Parsing ID3v2 tags in the MP3 files

This simple tag parser is very useful when you just need to get the basic information about the MP3 files, such as the title and the artist. Of course it could be extended to extract more information if necessary.

Apache license.

/*
 * Copyright (C) 2011-2012 George Yunaev
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 */
public class MetaInfoParser_MP3
{
	// Stores the data
	private String	m_artist;
	private String	m_title;

	public String getArtist()
	{
		return m_artist == null ? "Unknown" : m_artist;
	}

	public String getTitle()
	{
		return m_title == null ? "Unknown" : m_title;
	}

	// Same as parse but doesn't throw exceptions  
	public boolean parseNoThrow( File mp3file )
	{
		try
		{
			return parse( mp3file );
		}
		catch ( Exception ex )
		{
			return false;
		}
	}
	
	// Parses the MP3 file, and sets up artist and/or title.
	// Returns true if the file is MP3 file, it contains the ID3 tag, and at least artist or title is parsed  
	public boolean parse( File mp3file ) throws IOException
	{
		m_artist = null;
		m_title = null;
		
		// Read the first five bytes of the ID3 header - http://id3.org/id3v2-00
		RandomAccessFile file = new RandomAccessFile( mp3file, "r" );
		byte[] headerbuf = new byte[10];
		file.read( headerbuf );

		// Parse it quickly
		if ( headerbuf[0] != 'I' || headerbuf[1] != 'D' || headerbuf[2] != '3' )
		{
			file.close();
			return false;
		}

		// True if the tag is pre-V3 tag (shorter headers)
		final int TagVersion = headerbuf[3];
		
		// Check the version
		if ( TagVersion < 0 || TagVersion > 4 )
		{
			file.close();
			return false;
		}

		// Get the ID3 tag size and flags; see 3.1
		int tagsize = (headerbuf[9] & 0xFF) | ((headerbuf[8] & 0xFF) << 7 ) | ((headerbuf[7] & 0xFF) << 14 ) | ((headerbuf[6] & 0xFF) << 21 ) + 10;
		boolean uses_synch = (headerbuf[5] & 0x80) != 0 ? true : false;
		boolean has_extended_hdr = (headerbuf[5] & 0x40) != 0 ? true : false;
		
		// Read the extended header length and skip it
		if ( has_extended_hdr )
		{
			int headersize = file.read() << 21 | file.read() << 14 | file.read() << 7 | file.read(); 
			file.skipBytes( headersize - 4 );
		}
		
		// Read the whole tag
		byte[] buffer = new byte[ tagsize ];
		file.read( buffer );
		file.close();

		// Prepare to parse the tag
		int length = buffer.length;
		
		// Recreate the tag if desynchronization is used inside; we need to replace 0xFF 0x00 with 0xFF
		if ( uses_synch )
		{
			int newpos = 0;
			byte[] newbuffer = new byte[ tagsize ];
			
			for ( int i = 0; i < buffer.length; i++ )
			{
				if ( i < buffer.length - 1 && (buffer[i] & 0xFF) == 0xFF && buffer[i+1] == 0 )
				{
					newbuffer[newpos++] = (byte) 0xFF;
					i++;
					continue;
				}
				
				newbuffer[newpos++] = buffer[i];
			}

			length = newpos;
			buffer = newbuffer;
		}

		// Set some params
		int pos = 0;
		final int ID3FrameSize = TagVersion < 3 ? 6 : 10;
		
		// Parse the tags
		while ( true )
		{
			int rembytes = length - pos;
				
			// Do we have the frame header?
			if ( rembytes < ID3FrameSize )
				break;
			
			// Is there a frame?
			if ( buffer[pos] < 'A' || buffer[pos] > 'Z' )
				break;
			
			// Frame name is 3 chars in pre-ID3v3 and 4 chars after
			String framename;
			int framesize;
			
			if ( TagVersion < 3 )
			{
				framename = new String( buffer, pos, 3 );
				framesize = ((buffer[pos+5] & 0xFF) << 8 ) | ((buffer[pos+4] & 0xFF) << 16 ) | ((buffer[pos+3] & 0xFF) << 24 );
			}
			else
			{
				framename = new String( buffer, pos, 4 );
				framesize = (buffer[pos+7] & 0xFF) | ((buffer[pos+6] & 0xFF) << 8 ) | ((buffer[pos+5] & 0xFF) << 16 ) | ((buffer[pos+4] & 0xFF) << 24 );
			}
			
			if ( pos + framesize > length )
				break;
			
			if ( framename.equals( "TPE1" ) || framename.equals( "TPE2" ) || framename.equals( "TPE3" ) || framename.equals( "TPE" ) )
			{
				if ( m_artist == null )
					m_artist = parseTextField( buffer, pos + ID3FrameSize, framesize );
			}
				  
			if ( framename.equals( "TIT2" ) || framename.equals( "TIT" ) )
			{
				if ( m_title == null )
					m_title = parseTextField( buffer, pos + ID3FrameSize, framesize );
			}
					
			pos += framesize + ID3FrameSize;
			continue;
		}
		
		return m_title != null || m_artist != null;
	}

	private String parseTextField( final byte[] buffer, int pos, int size )
	{
		if ( size < 2 )
			return null;
		
		Charset charset;
		int charcode = buffer[pos]; 
		
		if ( charcode == 0 )
			charset = Charset.forName( "ISO-8859-1" );
		else if ( charcode == 3 )
			charset = Charset.forName( "UTF-8" );
		else
			charset = Charset.forName( "UTF-16" );
			
		return charset.decode( ByteBuffer.wrap( buffer, pos + 1, size - 1) ).toString();
	}
}