以绑定式方式启动service实现后台音乐播放的功能，界面简单，给出代码

好的，下面是一个简单的Android后台音乐播放器示例代码：

1. 创建一个后台Service

public class MusicService extends Service implements MediaPlayer.OnPreparedListener {

    private MediaPlayer mediaPlayer;

    @Override
    public void onCreate() {
        super.onCreate();
        mediaPlayer = new MediaPlayer();
        mediaPlayer.setOnPreparedListener(this);
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if (intent != null && intent.getAction() != null) {
            switch (intent.getAction()) {
                case "play":
                    String url = intent.getStringExtra("url");
                    playMusic(url);
                    break;
                case "pause":
                    pauseMusic();
                    break;
                case "resume":
                    resumeMusic();
                    break;
                case "stop":
                    stopMusic();
                    break;
            }
        }
        return super.onStartCommand(intent, flags, startId);
    }

    private void playMusic(String url) {
        try {
            mediaPlayer.reset();
            mediaPlayer.setDataSource(url);
            mediaPlayer.prepareAsync();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void pauseMusic() {
        if (mediaPlayer.isPlaying()) {
            mediaPlayer.pause();
        }
    }

    private void resumeMusic() {
        if (!mediaPlayer.isPlaying()) {
            mediaPlayer.start();
        }
    }

    private void stopMusic() {
        mediaPlayer.stop();
        mediaPlayer.reset();
    }

    @Override
    public void onPrepared(MediaPlayer mp) {
        mediaPlayer.start();
    }

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

2. 在AndroidManifest.xml中声明Service

<service android:name=".MusicService"/>

3. 在Activity中启动Service并发送命令

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText etUrl;
    private Button btnPlay;
    private Button btnPause;
    private Button btnResume;
    private Button btnStop;

    private Intent musicServiceIntent;

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

        etUrl = findViewById(R.id.et_url);
        btnPlay = findViewById(R.id.btn_play);
        btnPause = findViewById(R.id.btn_pause);
        btnResume = findViewById(R.id.btn_resume);
        btnStop = findViewById(R.id.btn_stop);

        btnPlay.setOnClickListener(this);
        btnPause.setOnClickListener(this);
        btnResume.setOnClickListener(this);
        btnStop.setOnClickListener(this);

        musicServiceIntent = new Intent(this, MusicService.class);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn_play:
                String url = etUrl.getText().toString();
                musicServiceIntent.setAction("play");
                musicServiceIntent.putExtra("url", url);
                startService(musicServiceIntent);
                break;
            case R.id.btn_pause:
                musicServiceIntent.setAction("pause");
                startService(musicServiceIntent);
                break;
            case R.id.btn_resume:
                musicServiceIntent.setAction("resume");
                startService(musicServiceIntent);
                break;
            case R.id.btn_stop:
                musicServiceIntent.setAction("stop");
                startService(musicServiceIntent);
                break;
        }
    }
}

这样就可以实现一个简单的后台音乐播放器了。注意，此示例代码并没有考虑到后台播放时的系统通知等功能，如需要可以自行添加。同时，由于示例代码没有使用前台服务，为了避免被系统杀死，需要在Service中定期调用startForeground方法。