Android : Generating a FileDescriptor on Android without first opening a file

on Friday, October 31, 2014


In Android, is it possible to generate a FileDescriptor directly from a byte array, without having to open a file first?


In Android 2.2, I am generating a MIDI file on the fly, and then playing it back using MediaPlayer. I've included the text of the Main.java file that does this successfully below. So far so good.


However, this process first calls...



FileOutputStream outputStream = openFileOutput(file, MODE_PRIVATE);
outputStream.write(byteStream);
outputStream.close();


... to write out the file, and then calls...



FileInputStream inputStream = new FileInputStream(midifile);
FileDescriptor fileDescriptor = inputStream.getFD();


... to read it back in, before calling:



mediaPlayer.setDataSource(fileDescriptor);


This seems to me to be wasteful. Can I create the FileDescriptor directly from the byteArray, so that the MIDI stream can be played immediately?




== Working code ==



package com.example.midi;

import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;

import android.app.Activity;
import android.media.AudioManager;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.View;

public class Main extends Activity {

private String file = "midi.mid";
MediaPlayer mediaPlayer;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mediaPlayer = new MediaPlayer();
mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);

createNewMIDIFile();
playNewMIDIFile();
}

public void createNewMIDIFile() {
Integer[] stream = new Integer[]{
//
0x4d, 0x54, 0x68, 0x64, // MThd = MIDI file designator
0x00, 0x00, 0x00, 0x06, // Standard MIDI File (SMF)
0x00, 0x01, 0x00, 0x02, // multiple-track format: 2 tracks
0x00, 0x40, // 64 ticks per beat (quarter note)
0x4D, 0x54, 0x72, 0x6B, // Header for track 1
0x00, 0x00, 0x00, 0x0B, // 11 bytes to describe the track
0x00, 0xFF, 0x51, 0x03, // set tempo:
0x0F, 0x42, 0x40, // 1,000,000 microseconds / beat: 60 bpm
0x00, 0xFF, 0x2F, 0x00, // End of track 1
0x4D, 0x54, 0x72, 0x6B, // Header for track 2
0x00, 0x00, 0x00, 0x0F, // 15 bytes to describe the track
0x00, // Immediately
0xC1, 0x01, // change instrument for track 2 to piano
0x00, // Immediately
0x91, 0x3C, 0x7F, // play middle C with a velocity of 127
0x30, // 48 ticks later (dotted eighth note)
0x81, 0x3C, 0x00, // stop playing the middle C
0x00, 0xFF, 0x2F, 0x00 // End of track 2
};

int length = stream.length;
byte[] byteStream = new byte[length];
for (int ii = 0; ii < length; ii++) {
byteStream[ii] = (byte) (stream[ii] % 256);
}

try {
FileOutputStream outputStream = openFileOutput(file, MODE_PRIVATE);
outputStream.write(byteStream);
outputStream.close();
} catch (Exception e) {
e.printStackTrace();
}
}

public void play(View view) {
/* Triggered by a button defined in activity_main.xml as
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="play"
android:text="Play MIDI" />
*/
playNewMIDIFile();
}

public void playNewMIDIFile() {
try {
String filename = getFilesDir() + "/" + file;
File midifile = new File(filename);
FileInputStream inputStream = new FileInputStream(midifile);
FileDescriptor fileDescriptor = inputStream.getFD();
mediaPlayer.reset();
mediaPlayer.setDataSource(fileDescriptor);
inputStream.close();
mediaPlayer.prepare();
mediaPlayer.start();
} catch (Exception e) {
e.printStackTrace();
}
}
}

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