服務(wù)（Service）是Android中實(shí)現(xiàn)程序后臺(tái)運(yùn)行的解決方案，它的運(yùn)行不依賴于任何用戶界面，適合去執(zhí)行那些不需要和用戶交互且需要長期運(yùn)行的任務(wù)。

注意

• Service并不是運(yùn)行在一個(gè)獨(dú)立的進(jìn)程中，而是依賴于創(chuàng)建服務(wù)時(shí)所在的應(yīng)用程序進(jìn)程。當(dāng)某個(gè)應(yīng)用程序進(jìn)程被殺掉時(shí)，所有依賴于該進(jìn)程的服務(wù)也會(huì)停止運(yùn)行。
• 服務(wù)并不會(huì)自動(dòng)開啟線程，所有的代碼都默認(rèn)運(yùn)行在主線程中。所以，我們需要在服務(wù)內(nèi)部手動(dòng)創(chuàng)建子線程，并在這里執(zhí)行具體的任務(wù)，避免出現(xiàn)主線程阻塞的情況。

生命周期
onCreate()：服務(wù)第一次創(chuàng)建的時(shí)候調(diào)用的
onStartCommand()：每次啟動(dòng)服務(wù)的時(shí)候都會(huì)調(diào)用
onBind()：綁定服務(wù)的時(shí)候調(diào)用
onUnBind()：解綁服務(wù)的時(shí)候調(diào)用
onDestory()：銷毀服務(wù)的時(shí)候調(diào)用

Service生命周期.png

Service生命周期測(cè)試Log結(jié)果.jpg

補(bǔ)充：

• 每個(gè)服務(wù)都只會(huì)存在一個(gè)實(shí)例，不管調(diào)用了多少次startService or bindService方法，只需要調(diào)用一次stopService（或stopSelf）or unbindService方法，服務(wù)就會(huì)停止。
• 一個(gè)服務(wù)只要被啟動(dòng)或被綁定之后，就會(huì)一直處于運(yùn)行狀態(tài)，必須讓上述兩種條件同時(shí)不滿足，服務(wù)才能被銷毀。

啟動(dòng)方式對(duì)比

注意：

• 沒有調(diào)用bindService直接調(diào)用unbindService會(huì)出現(xiàn)異常：
  java.lang.IllegalArgumentException: Service not registered；
  沒有調(diào)用startService直接調(diào)用stopService則沒事

服務(wù)的分類

Service分類.png

前臺(tái)服務(wù)了解
前臺(tái)服務(wù)會(huì)一直有一個(gè)正在運(yùn)行的圖標(biāo)在系統(tǒng)的狀態(tài)欄顯示，下拉狀態(tài)欄可以看到更加詳細(xì)的信息，類似于通知的效果。使用前臺(tái)服務(wù)，可以防止服務(wù)由于系統(tǒng)內(nèi)存不足等原因被回收掉，而且可以滿足一些特殊需求顯示信息。

public class MyService extends Service {
   
    @Override
    public void onCreate() {
        super.onCreate();
        
        Intent intent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
        Notification notification = new NotificationCompat.Builder(this)
                .setContentTitle("this is title")
                .setContentText("I'm SO HAPPY")
                .setWhen(System.currentTimeMillis())
                .setSmallIcon(R.mipmap.smile)
                .setContentIntent(pendingIntent)
                .build();
        startForeground((int) (Math.random() * 100), notification);
    }
}

IntentService了解
IntentService是一個(gè)用于處理異步請(qǐng)求的基類，客戶端通過startService發(fā)送請(qǐng)求。該服務(wù)按需啟動(dòng)，用子線程依次處理每個(gè)Intent，并且當(dāng)它執(zhí)行完成時(shí)自動(dòng)停止。內(nèi)部實(shí)現(xiàn)機(jī)制為Handler。

/**
 * IntentService is a base class for {@link Service}s that handle asynchronous
 * requests (expressed as {@link Intent}s) on demand.  Clients send requests
 * through {@link android.content.Context#startService(Intent)} calls; the
 * service is started as needed, handles each Intent in turn using a worker
 * thread, and stops itself when it runs out of work.
 *
 * <p>This "work queue processor" pattern is commonly used to offload tasks
 * from an application's main thread.  The IntentService class exists to
 * simplify this pattern and take care of the mechanics.  To use it, extend
 * IntentService and implement {@link #onHandleIntent(Intent)}.  IntentService
 * will receive the Intents, launch a worker thread, and stop the service as
 * appropriate.
 *
 * <p>All requests are handled on a single worker thread -- they may take as
 * long as necessary (and will not block the application's main loop), but
 * only one request will be processed at a time.
 *
 * @see android.os.AsyncTask
 */
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);
        }
    }

    /**
     * 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(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    /**
     * You should not override this method for your IntentService. Instead,
     * override {@link #onHandleIntent}, which the system calls when the IntentService
     * receives a start request.
     * @see android.app.Service#onStartCommand
     */
    @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();
    }

    /**
     * Unless you provide binding for your service, you don't need to implement this
     * method, because the default implementation returns null.
     * @see android.app.Service#onBind
     */
    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     * 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);
}