服务（Service）是Android中实现程序后台运行的解决方案，它的运行不依赖于任何用户界面，适合去执行那些不需要和用户交互且需要长期运行的任务。

注意

• Service并不是运行在一个独立的进程中，而是依赖于创建服务时所在的应用程序进程。当某个应用程序进程被杀掉时，所有依赖于该进程的服务也会停止运行。
• 服务并不会自动开启线程，所有的代码都默认运行在主线程中。所以，我们需要在服务内部手动创建子线程，并在这里执行具体的任务，避免出现主线程阻塞的情况。

生命周期
onCreate()：服务第一次创建的时候调用的
onStartCommand()：每次启动服务的时候都会调用
onBind()：绑定服务的时候调用
onUnBind()：解绑服务的时候调用
onDestory()：销毁服务的时候调用

Service生命周期.png

Service生命周期测试Log结果.jpg

补充：

• 每个服务都只会存在一个实例，不管调用了多少次startService or bindService方法，只需要调用一次stopService（或stopSelf）or unbindService方法，服务就会停止。
• 一个服务只要被启动或被绑定之后，就会一直处于运行状态，必须让上述两种条件同时不满足，服务才能被销毁。

启动方式对比

注意：

• 没有调用bindService直接调用unbindService会出现异常：
  java.lang.IllegalArgumentException: Service not registered；
  没有调用startService直接调用stopService则没事

服务的分类

Service分类.png

前台服务了解
前台服务会一直有一个正在运行的图标在系统的状态栏显示，下拉状态栏可以看到更加详细的信息，类似于通知的效果。使用前台服务，可以防止服务由于系统内存不足等原因被回收掉，而且可以满足一些特殊需求显示信息。

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是一个用于处理异步请求的基类，客户端通过startService发送请求。该服务按需启动，用子线程依次处理每个Intent，并且当它执行完成时自动停止。内部实现机制为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);
}