前文提到解析MP3標籤，程序源碼中也已經出現了調用解析MP3標籤、打印MP3文件信息的功能，這兒先說說MP3文件信息的解析。html
解析MP3的文件信息對MP3解碼器來講只是一個附加功能，若是不加入這部分源碼，同時刪除掉前文源碼中的相關調用，不影響解碼播放。若是你想編寫「迷你」型的MP3解碼器，能夠忽略這些附加的功能。java
MP3的標籤信息位於文件開始處或結尾處，用於表達MP3文件的相關信息，常見的有ID三、APE等。redis
ID3 V1 位於文件最後的128字節，若是讀取的是網絡文件而服務器又不支持隨機讀取的話，意味着不對對其解析這部分信息。這128字節共表示7個信息：服務器
[0..2] 3 bytes: ID3 v1標識 -- 'TAG'
[3..32] 30 bytes: 標題
[33..62] 30 bytes: 藝術家
[63..92] 30 bytes: 專輯名
[93..96] 4 bytes: 發行年份
[97..126] 30 bytes: v1.0 -- 註釋/附加/備註信息
v1.1 -- 前29 bytes註釋/附加/備註信息，最後1 byte音軌信息網絡
[127] 1 byte : 流派數據結構
從「標題」開始，每部份內容之間用'\0'（字符串結束標誌）或'\20'（空格）隔開。dom
ID3 V2 表示的信息更豐富，結構更復雜，位於文件開始處或位於APE標籤以後。ID3 V2的詳細內容請參見http://www.id3.org/id3v2.3.0 this
APE V1 & V2 位於文件開始處或ID3 V2以後。詳細內容請參見http://cn.bing.com/reference/semhtml/APE_tag （External links下的連接就是APE V2）。有不少MP3的標籤信息很混亂，內容重複。因爲APE標籤出現並在MP3中大量應用得比ID3晚，MP3文件的「有利」位置都被ID3佔用，因此APE標籤位於文件中的位置讓人捉摸不透，狀況很複雜，對網絡文件來講，判斷APE標籤的位置要反覆在文件中定位，何況有的服務器根本就不支持隨機訪問，因此我這兒就放棄了對APE的解析，儘管APE的解析過程並不複雜。spa
本文只解析ID3 V1的具備的那幾項簡單的內容，JAVA的字符集轉換很方便，因此解析ID3 V2的代碼很簡潔。ID3 V2的每一幀都以「Frame ID」開始，例如TT2或TIT2表示「標題」，程序中經過計算ID的哈希值來識別不一樣的幀。須要指出的是，在解碼器讀入文件時自動對標籤信息進行解析，調用IRandomAccess接口的tagAvailable()方法查詢是否已經完成對tag的解析完畢，對網絡文件，是開線程之後臺方式解析。若是對其解析完畢，調用getTitle()等方法就能夠返回其內容，若是MP3文件自己沒有標籤信息，返回值爲null。具體調用方法見http://jmp123.sf.net/ 下的API文檔。.net
ID3Tag.java源碼：
/*
* ID3Tag.java -- 解析MP3文件的ID3 v1/v2 tag
* Copyright (C) 2010
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* If you would like to negotiate alternate licensing terms, you may do
* so by contacting the author: <http://jmp123.sourceforge.net/>
*/
package jmp123.tag;
import java.io.UnsupportedEncodingException;
/*
ID3v1:
[0-2] 3 bytes: ID3 v1標識 -- 'TAG'
[3—32] 30 bytes: 標題
[33—62] 30 bytes: 藝術家
[63—92] 30 bytes: 專輯名
[93—96] 4 bytes: 發行年份
[97—126] 30 bytes: v1.0 -- 註釋/附加/備註信息
v1.1 -- 前29 bytes註釋/附加/備註信息，最後1 byte音軌信息
[127] 1 byte : 流派
*/
public final class ID3Tag {
// ID3v1 & ID3v2
private String strTitle;
private String strArtist;
private String strAlbum;
private String strYear;
// ID3v2
//private String strLyrics; // (內嵌)歌詞
private int intVersion;
private int intExHeaderSize;
private boolean boolID3v2Footer;
//TEXT_ENCODING[0]應由 "ISO-8859-1" 改成 "GBK". ??
private static String[] TEXT_ENCODING = {"GBK", "UTF-16", "UTF-16BE", "UTF-8"};
//--------------------------------------------------------------------
// ID3v1 & ID3v2
public void printTag() {
//if (strLyrics != null)
// System.out.println("\r" + strLyrics + "\n");
if (strTitle != null)
System.out.println("\r 標題: " + strTitle);
if (strArtist != null)
System.out.println("\r 藝術家: " + strArtist);
if (strAlbum != null)
System.out.println("\r 唱片集: " + strAlbum);
if (strYear != null)
System.out.println("\r 發行年: " + strYear);
}
public void destroy() {
strTitle = strArtist = strAlbum = strYear = null;
//strLyrics = null;
intVersion = intExHeaderSize = 0;
boolID3v2Footer = false;
}
public String getTitle() {
return strTitle;
}
public String getArtist() {
return strArtist;
}
public String getAlbum() {
return strAlbum;
}
public String getYear() {
return strYear;
}
//--------------------------------------------------------------------
// ID3v1
public boolean checkID3V1(byte[] b) {
return b[0] == 'T' && b[1] == 'A' && b[2] == 'G';
}
public void parseID3V1(byte[] b) {
int i;
if (b.length < 128 || checkID3V1(b) == false)
return;
byte[] buf = new byte[125];
System.arraycopy(b, 3, buf, 0, 125);
for (i = 0; i < 30 && buf[i] != 0; i++);
if (strTitle == null)
strTitle = new String(buf, 0, i).trim();
if (strTitle.length() == 0)
strTitle = null;
for (i = 30; i < 60 && buf[i] != 0; i++);
if (strArtist == null)
strArtist = new String(buf, 30, i-30).trim();
if (strArtist.length() == 0)
strArtist = null;
for (i = 60; i < 90 && buf[i] != 0; i++);
if (strAlbum == null)
strAlbum = new String(buf, 60, i-60).trim();
if (strAlbum.length() == 0)
strAlbum = null;
for (i = 90; i < 94 && buf[i] != 0; i++);
if (strYear == null)
strYear = new String(buf, 90, i-90).trim();
if (strYear.length() == 0)
strYear = null;
buf = null;
}
//--------------------------------------------------------------------
// ID3v2
public int checkID3V2(byte[] b, int off) {
if(b.length - off < 10)
return 0;
if(b[off] != 'I' || b[off+1] != 'D' || b[off+2] != '3')
return 0;
intVersion = b[off+3] & 0xff;
if(intVersion > 2 && (b[off+5] & 0x40) != 0)
intExHeaderSize = 1; //設置爲1表示有擴展頭
boolID3v2Footer = (b[off+5] & 0x10) != 0;
int size = synchSafeInt(b, off+6);
size += 10; // ID3 header:10bytes
return size;
}
//b[off..]不含ID3v2 頭(10 bytes)
public void parseID3V2(byte[] b, int off) {
int max_size = b.length;
int pos = off;
if(intExHeaderSize == 1) {
intExHeaderSize = synchSafeInt(b, off);
pos += intExHeaderSize;
}
max_size -= 10; //1 frame header: 10 bytes
if(boolID3v2Footer)
max_size -= 10;
//System.out.println("ID3 v2." + intVersion);
while(pos < max_size)
pos += getText(b, pos, max_size);
}
public static int synchSafeInt(byte[] b, int off) {
int i = (b[off] & 0x7f) << 21;
i |= (b[off+1] & 0x7f) << 14;
i |= (b[off+2] & 0x7f) << 7;
i |= b[off+3] & 0x7f;
return i;
}
private int makeInt(byte[] b, int off, int len) {
int i, ret = b[off] & 0xff;
for (i = 1; i < len; i++) {
ret <<= 8;
ret |= b[off + i] & 0xff;
}
return ret;
}
private int getText(byte[] b, int off, int max_size) {
int id_part = 4, frame_header = 10;
if(intVersion == 2) {
id_part = 3;
frame_header = 6;
}
String id = new String(b, off, id_part);
off += id_part;
int fsize, len;
fsize = len = makeInt(b, off, id_part);
off += id_part; // frame size = frame id bytes
if (intVersion > 2)
off += 2; // flag: 2 bytes
int enc = b[off];
len--; // Text encoding: 1 byte
off++; // Text encoding: 1 byte
if (len <= 0 || off + len > max_size || enc < 0 || enc >= TEXT_ENCODING.length)
return fsize + frame_header;
//System.out.println(len+" ------------------------------------ off = " + off);
//System.out.println("ID: " + id + ", id.hashCode()=" + id.hashCode());
//System.out.println("text encoding: " + TEXT_ENCODING[enc]);
//System.out.println("frame size: " + fsize);
try {
switch(id.hashCode()) {
case 83378: //TT2 v2.2
case 2575251: //TIT2 標題
if (strTitle == null)
strTitle = new String(b, off, len, TEXT_ENCODING[enc]).trim();
break;
case 83552:
case 2590194: //TYER 發行年
if (strYear == null)
strYear = new String(b, off, len, TEXT_ENCODING[enc]).trim();
break;
case 2569358: //TCON 流派
break;
case 82815:
case 2567331: //TALB 唱片集
if (strAlbum == null)
strAlbum = new String(b, off, len, TEXT_ENCODING[enc]).trim();
break;
case 83253:
case 2581512: //TPE1 藝術家
if (strArtist == null)
strArtist = new String(b, off, len, TEXT_ENCODING[enc]).trim();
break;
case 2583398: //TRCK 音軌
break;
/*case 2614438: //USLT 歌詞
off += 4; //Languge: 4 bytes
len -= 4;
strLyrics = new String(b, off, len, TEXT_ENCODING[enc]);
break;*/
}
} catch (UnsupportedEncodingException e) {
return fsize + frame_header;
} finally {
id = null;
}
return fsize + frame_header;
}
}
【本程序下載地址】http://jmp123.sourceforge.net/