用JAVA編寫(xiě)MP3解碼器——解析文件信息

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前文提到解析MP3標(biāo)簽，程序源碼中也已經(jīng)出現(xiàn)了調(diào)用解析MP3標(biāo)簽、打印MP3文件信息的功能，這兒先說(shuō)說(shuō)MP3文件信息的解析。

解析MP3的文件信息對(duì)MP3解碼器來(lái)說(shuō)只是一個(gè)附加功能，如果不加入這部分源碼，同時(shí)刪除掉前文源碼中的相關(guān)調(diào)用，不影響解碼播放。如果你想編寫(xiě)“迷你”型的MP3解碼器，可以忽略這些附加的功能。

MP3的標(biāo)簽信息位于文件開(kāi)始處或結(jié)尾處，用于表達(dá)MP3文件的相關(guān)信息，常見(jiàn)的有ID3、APE等。

ID3 V1 位于文件最后的128字節(jié)，共表示7個(gè)信息：

[0-2] 3 bytes: ID3 v1標(biāo)識(shí) -- 'TAG'
[3—32] 30 bytes: 標(biāo)題
[33—62] 30 bytes: 藝術(shù)家
[63—92] 30 bytes: 專輯名
[93—96] 4 bytes: 發(fā)行年份
[97—126] 30 bytes: v1.0 -- 注釋/附加/備注信息
v1.1 -- 前29 bytes注釋/附加/備注信息，最后1 byte音軌信息

[127] 1 byte : 流派

從“標(biāo)題”開(kāi)始，每部分內(nèi)容之間用'\0'（字符串結(jié)束標(biāo)志）或'\20'（空格）隔開(kāi)。

ID3 V2 表示的信息更豐富，結(jié)構(gòu)更復(fù)雜，位于文件開(kāi)始處或位于APE標(biāo)簽之后。ID3 V2的詳細(xì)內(nèi)容請(qǐng)參見(jiàn)http://www.id3.org/id3v2.3.0

APE V1 & V2 位于文件開(kāi)始處或ID3 V2之后。詳細(xì)內(nèi)容請(qǐng)參見(jiàn)http://cn.bing.com/reference/semhtml/APE_tag（External links下的鏈接就是APE V2）

本文只解析ID3 V1的那幾項(xiàng)簡(jiǎn)單的內(nèi)容，JAVA的字符集轉(zhuǎn)換很方便，所以解析ID3 V2的代碼很簡(jiǎn)潔。ID3 V2的每一幀都以“Frame ID”開(kāi)始，例如TT2或TIT2表示“標(biāo)題”，程序中通過(guò)計(jì)算ID的哈希值來(lái)識(shí)別不同的幀。ID3Tag.java源碼：

Java代碼
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; // (內(nèi)嵌)歌詞
private int intVersion;
private int intExHeaderSize;
private boolean boolID3v2Footer;
//TEXT_ENCODING[0]應(yīng)由 "ISO-8859-1" 改為 "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(biāo)題: " + strTitle);
if (strArtist != null)
System.out.println("\r 藝術(shù)家: " + strArtist);
if (strAlbum != null)
System.out.println("\r 唱片集: " + strAlbum);
if (strYear != null)
System.out.println("\r 發(fā)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; //設(shè)置為1表示有擴(kuò)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);
}

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 標(biāo)題
if (strTitle == null)
strTitle = new String(b, off, len, TEXT_ENCODING[en]).trim();
break;
case 83552:
case 2590194: //TYER 發(fā)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 藝術(shù)家
if (strArtist == null)
strArtist = new String(b, off, len, TEXT_ENCODING[en]).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[en]);
break;*/
}
} catch (UnsupportedEncodingException e) {
return fsize + frame_header;
} finally {
id = null;
}
return fsize + frame_header;
}
}