class LoginFilter(url: String, authManager: AuthenticationManager) :
    AbstractAuthenticationProcessingFilter(AntPathRequestMatcher(url))
{
    init {
        authenticationManager = authManager
    }

    override fun attemptAuthentication(request: HttpServletRequest?, response: HttpServletResponse?): Authentication {
        val userCredentials: UserCredentials = jacksonObjectMapper()
            .readValue(request!!.inputStream) // this part throws exception

        return authenticationManager.authenticate(
            UsernamePasswordAuthenticationToken(
                userCredentials.userName,
                userCredentials.password,
                emptyList()
            )
        )
    }
	// ...
}