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，如果读取的是网络文件而服务器又不支持随机读取的话，意味着不对对其解析这部分信息。这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

      APE V1 & V2 位于文件开始处或ID3 V2之后。详细内容请参见 （External links下的链接就是APE V2）。有很多MP3的标签信息很混乱，内容重复。由于APE标签出现并在MP3中大量应用得比ID3晚，MP3文件的“有利”位置都被ID3占用，所以APE标签位于文件中的位置让人捉摸不透，情况很复杂，对网络文件来说，判断APE标签的位置要反复在文件中定位，况且有的服务器根本就不支持随机访问，所以我这儿就放弃了对APE的解析，尽管APE的解析过程并不复杂。

 

       本文只解析ID3 V1的具有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。需要指出的是，在解码器读入文件时自动对标签信息进行解析，调用IRandomAccess接口的tagAvailable()方法查询是否已经完成对tag的解析完毕，对网络文件，是开线程以后台方式解析。如果对其解析完毕，调用getTitle()等方法就可以返回其内容，如果MP3文件本身没有标签信息，返回值为null。具体调用方法见 下的API文档。

 

ID3Tag.java源码：

/** 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 
     .** If you would like to negotiate alternate licensing terms, you may do* so by contacting the author: 
     */package jmp123.tag;import java.io.UnsupportedEncodingException;/* ID3v1: [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*/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;	}	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;		//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);	}	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  标题				if (strTitle == null)					strTitle = new String(b, off, len, TEXT_ENCODING[enc]).trim();				break;			case 83552:			case 2590194:	//TYER  发行年				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  艺术家				if (strArtist == null)					strArtist = new String(b, off, len, TEXT_ENCODING[enc]).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[enc]);				break;*/			}		} catch (UnsupportedEncodingException e) {			return fsize + frame_header;		} finally {			id = null;		}		return fsize + frame_header;	}}

 

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