import java.util.Arrays;
public class ChangeBytesToNumber
{
    public static void main(String[] args)
    {
        long l = -13912345678L;
        byte[] b = numberToBytes(l);            //数转成数组。
        long number=bytesToNumber(b);            //还原成数。
        System.out.println("l is  "+l);
        System.out.println("number is  "+number);
    }

    public static long bytesToNumber(byte[] b)
    {
        byte[] b1=Arrays.copyOfRange(b,0,8);        //复制数值部分。
        StringBuffer sb=new StringBuffer();
        String tem=null;
        int length=b1.length;
        for(int i=0;i<length;i++)            //组成16进制字符串。
        {
            tem=(b1[length-i-1]>=0)?Integer.toHexString(b1[length-i-1]):
                Integer.toHexString(b1[length-i-1]).substring(6);
            sb.append(tem);
        }
        long result=Long.parseLong(sb.toString(),16);    //转成long型值。
        result=(b[8]==0)?result:0-result;        //处理符号。            
        return result;
    }
    public static byte[] numberToBytes(long l)
    {
        byte [] b = new byte[9];             
        b[8]=(l>0)?(byte)0:(byte)1;        //l>0,b[8]=0；l<0，b[8]=1。
        l=(l>0)?l:0-l;                //处理l的绝对值。
        b[0] = (byte) (l & 0x000000000000FFL);  
        b[1] = (byte) ((l & 0x0000000000FF00L) >> 8);  
        b[2] = (byte) ((l & 0x0000000000FF0000L) >> 16);  
        b[3] = (byte) ((l & 0x00000000FF000000L) >> 24);  
        b[4] = (byte) ((l & 0x000000FF00000000L) >> 32);  
        b[5] = (byte) ((l & 0x0000FF0000000000L) >> 40);  
        b[6] = (byte) ((l & 0x00FF000000000000L) >> 48);  
        b[7] = (byte) ((l & 0xFF00000000000000L) >> 56);
        return b;
    }
}

------解决方案--------------------
Java code

/**
 * 各基础类型与byte之间的转换
 * @author shanl
 *
 */
public class Utility {
    
    /**
     * 将short转成byte[2]
     * @param a
     * @return
     */
    public static byte[] short2Byte(short a){
        byte[] b = new byte[2];
        
        b[0] = (byte) (a >> 8);
        b[1] = (byte) (a);
        
        return b;
    }
    
    /**
     * 将short转成byte[2]
     * @param a
     * @param b
     * @param offset b中的偏移量
     */
    public static void short2Byte(short a, byte[] b, int offset){
        b[offset] = (byte) (a >> 8);
        b[offset+1] = (byte) (a);
    }
    
    /**
     * 将byte[2]转换成short
     * @param b
     * @return
     */
    public static short byte2Short(byte[] b){
        return (short) (((b[0] & 0xff) << 8) | (b[1] & 0xff));
    }
    
    /**
     * 将byte[2]转换成short
     * @param b
     * @param offset
     * @return 
     */
    public static short byte2Short(byte[] b, int offset){
        return (short) (((b[offset] & 0xff) << 8) | (b[offset+1] & 0xff));
    }

    /**
     * long转byte[8]
     * 
     * @param a
     * @param b
     * @param offset
     *            b的偏移量
     */
    public static void long2Byte(long a, byte[] b, int offset) {        
        b[offset + 0] = (byte) (a >> 56);
        b[offset + 1] = (byte) (a >> 48);
        b[offset + 2] = (byte) (a >> 40);
        b[offset + 3] = (byte) (a >> 32);

        b[offset + 4] = (byte) (a >> 24);
        b[offset + 5] = (byte) (a >> 16);
        b[offset + 6] = (byte) (a >> 8);
        b[offset + 7] = (byte) (a);
    }

    /**
     * byte[8]转long
     * 
     * @param b
     * @param offset
     *            b的偏移量
     * @return
     */
    public static long byte2Long(byte[] b, int offset) {
         return ((((long) b[offset + 0] & 0xff) << 56)
         | (((long) b[offset + 1] & 0xff) << 48)
         | (((long) b[offset + 2] & 0xff) << 40)
         | (((long) b[offset + 3] & 0xff) << 32)
         
         | (((long) b[offset + 4] & 0xff) << 24)
         | (((long) b[offset + 5] & 0xff) << 16)
         | (((long) b[offset + 6] & 0xff) << 8)
         | (((long) b[offset + 7] & 0xff) << 0));
    }

    /**
     * byte[8]转long
     * 
     * @param b
     * @return
     */
    public static long byte2Long(byte[] b) {
         return
         ((b[0]&0xff)<<56)|
         ((b[1]&0xff)<<48)|
         ((b[2]&0xff)<<40)|
         ((b[3]&0xff)<<32)|
        
         ((b[4]&0xff)<<24)|
         ((b[5]&0xff)<<16)|
         ((b[6]&0xff)<<8)|
         (b[7]&0xff);
    }

    /**
     * long转byte[8]
     * 
     * @param a
     * @return
     */
    public static byte[] long2Byte(long a) {
        byte[] b = new byte[4 * 2];

        b[0] = (byte) (a >> 56);
        b[1] = (byte) (a >> 48);
        b[2] = (byte) (a >> 40);
        b[3] = (byte) (a >> 32);
        
        b[4] = (byte) (a >> 24);
        b[5] = (byte) (a >> 16);
        b[6] = (byte) (a >> 8);
        b[7] = (byte) (a >> 0);

        return b;
    }

    /**
     * byte数组转int
     * 
     * @param b
     * @return
     */
    public static int byte2Int(byte[] b) {
        return ((b[0] & 0xff) << 24) | ((b[1] & 0xff) << 16)
                | ((b[2] & 0xff) << 8) | (b[3] & 0xff);
    }

    /**
     * byte数组转int
     * 
     * @param b
     * @param offset
     * @return
     */
    public static int byte2Int(byte[] b, int offset) {
        return ((b[offset++] & 0xff) << 24) | ((b[offset++] & 0xff) << 16)
                | ((b[offset++] & 0xff) << 8) | (b[offset++] & 0xff);
    }

    /**
     * int转byte数组
     * 
     * @param a
     * @return
     */
    public static byte[] int2Byte(int a) {
        byte[] b = new byte[4];
        b[0] = (byte) (a >> 24);
        b[1] = (byte) (a >> 16);
        b[2] = (byte) (a >> 8);
        b[3] = (byte) (a);

        return b;
    }

    /**
     * int转byte数组
     * 
     * @param a
     * @param b
     * @param offset
     * @return
     */
    public static void int2Byte(int a, byte[] b, int offset) {        
        b[offset++] = (byte) (a >> 24);
        b[offset++] = (byte) (a >> 16);
        b[offset++] = (byte) (a >> 8);
        b[offset++] = (byte) (a);
    }
}