Untitled

<html>
<head>
    <title></title>
    <script src="jquery-1.11.2.js"></script>
    <script>
        $(function() {
            $('#chek').click(function () {
                if ($(this).attr('checked')) {
                    $("#btnSubmit").html("true");
                }
                else {
                    $("#btnSubmit").html("false");
                }
            });
        });
    </script>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>

<body>
<div>
    <form method="post">

       <input type="checkbox" id = "chek" name="readAlread" value="true"> true
        <input type="checkbox" name="readAlread" value="false"> false
        <br>
        <br>
        <input id ="btnSubmit" type="submit" value="submit" onclick="chek">Добавить
    </form>
</div>
</body>
</html>

@PostMapping("/add")
    public String add(@RequestParam Boolean readAlready, Map<String, Object> model) {
     ......
        return "main";
    }

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Sat May 26 20:09:56 MSK 2018
There was an unexpected error (type=Bad Request, status=400).
Required Boolean parameter 'readAlready' is not present