怎么用JAVA编写MP3解码器？解析文件信息

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;
}
}