首先要引入Apache下的commons-codec.jar

import org.apache.commons.codec.digest.DigestUtils;

public class MD5Util {

	public static String md5(String str) {
		return DigestUtils.md5Hex(str);
	}
	
	private static final String salt = "1a2b3c4d";
	
	public static String inputPassToFormPass(String inputPass, String salt) {
		String str = "" + salt.charAt(0) + salt.charAt(2) + inputPass + salt.charAt(5) + salt.charAt(4);
		return md5(str);
	}
	
	public static String formPassToDBPass(String formPass, String saltDB) {
		String str = "" + saltDB.charAt(0) + saltDB.charAt(2) + formPass + saltDB.charAt(5) + saltDB.charAt(4);
		return md5(str);
	}
	
	public static void main(String[] args) {
		System.out.println(inputPassToFormPass("123456", salt));//d3b1294a61a07da9b49b6e22b2cbd7f9
		System.out.println(formPassToDBPass(inputPassToFormPass("123456", salt), salt));//b7797cce01b4b131b433b6acf4add449
	}

}