function validateLoginForm() {

    var url = "<%=(request.getContextPath())%>/Login.do";
    var username = document.getElementById("username").value;
    var password = document.getElementById("password").value;
    var params = {  "username":username,
                    "password":password
                };

    var http = new XMLHttpRequest();
    http.open("POST", url, false);
    http.setRequestHeader("X-Requested-With","XMLHttpRequest");
    http.setRequestHeader("Content-type", "application/json");
    http.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            $("html").html(this.responseText);
        }
    };
    http.send(JSON.stringify(params));
    return true;
}


      <form name="loginForm" method="POST" autocomplete="off" onsubmit="return validateLoginForm()">
                                  ....
                                  ....
                                  ....

                                  <tr id="userNameRow">
                                    <th id="usernameLabel">Username</th>
                                    <td align="left"><input name="username" id="username" autocomplete="off" type="text" aria-describedby="loginError"></td>
                                  </tr>
                                  <tr id="userPwdRow">
                                    <th id="passwordLabel">Password</th>
                                    <td align="left"><input name="password" id="password" autocomplete="off" type="password"></td>
                                  </tr>

JSONObject jsonObj = new JSONObject(requestBody);
    String username = (String) jsonObj.get("username");
    String password = (String) jsonObj.get("password");