目標：在後臺定時執行列印當前時間的log
Android定時任務一般有2種實現方式，一種是使用Java API裡面提供的Timer類，另一種是Android的Alarm機制。但是前者無法喚醒CPU，可能會導致後臺執行的任務無法正常執行，所以建議使用後者。
獲取一個Alarm例項：

AlarmManager manager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);

之後呼叫AlarmManager的set()方法就可以設定一個定時任務，比如想要設定一個在10s中後執行就可以寫成：

long time = SystemClock.elapsedRealtime() + 10 * 1000;
manager.set
 
(AlarmManager.ELAPSED_REALTIME_WAKEUP, time, PendingIntent);

其中set()方法中有3個引數，第一個引數有4個可選，具體含義感興趣的同學可以去查一下，第二個就是時間，最後一個引數就是PendingIntent,這裡一般會呼叫getBroadcast()方法來獲取一個能夠執行廣播的PendingIntent。這樣當定時任務被處罰的時候，廣播接收器的onReceiver()方法就可以得到執行。
下面看一個小例子：

MainActivity.class:

package com.superxingyun.servicebestpractice;

import
 
 android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Intent i = new
 
 Intent(this, LongRunningService.class);
        startService(i);//啟動LongRunningService
    }
}

LongRunningService.class:

package com.superxingyun.servicebestpractice;

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.os.SystemClock;
import android.support.annotation.Nullable;
import android.util.Log;

import java.util.Date;

/**
 * Created by 月滿軒尼詩丶大帥~~ on 2016/11/8.
 */

public class LongRunningService extends Service {
    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.d("LongRunningService", "executed at" + new Date().toString());
            }
        }).start();
        AlarmManager manager = (AlarmManager) getSystemService(ALARM_SERVICE);
        int aHour = 60 * 1000;//這是一分鐘毫秒數
        long triggerAtTime = SystemClock.elapsedRealtime() + aHour;
        Intent i = new Intent(this, AlarmReceiver.class);
        PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, 0);
        manager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, triggerAtTime, pi);
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }
}

AlarmReceiver.class:

package com.superxingyun.servicebestpractice;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

/**
 * Created by 月滿軒尼詩丶大帥~~ on 2016/11/9.
 */
public class AlarmReceiver extends BroadcastReceiver{
    @Override
    public void onReceive(Context context, Intent intent) {
        Intent i = new Intent(context, LongRunningService.class);
        context.startService(i);
    }
}