初學者的一個簡單實現，感覺比較容易理解  

 public String md5Digest(String src) throws Exception {  
// 定義數字簽名方法, 可用：MD5, SHA-1           MessageDigest md = MessageDigest.getInstance("MD5");  
byte[] b = md.digest(src.getBytes("utf-8"));  

return this.byte2HexStr(b);  
    }


private String byte2HexStr(byte[] b) {  
         StringBuilder sb = new
 
 StringBuilder();  
for (int i = 0; i < b.length; i++) {  
             String s = Integer.toHexString(b[i] & 0xFF);  
if (s.length() == 1) {  
                  sb.append("0");  
             }  

             sb.append(s.toUpperCase());  
          }  

return sb.toString();  
     }