Azureus源码剖析（二） ---解析Torrent种子文件（下）

     对Azureus中解析Torrent种子文件的源代码进行了适度裁剪，得到下面这样一个解析torrent文件的示例代码，如下所示：

/*
 * BeDecoder.java
 *
 */

package com.vista.test;

import java.io.*;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

/**
 * A set of utility methods to decode a bencoded array of byte into a Map.
 * integer are represented as Long, String as byte[], dictionnaries as Map, and list as List.
 *
 */
public class BDecoder
{
    //字符集
    public static final String BYTE_ENCODING = "UTF8";
    public static Charset    BYTE_CHARSET;
    static
    {
        try
        {
            BYTE_CHARSET     = Charset.forName(BYTE_ENCODING);
        }
        catch( Throwable e )
        {
            e.printStackTrace();
        }
    }
    private static final boolean TRACE    = true;

    private boolean recovery_mode;

    public static Map decode(BufferedInputStream is) throws Exception
    {//解码
        return( new BDecoder().decodeStream( is ));
    }

    public BDecoder()
    {
    }

    public Map decodeStream(BufferedInputStream data ) throws Exception
    {
        Object    res = decodeInputStream(new BDecoderInputStreamStream(data), 0);//0指定递归层次从第一层开始
        if ( res == null )
        {
            throw( new Exception( "BDecoder: zero length file" ));

        }
        else if ( !(res instanceof Map ))
        {
            throw( new Exception( "BDecoder: top level isn't a Map" ));
        }
        return((Map)res );
    }

    /**
     *
     * @param dbis
     * @param nesting 递归层次
     * @throws Exception
     */
    private Object decodeInputStream(BDecoderInputStream dbis,int nesting ) throws Exception
    {
        if (nesting == 0 && !dbis.markSupported())
        {
            throw new IOException("InputStream must support the mark() method");
        }
        //set a mark
        dbis.mark(Integer.MAX_VALUE);

        //read a byte
        int tempByte = dbis.read();//读一个字节

        //decide what to do
        switch (tempByte)
        {
            case 'd' :
            {//是字典
                //create a new dictionary object
                Map tempMap = new HashMap();
                try
                {
                    //get the key
                    byte[] tempByteArray = null;
                    while ((tempByteArray = (byte[]) decodeInputStream(dbis, nesting+1)) != null)
                    {
                        //decode some more
                        Object value = decodeInputStream(dbis,nesting+1);//读值
                        // value interning is too CPU-intensive, let's skip that for now
                        //if(value instanceof byte[] && ((byte[])value).length < 17)
                        //value = StringInterner.internBytes((byte[])value);
                        // keys often repeat a lot - intern to save space
                        String    key = null;

                        if ( key == null )
                        {
                            CharBuffer    cb = BYTE_CHARSET.decode(ByteBuffer.wrap(tempByteArray));
                            key = new String(cb.array(),0,cb.limit());//键
                        }
                        if ( TRACE )
                        {
                            System.out.println( key + "->" + value + ";" );
                        }

                            // recover from some borked encodings that I have seen whereby the value has
                            // not been encoded. This results in, for example,
                            // 18:azureus_propertiesd0:e
                            // we only get null back here if decoding has hit an 'e' or end-of-file
                            // that is, there is no valid way for us to get a null 'value' here

                        if ( value == null )
                        {
                            //Debug.out( "Invalid encoding - value not serialsied for '" + key + "' - ignoring" );
                            break;
                        }
                        tempMap.put( key, value);//放入结果集中
                    }

                    dbis.mark(Integer.MAX_VALUE);
                    tempByte = dbis.read();
                    dbis.reset();
                    if ( nesting > 0 && tempByte == -1 )
                    {
                        throw( new Exception( "BDecoder: invalid input data, 'e' missing from end of dictionary"));
                    }
                }catch( Throwable e )
                {
                    if ( !recovery_mode )
                    {
                        if ( e instanceof IOException )
                        {
                            throw((IOException)e);
                        }

                        throw( new IOException(e.getMessage()));
                    }
                }
                return tempMap;
            }
        case 'l' :
        {
            //create the list
            ArrayList tempList = new ArrayList();
            try
            {
                //create the key
                Object tempElement = null;
                while ((tempElement = decodeInputStream(dbis, nesting+1)) != null)
                {
                    //add the element
                    tempList.add(tempElement);//读取列表元素并加入列表中
                }

                tempList.trimToSize();
                dbis.mark(Integer.MAX_VALUE);
                tempByte = dbis.read();
                dbis.reset();
                if ( nesting > 0 && tempByte == -1 )
                {
                    throw( new Exception( "BDecoder: invalid input data, 'e' missing from end of list"));
                }
            }
            catch( Throwable e )
            {
                if ( !recovery_mode )
                {
                    if ( e instanceof IOException )
                    {
                        throw((IOException)e);
                    }
                    throw( new IOException(e.getMessage()));
                }
            }
                //return the list
            return tempList;
        }
        case 'e' :
        case -1 :
            return null;//当前结束

        case 'i' :
            return new Long(getNumberFromStream(dbis, 'e'));//整数

        case '0' :
        case '1' :
        case '2' :
        case '3' :
        case '4' :
        case '5' :
        case '6' :
        case '7' :
        case '8' :
        case '9' :
                //move back one
            dbis.reset();
                //get the string
            return getByteArrayFromStream(dbis);//读取指定长度字符串

        default :
        {
            int    rem_len = dbis.available();
            if ( rem_len > 256 )
            {
                rem_len    = 256;
            }

            byte[] rem_data = new byte[rem_len];
            dbis.read( rem_data );
            throw( new Exception("BDecoder: unknown command '" + tempByte + ", remainder = " + new String( rem_data )));
        }
        }
    }

    /** only create the array once per decoder instance (no issues with recursion as it's only used in a leaf method)
     */
    private final char[] numberChars = new char[32];

    private long getNumberFromStream(BDecoderInputStream dbis, char parseChar) throws IOException
    {
        int tempByte = dbis.read();
        int pos = 0;
        while ((tempByte != parseChar) && (tempByte >= 0))
        {//读取整数字节，直到终结字符'e'
            numberChars[pos++] = (char)tempByte;
            if ( pos == numberChars.length )
            {
                throw( new NumberFormatException( "Number too large: " + new String(numberChars,0,pos) + "" ));
            }
            tempByte = dbis.read();
        }

        //are we at the end of the stream?
        if (tempByte < 0)
        {
            return -1;
        }
        else if ( pos == 0 )
        {
            // support some borked impls that sometimes don't bother encoding anything
            return(0);
        }
        return( parseLong( numberChars, 0, pos ));//转换为Long型整数
    }

    public static long parseLong(char[]    chars,int start,int    length )
    {//转换为Long型整数
        long result = 0;
        boolean negative = false;
        int     i     = start;
        int    max = start + length;
        long limit;
        if ( length > 0 )
        {
            if ( chars[i] == '-' )
            {
                negative = true;
                limit = Long.MIN_VALUE;
                i++;
            }
            else
            {
                limit = -Long.MAX_VALUE;
            }

            if ( i < max )
            {
                int digit = chars[i++] - '0';
                if ( digit < 0 || digit > 9 )
                {
                    throw new NumberFormatException(new String(chars,start,length));
                }
                else
                {
                    result = -digit;
                }
            }

            long multmin = limit / 10;

            while ( i < max )
            {
                // Accumulating negatively avoids surprises near MAX_VALUE
                int digit = chars[i++] - '0';
                if ( digit < 0 || digit > 9 )
                {
                    throw new NumberFormatException(new String(chars,start,length));
                }
                if ( result < multmin )
                {
                    throw new NumberFormatException(new String(chars,start,length));
                }
                result *= 10;
                if ( result < limit + digit )
                {
                    throw new NumberFormatException(new String(chars,start,length));
                }

                result -= digit;
            }
        }
        else
        {
            throw new NumberFormatException(new String(chars,start,length));
        }

        if ( negative )
        {
            if ( i > start+1 )
            {
                return result;

            }
            else
            {    /* Only got "-" */
                throw new NumberFormatException(new String(chars,start,length));
            }
        }
        else
        {
            return -result;
        }
    }

    private byte[] getByteArrayFromStream(BDecoderInputStream dbis ) throws IOException
    {
        int length = (int) getNumberFromStream(dbis, ':');
        if (length < 0)
        {
            return null;
        }
        // note that torrent hashes can be big (consider a 55GB file with 2MB pieces
        // this generates a pieces hash of 1/2 meg
        if ( length > 8*1024*1024 )
        {
            throw( new IOException( "Byte array length too large (" + length + ")"));
        }

        byte[] tempArray = new byte[length];
        int count = 0;
        int len = 0;
        //get the string
        while (count != length && (len = dbis.read(tempArray, count, length - count)) > 0)
        {
            count += len;
        }
        if ( count != tempArray.length )
        {
            throw( new IOException( "BDecoder::getByteArrayFromStream: truncated"));
        }
        return tempArray;
    }

    public void setRecoveryMode(boolean    r )
    {
        recovery_mode    = r;
    }

    public static void print(PrintWriter writer,Object obj )
    {
        print( writer, obj, "", false );
    }

    private static void print(PrintWriter writer,Object obj,String indent,boolean    skip_indent )
    {
        String    use_indent = skip_indent?"":indent;
        if ( obj instanceof Long )
        {
            writer.println( use_indent + obj );

        }
        else if ( obj instanceof byte[])
        {
            byte[]    b = (byte[])obj;
            if ( b.length==20 )
            {
                writer.println( use_indent + " { "+ ByteFormatter.nicePrint( b )+ " }" );
            }
            else if ( b.length < 64 )
            {
                writer.println( new String(b) + " [" + ByteFormatter.encodeString( b ) + "]" );
            }else{
                writer.println( "[byte array length " + b.length );
            }
        }else if ( obj instanceof String )
        {
            writer.println( use_indent + obj );

        }
        else if ( obj instanceof List )
        {
            List    l = (List)obj;
            writer.println( use_indent + "[" );
            for (int i=0;i<l.size();i++)
            {
                writer.print( indent + "  (" + i + ") " );
                print( writer, l.get(i), indent + "    ", true );
            }
            writer.println( indent + "]" );

        }
        else
        {
            Map    m = (Map)obj;
            Iterator    it = m.keySet().iterator();
            while( it.hasNext())
            {
                String    key = (String)it.next();
                if ( key.length() > 256 )
                {
                    writer.print( indent + key.substring(0,256) + " = " );
                }
                else
                {
                    writer.print( indent + key + " = " );
                }
                print( writer, m.get(key), indent + "  ", true );
            }
        }
    }

    private static void print(File f,File output)
    {
        try
        {
            BDecoder    decoder = new BDecoder();//解码器
            PrintWriter    pw = new PrintWriter( new FileWriter( output ));//输出结果
            print( pw, decoder.decodeStream( new BufferedInputStream( new FileInputStream( f ))));
            pw.flush();
        }
        catch( Throwable e )
        {
            e.printStackTrace();
        }
    }

    private interface BDecoderInputStream
    {
        public int read() throws IOException;
        public int read(byte[] buffer)throws IOException;
        public int read(byte[]     buffer,int    offset,int    length )throws IOException;
        public int available()throws IOException;
        public boolean markSupported();
        public void mark(int limit );
        public void reset() throws IOException;
    }

    private class BDecoderInputStreamStream implements BDecoderInputStream
    {
        final private BufferedInputStream is;
        private BDecoderInputStreamStream(BufferedInputStream _is )
        {
            is    = _is;
        }
        /**
         * 从此输入流中读取下一个数据字节。返回一个 0 到 255 范围内的 int 字节值。
         * 如果因为已经到达流末尾而没有字节可用，则返回 -1。
         * 在输入数据可用、检测到流末尾或抛出异常之前，此方法将一直阻塞。
         */
        public int read()throws IOException
        {
            return( is.read());
        }
        /**
         * 从此输入流中将 byte.length 个字节的数据读入一个 byte 数组中。在某些输入可用之前，此方法将阻塞。
         */
        public int read(byte[] buffer )throws IOException
        {
            return( is.read( buffer ));
        }
        /**
         * 从此字节输入流中给定偏移量处开始将各字节读取到指定的 byte 数组中。
         */
        public int read(byte[]     buffer,int    offset,int    length)throws IOException
        {
            return( is.read( buffer, offset, length ));
        }
        /**
         * 返回可以从此输入流读取（或跳过）、且不受此输入流接下来的方法调用阻塞的估计字节数。
         */
        public int available() throws IOException
        {
            return( is.available());
        }

        /**
         * 测试此输入流是否支持 mark 和 reset 方法。
         */
        public boolean markSupported()
        {
            return( is.markSupported());
        }

        /**
         * 在输入流中的当前位置上作标记。reset 方法的后续调用将此流重新定位在最后标记的位置上，以便后续读取操作重新读取相同的字节。
         * @param limit 在标记位置变为无效之前可以读取字节的最大限制。
         */
        public void mark(int limit )
        {
            is.mark( limit );
        }
        /**
         * 将此流重新定位到对此输入流最后调用 mark 方法时的位置。
         */
        public void reset()throws IOException
        {
            is.reset();
        }
    }

    public static void main(String[]    args )
    {
        print(new File( "C:\\1001.torrent" ),new File( "C:\\tables.txt" ));
    }
}