问Servlet到jsp的按钮点击-错误是什么？

EN

  <form name="login_form" action="LoginValidation" method="get" onsubmit="return func()" >
      <input type="text" placeholder="Username" id="username" class="style-4" />
      <input type="password" placeholder="Password" id="pass" class="style-4" />
      <input type="submit" class="button" name="Sign_button" "/>
  </form>

   function func()
        {  
            var filter="";
            var uname = document.getElementById('username').value;
            var pass = document.getElementById('pass').value;
            if(uname==filter && pass==filter)
                {
                    alert('Please enter your username and password to sign in.');
                    return false;

                }
            else if(uname==filter)
                {
                    alert('Please enter your username to sign in.');
                    return false;
                }
            else if(pass==filter)
                {
                    alert('Please enter your password to sign in.');
                    return false;  
                }
                else
                    return true;
        }

 <servlet>
    <servlet-name>LoginValidation</servlet-name>
    <servlet-class>Controller.LoginValidation</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>LoginValidation</servlet-name>
    <url-pattern>/LoginValidation</url-pattern>
</servlet-mapping>