你执行你的耗时操作。

这里我 需要解释以下几个方法，也许大家都已经很清楚了，不过为了抛砖引玉，我还是要提一嘴。

Service中提供了一个方法：

public int onStartCommand(Intent intent, int flags, int startId) {
     onStart(intent, startId);
     return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY;
 }

   public int onStartCommand(Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY;
    }

这个方法的具体含义是，当你的需要这个service启动的时候，或者调用这个servcie的时候，那么这个方法首先是要被回调的。

同时IntentService中提供了这么一个方法：

protected abstract void onHandleIntent(Intent intent);

protected abstract void onHandleIntent(Intent intent);

这是一个抽象方法，也就是说具体的实现需要被延伸到子类。

子类的声明：

public class ChargeService extends IntentService

public class ChargeService extends IntentService 

上面提到过IntentService是继承Service的，那么这个子类也肯定继承service，那么onHandleIntent()方法是什么时候被调用的呢？让我们具体看IntentService的内部实现：

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);
    }
}

/**
 * Creates an IntentService.  Invoked by your subclass's constructor.
 *
 * @param name Used to name the worker thread, important only for debugging.
 */
public IntentService(String name) {
    super();
    mName = name;
}

/**
 * Sets intent redelivery preferences.  Usually called from the constructor
 * with your preferred semantics.
 *
 * <p>If enabled is true,
 * {@link #onStartCommand(Intent, int, int)} will return
 * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
 * {@link #onHandleIntent(Intent)} returns, the process will be restarted
 * and the intent redelivered.  If multiple Intents have been sent, only
 * the most recent one is guaranteed to be redelivered.
 *
 * <p>If enabled is false (the default),
 * {@link #onStartCommand(Intent, int, int)} will return
 * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
 * dies along with it.
 */
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(Intent intent, int startId) {
    Message msg = mServiceHandler.obtainMessage();
    msg.arg1 = startId;
    msg.obj = intent;
    mServiceHandler.sendMessage(msg);
}

    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);
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public IntentService(String name) {
        super();
        mName = name;
    }

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     * with your preferred semantics.
     *
     * <p>If enabled is true,
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
     * {@link #onHandleIntent(Intent)} returns, the process will be restarted
     * and the intent redelivered.  If multiple Intents have been sent, only
     * the most recent one is guaranteed to be redelivered.
     *
     * <p>If enabled is false (the default),
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     */
    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(Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

在这里我们可以清楚的看到其实IntentService在执行onCreate的方法的时候，其实开了一个线程HandlerThread,并获得了当前线程队列管理的looper，并且在onStart的时候，把消息置入了消息队列，

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

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

在消息被handler接受并且回调的时候，执行了onHandlerIntent方法，该方法的实现是子类去做的。

结论：

IntentService是通过Handler looper message的方式实现了一个多线程的操作，同时耗时操作也可以被这个线程管理和执行，同时不会产生ANR的情况。