无需先打开文件即可在 Android 上生成 FileDescriptor

Question

在Android中，是否可以直接从字节数组生成FileDescriptor，而不必先打开文件？

在 Android 2.2 中，我即时生成一个 MIDI 文件，然后使用 MediaPlayer 播放它。我在下面包含了成功执行此操作的 Main.java 文件的文本。到现在为止还挺好。

然而，这个过程首先调用...

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

...写出文件，然后调用...

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

...在调用之前重新阅读它：

mediaPlayer.setDataSource(fileDescriptor);

在我看来，这很浪费。我可以直接从 byteArray 创建 FileDescriptor，以便立即播放 MIDI 流吗？

---

== 工作代码 ==

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";
  private 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();
    }
  }
}

有关即时构建 MIDI 文件的更多信息，请参阅kevinboone.net、skytopia和sonicspot

Answer 1

MediaPlayer 将从文件或 HTTP url 播放。

因此，这里的技巧是从指向本地主机服务器的 url 进行播放，您可以在其中从内存或任何其他方式动态生成数据。

请参阅我的答案如何做到这一点： Android ServerSocketprogramming with jCIFS Streaming Files