Um pré-requisito do texto é ter o Spring Security configurado. Você já fez?

Onde está <bean> no XML você pode criar um método que retorna o objeto e tem a anotação @Bean.

@Bean
public ReCaptchaAuthenticationFilter captcha(AuthenticationManager authenticationManager) {
ReCaptchaAuthenticationFilter recaptcha = new ReCaptchaAuthenticationFilter();
recaptcha.setAuthenticationManager(authenticationManager);
recaptcha.setFilterProcessesUrl("/authentication");
recaptcha.setPrivateKey("your_private_recaptcha_key");

SavedRequestAwareAuthenticationSuccessHandler success = new SavedRequestAwareAuthenticationSuccessHandler();
success.setAlwaysUseDefaultTargetUrl(true);
success.setDefaultTargetUrl("/home");
recaptcha.setAuthenticationSuccessHandler(success);

SimpleUrlAuthenticationFailureHandler failure = new SimpleUrlAuthenticationFailureHandler();
failure.setDefaultFailureUrl("/login?error=1");
recaptcha.setAuthenticationFailureHandler(failure);
return recaptcha;
}

Spring security, já está configurado no projeto.

Tenho dois arquivos:

primeiro:

@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(securedEnabled = true, prePostEnabled = true)
public class SecurityWebConfig extends WebSecurityConfigurerAdapter {

E este:

package br.com.netsoft;

import org.springframework.security.web.context.AbstractSecurityWebApplicationInitializer;

public class SpringWebSecurityInitializer extends AbstractSecurityWebApplicationInitializer {

}

Ele reclama desta linha do que você informou.

ReCaptchaAuthenticationFilter, Entendo que esteja faltando esta classe.

O que é esta classe:

https://github.com/mkopylec/recaptcha-spring-boot-starter/blob/master/src/main/java/com/github/mkopylec/recaptcha/security/RecaptchaAuthenticationFilter.java

Ah, a classe pede alguns argumentos no construtor:

public RecaptchaAuthenticationFilter(
            RecaptchaValidator recaptchaValidator,
            RecaptchaProperties recaptcha,
            LoginFailuresManager failuresManager
    )

Está diferente do POST!

Não entendi....

No código do blog que você colocou no início, não tinha esse construtor.

A ta, então eu poderia usar o exemplo do link que postei ?

Isso!

Essa classe:

public class ReCaptchaAuthenticationFilter extends UsernamePasswordAuthenticationFilter implements InitializingBean
{
    private final String CAPTCHA_CHALLENGE_FIELD = "recaptcha_challenge_field";
    private final String CAPTCHA_RESPONSE_FIELD = "recaptcha_response_field";
    private final ReCaptchaImpl reCaptcha;
    private String privateKey;
    private final Logger log = LoggerFactory.getLogger(ReCaptchaAuthenticationFilter.class);

    public ReCaptchaAuthenticationFilter() {
        this.reCaptcha = new ReCaptchaImpl();
    }

    public Authentication attemptAuthentication(HttpServletRequest request, HttpServletResponse response) throws AuthenticationException
    {

        String reCaptchaChallenge = request.getParameter(CAPTCHA_CHALLENGE_FIELD);
        String reCaptchaResponse = request.getParameter(CAPTCHA_RESPONSE_FIELD);
        String remoteAddress = request.getRemoteAddr();

        if (!StringUtils.isEmpty(reCaptchaChallenge))
        {
            log.debug("ReCaptcha Challenge not null");
            if (!StringUtils.isEmpty(reCaptchaResponse))
            {
                log.debug("ReCaptcha Answser not null, call ReCaptcha to verify it");
                ReCaptchaResponse reCaptchaCheck = reCaptcha.checkAnswer(remoteAddress, reCaptchaChallenge, reCaptchaResponse);

                if (reCaptchaCheck.isValid())
                {
                    log.debug("ReCaptcha answer is valid, attempt authentication");
                    return super.attemptAuthentication(request, response);
                }
                else
                {
                    this.reCaptchaError(request, response, "ReCaptcha failed : " + reCaptchaCheck.getErrorMessage());
                    return null;
                }
            }
            else
            {
                this.reCaptchaError(request, response, "ReCaptcha failed : empty answer");
                return null;
            }

        }
        else
        {
            return super.attemptAuthentication(request, response);
        }
    }

    private void reCaptchaError(HttpServletRequest request, HttpServletResponse response, String errorMsg)
    {
        log.error("ReCaptcha failed : " + errorMsg);
        try
        {

            RequestDispatcher dispatcher = request.getRequestDispatcher("/login?error=2");

            dispatcher.forward(request, response);
        }
        catch (ServletException e)
        {
            throw new AuthenticationServiceException("ReCaptcha failed : " + errorMsg);
        }
        catch (IOException e)
        {
            throw new AuthenticationServiceException("Recaptcha failed : " + errorMsg);
        }
    }

    public void setPrivateKey(String privateKey)
    {
        this.privateKey = privateKey;
    }

    public void afterPropertiesSet()
    {
        if (StringUtils.isEmpty(this.privateKey))
        {
            throw new IllegalArgumentException("The 'privateKey' should be set for the bean type 'ReCaptchaAuthenticationFilter'");
        }
        else
        {
            reCaptcha.setPrivateKey(this.privateKey);
        }
    }
}

Consegui não...

A sua classe dá erro na própria classe e em outras classes deste código do git.

Alguma ajuda ?