Custom Exception thrown in Java should be shown in Thymeleaf

I want to show my custom exception that is being thrown in Java code in Thymeleaf.

@Override
public UserDetails loadUserByUsername(String benutzername) throws UsernameNotFoundException {
    Benutzer benutzer = userRepository.findByBenutzername(benutzername);
    if (benutzer == null || ! benutzer.isActive()) {
        throw new UsernameNotFoundException("Wrong username or password was provided!");
    }

<div th:if="${param.error}" >
    <div class="alert alert-danger">
        Here should my custom exception appear from above.
    </div>
</div>

I throw a new UsernameNotFoundException with a custom exception and this exception should be shown in my template, however, I do not know how this should be done.

Solution

Define the property in application.properties to enable the stacktrace to be included as expression attribute in Thymeleaf view.

server.error.include-stacktrace=always

and then create an error page named error.html like below.

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:th="http://www.thymeleaf.org">
<head>
    <style>
table td{
vertical-align:top;
border:solid 1px #888;
padding:10px;
}

    </style>
</head>
<body>
<h1>My Thymeleaf Error Page</h1>
<table>
    <tr>
        <td>Date</td>
        <td th:text="${timestamp}"/>
    </tr>
    <tr>
        <td>Path</td>
        <td th:text="${path}"/>
    </tr>
    <tr>
        <td>Error</td>
        <td th:text="${error}"/>
    </tr>
    <tr>
        <td>Status</td>
        <td th:text="${status}"/>
    </tr>
    <tr>
        <td>Message</td>
        <td th:text="${message}"/>
    </tr>
    <tr>
        <td>Exception</td>
        <td th:text="${exception}"/>
    </tr>
    <tr>
        <td>Trace</td>
        <td>
            <pre th:text="${trace}"/>
        </td>
    </tr>
</table>
</body>
</html>

Refer this for full example.