android - 음악플레이어, 서비스 (service), onStartCommand

package com.example.it.myapplication14_1;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    Intent intent;
    Button btnStart, btnStop;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        setTitle("서비스 테스트 예제");

        intent = new Intent(this, MusicService.class);
        btnStart = (Button) findViewById(R.id.btnStart);
        btnStop = (Button) findViewById(R.id.btnStop);

        btnStart.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                startService(intent);
                android.util.Log.i("서비스 테스트", "startService()");
            }
        });

        btnStop.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                stopService(intent);
                android.util.Log.i("서비스 테스트", "stopService()");
            }
        });
    }
}

package com.example.it.myapplication14_1;

import android.app.Service;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.IBinder;
import android.support.annotation.Nullable;

public class MusicService extends Service {

    MediaPlayer mp;

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate () {
        android.util.Log.i("서비스 테스트","onCreate()");
        super.onCreate();
    }

    @Override
    public void onDestroy() {
        android.util.Log.i("서비스 테스트","onDestroy()");
        mp.stop();
        super.onDestroy();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        android.util.Log.i("서비스 테스트","onStartCommand()");
        mp = MediaPlayer.create(this, R.raw.song1);
        mp.setLooping(true);
        mp.start();
        return super.onStartCommand(intent, flags, startId);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Button
        android:id="@+id/btnStart"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="음악서비스 시작" />

    <Button
        android:id="@+id/btnStop"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="음악서비스 중지" />

</LinearLayout>