public class MyService extends Service {
    String msg;
    public MyService() {
        msg = "Msg from Service";
    }
 
    @Override
    public IBinder onBind(Intent intent) {
        return new MyBinder();
    }
 
    public class MyBinder extends Binder{
        public String getMsg(){
            return msg;
        }
    }
 
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.i("Service","Service start");
        return super.onStartCommand(intent, flags, startId);
    }
}

在activity中绑定Service：

public class MainActivity extends AppCompatActivity {
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Intent intent = new Intent(this, MyService.class);
        startService(intent);
        bindService(intent, sc, Context.BIND_AUTO_CREATE);
    }
 
    private ServiceConnection sc = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            MyService.MyBinder myBinder = (MyService.MyBinder) service;
            Toast.makeText(MainActivity.this,myBinder.getMsg(),Toast.LENGTH_SHORT).show();
        }
 
        @Override
        public void onServiceDisconnected(ComponentName name) {
            // 连接出现了异常断开了，MyService被杀掉了
            Toast.makeText(getApplicationContext(),"Service被解绑",Toast.LENGTH_SHORT).show();
            Log.i("MainActivity","Service被解绑");
        }
    };
 
    @Override
    protected void onDestroy() {
        super.onDestroy();
        //退出记得解绑Service
        unbindService(sc);
    }
}