android编程在退出时停止mediaplayer流

所以我有这个代码来流式传输mp3，现在我想知道当有人用后退按钮/主页按钮退出应用程序时，我该如何让音乐停止。即使我退出应用程序并关闭它，音乐仍在后台播放，我也无法让它停止。

package com.hrupin.streamingmedia;
import android.app.Activity;
import android.media.MediaPlayer;
import android.media.MediaPlayer.OnBufferingUpdateListener;
import android.media.MediaPlayer.OnCompletionListener;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.SeekBar;
import com.hrupin.media.R;
public class StreamingMp3Player extends Activity implements OnClickListener, OnTouchListener, OnCompletionListener, OnBufferingUpdateListener{
    private ImageButton buttonPlayPause;
    private SeekBar seekBarProgress;
    public EditText editTextSongURL;
    private MediaPlayer mediaPlayer;
    private int mediaFileLengthInMilliseconds; // this value contains the song duration in milliseconds. Look at getDuration() method in MediaPlayer class
    private final Handler handler = new Handler();
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        initView();
    }
    /** This method initialise all the views in project*/
    private void initView() {
        buttonPlayPause = (ImageButton)findViewById(R.id.ButtonTestPlayPause);
        buttonPlayPause.setOnClickListener(this);
        seekBarProgress = (SeekBar)findViewById(R.id.SeekBarTestPlay);  
        seekBarProgress.setMax(99); // It means 100% .0-99
        seekBarProgress.setOnTouchListener(this);
        //editTextSongURL = (EditText)findViewById(R.id.EditTextSongURL);
        //editTextSongURL.setText(R.string.testsong_20_sec);
        mediaPlayer = new MediaPlayer();
        mediaPlayer.setOnBufferingUpdateListener(this);
        mediaPlayer.setOnCompletionListener(this);
    }
    /** Method which updates the SeekBar primary progress by current song playing position*/
    private void primarySeekBarProgressUpdater() {
        seekBarProgress.setProgress((int)(((float)mediaPlayer.getCurrentPosition()/mediaFileLengthInMilliseconds)*100)); // This math construction give a percentage of "was playing"/"song length"
        if (mediaPlayer.isPlaying()) {
            Runnable notification = new Runnable() {
                public void run() {
                    primarySeekBarProgressUpdater();
                }
            };
            handler.postDelayed(notification,1000);
        }
    }
    @Override
    public void onClick(View v) {
        if(v.getId() == R.id.ButtonTestPlayPause){
             /** ImageButton onClick event handler. Method which start/pause mediaplayer playing */
            try {
                mediaPlayer.setDataSource("http://www.metalgeardroid.com/music/OST.mp3"); // setup song from http://www.hrupin.com/wp-content/uploads/mp3/testsong_20_sec.mp3 URL to mediaplayer data source
                mediaPlayer.prepare(); // you must call this method after setup the datasource in setDataSource method. After calling prepare() the instance of MediaPlayer starts load data from URL to internal buffer. 
            } catch (Exception e) {
                e.printStackTrace();
            }
            mediaFileLengthInMilliseconds = mediaPlayer.getDuration(); // gets the song length in milliseconds from URL
            if(!mediaPlayer.isPlaying()){
                mediaPlayer.start();
                buttonPlayPause.setImageResource(R.drawable.button_pause);
            }else {
                mediaPlayer.pause();
                buttonPlayPause.setImageResource(R.drawable.button_play);
            }
            primarySeekBarProgressUpdater();
        }
    }
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        if(v.getId() == R.id.SeekBarTestPlay){
            /** Seekbar onTouch event handler. Method which seeks MediaPlayer to seekBar primary progress position*/
            if(mediaPlayer.isPlaying()){
                SeekBar sb = (SeekBar)v;
                int playPositionInMillisecconds = (mediaFileLengthInMilliseconds / 100) * sb.getProgress();
                mediaPlayer.seekTo(playPositionInMillisecconds);
            }
        }
        return false;
    }
    @Override
    public void onCompletion(MediaPlayer mp) {
         /** MediaPlayer onCompletion event handler. Method which calls then song playing is complete*/
        buttonPlayPause.setImageResource(R.drawable.button_play);
    }
    @Override
    public void onBufferingUpdate(MediaPlayer mp, int percent) {
        /** Method which updates the SeekBar secondary progress by current song loading from URL position*/
        seekBarProgress.setSecondaryProgress(percent);
    }
}