<section id="adapter_error_handling">
    <title>Error Handling</title>
    <para>
        Keycloak has some error handling facilities for servlet based client adapters.  When an error is encountered in
        authentication, keycloak will call <literal>HttpServletResponse.sendError()</literal>.  You can set up an error-page
        within your <literal>web.xml</literal> file to handle the error however you want.  Keycloak may throw
        400, 401, 403, and 500 errors.
    </para>
    <para>
<programlisting>
<![CDATA[
<error-page>
    <error-code>404</error-code>
    <location>/ErrorHandler</location>
</error-page>]]>
</programlisting>
    </para>
    <para>
        Keycloak also sets an <literal>HttpServletRequest</literal> attribute that you can retrieve.  The attribute name
        is <literal>org.keycloak.adapters.spi.AuthenticationError</literal>.  Typecast this object to:
        <literal>org.keycloak.adapters.OIDCAuthenticationError</literal>.  This class can tell you exactly what happened.
        If this attribute is not set, then the adapter was not responsible for the error code.
    </para>
    <para>
<programlisting>
public class OIDCAuthenticationError implements AuthenticationError {
    public static enum Reason {
        NO_BEARER_TOKEN,
        NO_REDIRECT_URI,
        INVALID_STATE_COOKIE,
        OAUTH_ERROR,
        SSL_REQUIRED,
        CODE_TO_TOKEN_FAILURE,
        INVALID_TOKEN,
        STALE_TOKEN,
        NO_AUTHORIZATION_HEADER
    }

    private Reason reason;
    private String description;

    public OIDCAuthenticationError(Reason reason, String description) {
        this.reason = reason;
        this.description = description;
    }

    public Reason getReason() {
        return reason;
    }

    public String getDescription() {
        return description;
    }
}

</programlisting>
    </para>
</section>