前文提到解析MP3标签，程序源码中也已经出现了调用解析MP3标签、打印MP3文件信息的功能，这儿先说说MP3文件信息的解析。

解析MP3的文件信息对MP3解码器来说只是一个附加功能，如果不加入这部分源码，同时删除掉前文源码中的相关调用，不影响解码播放。如果你想编写“迷你”型的MP3解码器，可以忽略这些附加的功能。

MP3的标签信息位于文件开始处或结尾处，用于表达MP3文件的相关信息，常见的有ID3、APE等。

ID3 V1 位于文件最后的128字节，共表示7个信息：

[0-2] 3 bytes: ID3 v1标识 -- 'TAG'[3—32] 30 bytes: 标题[33—62] 30 bytes: 艺术家[63—92] 30 bytes: 专辑名[93—96] 4 bytes: 发行年份[97—126] 30 bytes: v1.0 -- 注释/附加/备注信息v1.1 -- 前29 bytes注释/附加/备注信息，最后1 byte音轨信息

[127] 1 byte : 流派

从“标题”开始，每部分内容之间用'\0'（字符串结束标志）或'\20'（空格）隔开。

ID3 V2 表示的信息更丰富，结构更复杂，位于文件开始处或位于APE标签之后。ID3 V2的详细内容请参见http://www.id3.org/id3v2.3.0

APE V1 & V2 位于文件开始处或ID3 V2之后。详细内容请参见http://cn.bing.com/reference/semhtml/APE_tag（External links下的链接就是APE V2）

本文只解析ID3 V1的那几项简单的内容，JAVA的字符集转换很方便，所以解析ID3 V2的代码很简洁。ID3 V2的每一帧都以“Frame ID”开始，例如TT2或TIT2表示“标题”，程序中通过计算ID的哈希值来识别不同的帧。ID3Tag.java源码：

Java代码 package tag; import java.io.UnsupportedEncodingException; public final class ID3Tag { // ID3v1 & ID3v2 private String strTitle; private String strArtist; private String strAlbum; private String strYear; // ID3v2 //private String strLyrics; // (内嵌)歌词 private int intVersion; private int intExHeaderSize; private boolean boolID3v2Footer; //TEXT_ENCODING[0]应由 "ISO-8859-1" 改为 "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 标题: " + strTitle); if (strArtist != null) System.out.println("\r 艺术家: " + strArtist); if (strAlbum != null) System.out.println("\r 唱片集: " + strAlbum); if (strYear != null) System.out.println("\r 发行年: " + strYear); } public void destroy() { strTitle = strArtist = strAlbum = strYear = null; //strLyrics = null; intVersion = intExHeaderSize = 0; boolID3v2Footer = false; } //-------------------------------------------------------------------- // ID3v1 public boolean checkID3V1(byte[] b) { return b[0] == 'T' && b[1] == 'A' && b[2] == 'G'; } public void parseID3V1(byte[] b) { if (b.length < 128 || checkID3V1(b) == false) return; byte[] buf = new byte[125]; System.arraycopy(b, 3, buf, 0, 125); for (int i = 0; i < buf.length; i++) if (buf[i] == 0) buf[i] = 0x20; //空格 if (strTitle == null) strTitle = new String(buf, 0, 30).trim(); if (strTitle.length() == 0) strTitle = null; if (strArtist == null) strArtist = new String(buf, 30, 30).trim(); if (strArtist.length() == 0) strArtist = null; if (strAlbum == null) strAlbum = new String(buf, 60, 30).trim(); if (strAlbum.length() == 0) strAlbum = null; if (strYear == null) strYear = new String(buf, 90, 4).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; //设置为1表示有扩展头 boolID3v2Footer = (b[off+5] & 0x10) != 0; int size = synchSafeInt(b, off+6); size += 10; // ID3 header:10bytes return size; } //b[off..]不含ID3v2 头(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); } private 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 en = b[off]; len--; // Text encoding: 1 byte off++; // Text encoding: 1 byte if (len <= 0 || off + len > max_size || en < 0 || en >= 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[en]); //System.out.println("frame size: " + fsize); try { switch(id.hashCode()) { case 83378: //TT2 v2.2 case 2575251: //TIT2 标题 if (strTitle == null) strTitle = new String(b, off, len, TEXT_ENCODING[en]).trim(); break; case 83552: case 2590194: //TYER 发行年 if (strYear == null) strYear = new String(b, off, len, TEXT_ENCODING[en]).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[en]).trim(); break; case 83253: case 2581512: //TPE1 艺术家 if (strArtist == null) strArtist = new String(b, off, len, TEXT_ENCODING[en]).trim(); break; case 2583398: //TRCK 音轨 break; /*case 2614438: //USLT 歌词 off += 4; //Languge: 4 bytes len -= 4; strLyrics = new String(b, off, len, TEXT_ENCODING[en]); break;*/ } } catch (UnsupportedEncodingException e) { return fsize + frame_header; } finally { id = null; } return fsize + frame_header; } }