FULL PRODUCT VERSION :
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) Client VM (build 24.51-b03, mixed mode, sharing)

java version "1.8.0_11"
Java(TM) SE Runtime Environment (build 1.8.0_11-b12)
Java HotSpot(TM) Client VM (build 25.11-b03, mixed mode)

java version "1.8.0"
Java(TM) SE Runtime Environment (build 1.8.0-b132)
Java HotSpot(TM) Client VM (build 25.0-b70, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
window 7 professional 6.1.7601.17514

A DESCRIPTION OF THE PROBLEM :
when i try to hash a string pair and generate a byte array with encoding "GBK", and i test the function several times with four string pairs. But i get different result between jdk7 and jdk8
A ("superuser" and "") result:equal
B ("superuser" and "superuser") result:not equal
C ("test62" and "test62") result:not equal
D ("user6" and "user6") result:equal


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
all result will be same between jdk7 and jdk8 for every string pair

ERROR MESSAGES/STACK TRACES THAT OCCUR :
in jdk 7 i get :
C:\Java\jdk1.7.0_51\bin>java Program test62 test62
[-51, -126, -79, -62, 35, 41, 93, -102, -93, -21, -105, 58, 74, 102, 116, 17, 23, 14, 63]

in jdk 8 i get:
C:\Java\jdk\bin>java Program test62 test62
[-51, -126, -79, -62, 35, 41, 93, -102, -93, -21, -105, 58, 74, 102, 116, 17, 23, 14, 63, 51]

C:\Users\bfQ463\Downloads\jre-8u11-windows-i586\jre1.8.0_11\bin>java Program test62 test62
[-51, -126, -79, -62, 35, 41, 93, -102, -93, -21, -105, 58, 74, 102, 116, 17, 23, 14, 63, 51]

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.UnsupportedEncodingException;
import java.nio.charset.Charset;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.*;



public class Program
{
    public static void main(String[] args) throws UnsupportedEncodingException
    {

        byte[] userNameBytes = ServerUtil.String2ByteArray(args[0], "GBK");
        byte[] passwordBytes = ServerUtil.String2ByteArray(args[1], "GBK");
        byte[] hashedPasswordBytes = ServerUtil.digestPassword(userNameBytes, passwordBytes);
        String tmp = new String(hashedPasswordBytes,"GBK");
        hashedPasswordBytes = tmp.getBytes("GBK");
        System.out.println(Arrays.toString(hashedPasswordBytes));
        
    }
    
    private static class ServerUtil
    {

        public synchronized static byte[] digestPassword(byte[] username, byte[] password)
        {
            MessageDigest md = null;
            if (md == null)
            {
                try
                {
                    md = MessageDigest.getInstance("SHA");
                }
                catch (NoSuchAlgorithmException e)
                {

                }
            }
            md.reset();

            if (password != null && password.length != 0)
            {
                md.update(password);
            }
            if (username != null && username.length != 0)
            {
                md.update(username);
            }
            else
            {
                return new byte[] {};
            }

            return md.digest();
        }

        public static byte[] String2ByteArray(String source, String encoding)
                throws UnsupportedEncodingException
        {
            if (null == source || "".equals(source))
            {
                return new byte[] {};
            }
            if (null == encoding || "".equals(encoding))
            {
                return source.getBytes();
            }
            return source.getBytes(encoding);
        }
    }
}

---------- END SOURCE ----------