Android开发中，当需要创建在后台运行的程序的时候，就要使用到Service。Service 可以分为有无限生命和有限生命两种。特别需要注意的是Service跟Activities是不同的（简单来说可以理解为后台与前台的区别），例如，如果需要使用Service的话，需要调用startService(),从而利用startService()去调用Service中的OnCreate()和onStart()方法来启动一个后台的Service。
      启动一个Service的过程如下：context.startService()  ->onCreate()- >onStart()->Service running其中onCreate()可以进行一些服务的初始化工作，onStart()则启动服务。
      停止一个Service的过程如下：context.stopService() | ->onDestroy() ->Service stop
      接下来的实例是一个利用后台服务播放音乐的小例子，点击start运行服务，点击stop停止服务。
ServicesDemo.java（是一个Activity）

package com.android.myservice;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class ServiceDemo extends Activity implements OnClickListener {
    private static final String TAG = "ServiceDemo";
    Button buttonStart, buttonStop;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        buttonStart = (Button) findViewById(R.id.buttonStart);
        buttonStop = (Button) findViewById(R.id.buttonStop);

        buttonStart.setOnClickListener(this);
        buttonStop.setOnClickListener(this);
    }

    public void onClick(View src) {
        switch (src.getId()) {
            case R.id.buttonStart:
                Log.i(TAG, "onClick: starting service");
                startService(new Intent(this, MyService.class));
                break;
            case R.id.buttonStop:
                Log.i(TAG, "onClick: stopping service");
                stopService(new Intent(this, MyService.class));
                break;
        }
    }
}
除此之外还要在Manifest里面声明服务:(AndroidManifest.xml)
<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.android.myservice">
    <application android:label="@string/app_name">
        <activity android:name=".ServiceDemo" android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <service android:enabled="true" android:name=".MyService"/>
    </application>
</manifest>

定义Service(MyService.java)
package com.android.myservice;

import android.app.Service;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.IBinder;
import android.util.Log;
import android.widget.Toast;

public class MyService extends Service {
    private static final String TAG = "MyService";
    MediaPlayer player;

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

    @Override
    public void onCreate() {
        Toast.makeText(this, "My Service created", Toast.LENGTH_LONG).show();
        Log.i(TAG, "onCreate");

        player = MediaPlayer.create(this, R.raw.braincandy);
        player.setLooping(false);
    }

    @Override
    public void onDestroy() {
        Toast.makeText(this, "My Service Stoped", Toast.LENGTH_LONG).show();
        Log.i(TAG, "onDestroy");
        player.stop();
    }

    @Override
    public void onStart(Intent intent, int startid) {
        Toast.makeText(this, "My Service Start", Toast.LENGTH_LONG).show();
        Log.i(TAG, "onStart");
        player.start();
    }
}
layout文件夹中是main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center">
<TextView 
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" android:text="@string/services_demo" android:gravity="center" android:textSize="20sp" android:padding="20dp"/>
<Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/buttonStart" android:text="@string/start"></Button>
<Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/stop" android:id="@+id/buttonStop"></Button>
</LinearLayout>

values 文件夹中是strings.xml
<?xml version="1.0" encoding="utf-8"?>

<resources>
    <string name="start">Start</string>
    <string name="stop">Stop</string>
    <string name="services_demo">Service Demo</string>
</resources>