Android Mediaplayer无法播放tempfile

Question

对于Android应用程序，我尝试在tempfile中编写midifiles并使用 MediaPlayer 进行播放。 尝试并遵循了很多例子，但我无法让它发挥作用。 问题是 MediaPlayer 在创建时不断提供null。 有人可以看看这段代码并告诉我我做错了什么。 提前谢谢。

    package com.example.miditest;

    import java.io.DataOutputStream;
    import java.io.File;
    import java.io.FileOutputStream;
    import java.io.IOException;

    import android.media.MediaPlayer;
    import android.net.Uri;
    import android.os.Bundle;
    import android.annotation.SuppressLint;
    import android.app.Activity;
    import android.view.Menu;
    import android.view.View;
    import android.widget.Button;
    import android.widget.Toast;

    @SuppressLint("ShowToast")
    public class MainActivity extends Activity {



    public void init(){

    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button Playbutton1 = (Button)findViewById(R.id.button1);
        Playbutton1.setOnClickListener(
            new Button.OnClickListener(){
                @Override
                public void onClick(View v){
                    Button Playbutton1 =    (Button)findViewById(R.id.button1);
                    Playbutton1.setText("test1");
                    WriteMidiFile();
                    if(tempMid.exists()){
                        Toast.makeText(getApplication(), "File bestaat", Toast.LENGTH_LONG);
                    };
                    Playfile();
                }
            }
        );
    }

    MediaPlayer mediaPlayer;

    /*  This one with a stored file in raw folder worked!
     *  the file stored was generated in java with the same code to be sure there's no
     *  difference.

    public void Playfile(){
        mediaPlayer = MediaPlayer.create(this,R.raw.testmid);
        mediaPlayer.start();
        mediaPlayer.release();
    }
    */

    public void Playfile(){
        // test that the tempfile can be found.
        if(tempMid.exists()){
            Toast.makeText(getApplication(), "File bestaat", Toast.LENGTH_LONG);
        };
        // creating mediaPlayer gives NULL 
        Uri Uristring = Uri.fromFile(tempMid);
        mediaPlayer = MediaPlayer.create(getBaseContext(), Uristring);
        mediaPlayer.start();

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

    File tempMid;
    public void WriteMidiFile(){
    /*  Writing midi data to tempfile
     *  Tested positive to be a valid and playable midi file
     */

        try {
            tempMid = File.createTempFile("Chord",".mid",this.getCacheDir());
            DataOutputStream data;
            data = new DataOutputStream(new FileOutputStream(tempMid.getAbsolutePath()));
            data.write((byte)'M');
            data.write((byte)'T');
            data.write((byte)'h');
            data.write((byte)'d');
            data.writeInt(6);
            data.writeInt(1);
            data.writeShort((short)480);
            data.write((byte)'M');
            data.write((byte)'T');
            data.write((byte)'r');
            data.write((byte)'k');
            data.writeInt(50); //tracklengte
            data.write(0x00);
            data.writeShort((short)49176);//channel gitaar
            data.writeShort((short)144);//tijd voor note on 
            data.writeShort((short)16464);//40 50
            data.write(0x00);
            data.writeShort((short)15184);//3b 50
            data.write(0x00);
            data.writeShort((short)14160);//37 50
            data.write(0x00);
            data.writeShort((short)12880);//32 50
            data.write(0x00);
            data.writeShort((short)11600);//2d 50
            data.write(0x00);
            data.writeShort((short)10320);//28 50
            data.writeShort((short)36471);//tijd voor note off
            data.writeShort((short)16384);//64 00
            data.write(0x00);
            data.writeShort((short)15104);//59 00
            data.write(0x00);
            data.writeShort((short)14080);//55 00
            data.write(0x00);
            data.writeShort((short)12800);//50 00
            data.write(0x00);
            data.writeShort((short)11520);//45 00
            data.write(0x00);
            data.writeShort((short)10240);//40 00
            data.writeInt(16723712);
            data.close();



        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }   

    };

}