JAVA实现MD5算法

public class MD5 {  
    /* 
     * 四个链接变量 
     */  
    private final int A=0x67452301;  
    private final int B=0xefcdab89;  
    private final int C=0x98badcfe;  
    private final int D=0x10325476;  
    /* 
     * ABCD的临时变量 
     */  
    private int Atemp,Btemp,Ctemp,Dtemp;  
      
    /* 
     * 常量ti 
     * 公式:floor(abs(sin(i + 1)) × (2 pow 32) 
     */  
    private final int K[] = {   
            0xd76aa478,0xe8c7b756,0x242070db,0xc1bdceee,0xf57c0faf,0x4787c62a,0xa8304613,0xfd469501,0x698098d8,0x8b44f7af,0xffff5bb1,0x895cd7be,0x6b901122,0xfd987193,0xa679438e,0x49b40821,0xf61e2562,0xc040b340,0x265e5a51,0xe9b6c7aa,0xd62f105d,0x02441453,0xd8a1e681,0xe7d3fbc8,0x21e1cde6,0xc33707d6,0xf4d50d87,0x455a14ed,0xa9e3e905,0xfcefa3f8,0x676f02d9,0x8d2a4c8a,0xfffa3942,0x8771f681,0x6d9d6122,0xfde5380c,0xa4beea44,0x4bdecfa9,0xf6bb4b60,0xbebfbc70,0x289b7ec6,0xeaa127fa,0xd4ef3085,0x04881d05,0xd9d4d039,0xe6db99e5,0x1fa27cf8,0xc4ac5665,0xf4292244,0x432aff97,0xab9423a7,0xfc93a039,0x655b59c3,0x8f0ccc92,0xffeff47d,0x85845dd1,0x6fa87e4f,0xfe2ce6e0,0xa3014314,0x4e0811a1,0xf7537e82,0xbd3af235,0x2ad7d2bb,0xeb86d391 };  
    /* 
     * 向左位移数,计算方法未知 
     */  
    private final int s[] = { 7,12,17,22,7,5,9,14,20,4,11,16,23,6,10,15,21,21 };  
      
  
    /* 
     * 初始化函数 
     */  
    private void init(){  
        Atemp=A;  
        Btemp=B;  
        Ctemp=C;  
        Dtemp=D;  
    }  
    /* 
     * 移动一定位数 
     */  
    private int shift(int a,int s){  
        return (a<<s)|(a>>>(32-s));//右移的时候，高位一定要补零，而不是补充符号位  
    }  
    /* 
     * 主循环 
     */  
    private void MainLoop(int M[]){  
        int F,g;  
        int a=Atemp;  
        int b=Btemp;  
        int c=Ctemp;  
        int d=Dtemp;  
        for(int i=0;i<64;i++){  
            if(i<16){  
                F=(b&c)|((~b)&d);  
                g=i;  
            }  
            else if(i<32){  
                F=(d&b)|((~d)&c);  
                g=(5*i+1)%16;  
            }  
            else if(i<48){  
                F=b^c^d;  
                g=(3*i+5)%16;  
            }  
            else{  
                F=c^(b|(~d));  
                g=(7*i)%16;  
            }  
            int tmp=d;  
            d=c;  
            c=b;  
            b=b+shift(a+F+K[i]+M[g],s[i]);  
            a=tmp;            
        }  
        Atemp=a+Atemp;  
        Btemp=b+Btemp;  
        Ctemp=c+Ctemp;  
        Dtemp=d+Dtemp;  
          
    }  
    /* 
     * 填充函数 
     * 处理后应满足bits ≡ 448 (mod 512),字节就是bytes ≡ 56（mode64) 
     * 填充方式为先加一个0,其它位补零 
     * 最后加上64位的原来长度 
     */  
    private int[] add(String str){  
        int num=((str.length()+8)/64)+1;//以512位，64个字节为一组  
        int strByte[]=new int[num*16];//64/4=16，所以有16个整数  
        for(int i=0;i<num*16;i++){//全部初始化0  
            strByte[i]=0;  
        }  
        int i;  
        for(i=0;i<str.length();i++){  
            strByte[i>>2]|=str.charAt(i)<<((i%4)*8);//一个整数存储四个字节，小端序  
        }  
        strByte[i>>2]|=0x80<<((i%4)*8);//尾部添加1  
        /* 
         * 添加原长度，长度指位的长度，所以要乘8，然后是小端序，所以放在倒数第二个,这里长度只用了32位 
         */  
        strByte[num*16-2]=str.length() * 8;  
        return strByte;  
    }  
    /* 
     * 调用函数 
     */  
    public String getMD5(String source){  
        init();  
        int strByte[]=add(source);  
        for(int i=0;i<strByte.length/16;i+=16){  
            int num[]=new int[16];  
            for(int j=0;j<16;j++){  
                num[j]=strByte[i*16+j];  
            }  
            MainLoop(num);  
        }  
        return changeHex(Atemp)+changeHex(Btemp)+changeHex(Ctemp)+changeHex(Dtemp);  
          
    }  
    /* 
     * 整数变成16进制字符串 
     */  
    private String changeHex(int a){  
        String str="";  
        for(int i=0;i<4;i++){  
            str+=Integer.toHexString(((a>>i*8)%(1<<8))&0xff);  
        }  
        return str;  
    }  
    /* 
     * 单例 
     */  
    private static MD5 instance;  
    public static MD5 getInstance(){  
        if(instance==null){  
            instance=new MD5();  
        }  
        return instance;  
    }  
    private MD5(){};  
}  

import tools.com.edu.hdu.MD5;  
  
public class Test {  
    public static void main(String[] args) {  
        String str=MD5.getInstance().getMD5("");  
        System.out.println(str);  
      
    }  
}