Solutions to Java non thread safety problems

package ch14;
public class LoginCheck
{
    private static String username; 
    private static String password; 
    public static void doPost(String _username,String _password)
    { 
        try
        { 
            username=_username; 
            if (username.equals("admin"))
            { 
                Thread.sleep(5000); 
            } 
            password=_password; 
            System.out.println("username="+username+"password="+password); 
        }
        catch(InterruptedException e)
        { 
            // TODO Auto-generated catch block 
            e.printStackTrace(); 
        } 
    } 

package ch14;
public class LoginThreadA extends Thread
{
    public void run()
    { 
        LoginCheck.doPost("admin","admin"); 
    }
}

package ch14;
public class LoginThreadB extends Thread
{ 
    public void run()
    { 
        LoginCheck.doPost("root","root"); 
    }
}

package ch14;
public class Test07
{
    public static void main(String[] args)
    {
        LoginThreadA a=new LoginThreadA();
        a.run();    //启动线程LoginThreadA
        LoginThreadB b=new LoginThreadB();
        b.run();    //启动线程LoginThreadB
    }
}

username=root password=admin
username=root password=root

package ch14;
public class LoginCheck
{
    private static String username; 
    private static String password; 
    synchronized public static void doPost(String _username,String _password)
    { 
        try
        { 
            username=_username; 
            if (username.equals("admin"))
            { 
                Thread.sleep(5000); 
            } 
            password=_password; 
            System.out.println("username="+username+"password="+password); 
        }
        catch (InterruptedException e)
        { 
            // TODO Auto-generated catch block 
            e.printStackTrace(); 
        } 
    } 
}

username=admin password=admin
username=root password=root