Handle Security exceptions in Spring Boot Resource Server

If you're using token validation URL with config similar to Configuring resource server with RemoteTokenServices in Spring Security Oauth2 which returns HTTP status 401 in case of unauthorized:

@Primary
@Bean
public RemoteTokenServices tokenService() {
    RemoteTokenServices tokenService = new RemoteTokenServices();
    tokenService.setCheckTokenEndpointUrl("https://token-validation-url.com");
    tokenService.setTokenName("token");
    return tokenService;
}

Implementing custom authenticationEntryPoint as described in other answers (https://stackoverflow.com/a/44372313/5962766) won't work because RemoteTokenService use 400 status and throws unhandled exceptions for other statuses like 401:

public RemoteTokenServices() {
        restTemplate = new RestTemplate();
        ((RestTemplate) restTemplate).setErrorHandler(new DefaultResponseErrorHandler() {
            @Override
            // Ignore 400
            public void handleError(ClientHttpResponse response) throws IOException {
                if (response.getRawStatusCode() != 400) {
                    super.handleError(response);
                }
            }
        });
}

So you need to set custom RestTemplate in RemoteTokenServices config which would handle 401 without throwing exception:

@Primary
@Bean
public RemoteTokenServices tokenService() {
    RemoteTokenServices tokenService = new RemoteTokenServices();
    tokenService.setCheckTokenEndpointUrl("https://token-validation-url.com");
    tokenService.setTokenName("token");
    RestOperations restTemplate = new RestTemplate();
    restTemplate.setRequestFactory(new HttpComponentsClientHttpRequestFactory());
    ((RestTemplate) restTemplate).setErrorHandler(new DefaultResponseErrorHandler() {
            @Override
            // Ignore 400 and 401
            public void handleError(ClientHttpResponse response) throws IOException {
                if (response.getRawStatusCode() != 400 && response.getRawStatusCode() != 401) {
                    super.handleError(response);
                }
            }
        });
    }
    tokenService.setRestTemplate(restTemplate);
    return tokenService;
}

And add dependency for HttpComponentsClientHttpRequestFactory:

<dependency>
  <groupId>org.apache.httpcomponents</groupId>
  <artifactId>httpclient</artifactId>
</dependency>

You are not able to make use of Spring MVC Exception handler annotations such as @ControllerAdvice because spring security filters kicks in much before Spring MVC.

As noted in previous comments the request is rejected by the security framework before it reaches the MVC layer so @ControllerAdvice is not an option here.

There are 3 interfaces in the Spring Security framework that may be of interest here:

• org.springframework.security.web.authentication.AuthenticationSuccessHandler
• org.springframework.security.web.authentication.AuthenticationFailureHandler
• org.springframework.security.web.access.AccessDeniedHandler

You can create implementations of each of these Interfaces in order to customize the response sent for various events: successful login, failed login, attempt to access protected resource with insufficient permissions.

The following would return a JSON response on unsuccessful login attempt:

@Component
public class RestAuthenticationFailureHandler implements AuthenticationFailureHandler
{
  @Override
  public void onAuthenticationFailure(HttpServletRequest request, HttpServletResponse response,
      AuthenticationException ex) throws IOException, ServletException
  {
    response.setStatus(HttpStatus.FORBIDDEN.value());
    
    Map<String, Object> data = new HashMap<>();
    data.put("timestamp", new Date());
    data.put("status",HttpStatus.FORBIDDEN.value());
    data.put("message", "Access Denied");
    data.put("path", request.getRequestURL().toString());
    
    OutputStream out = response.getOutputStream();
    com.fasterxml.jackson.databind.ObjectMapper mapper = new ObjectMapper();
    mapper.writeValue(out, data);
    out.flush();
  }
}

You also need to register your implementation(s) with the Security framework. In Java config this looks like the below:

@Configuration
@EnableWebSecurity
@ComponentScan("...")
public class SecurityConfiguration extends WebSecurityConfigurerAdapter
{
  @Override
  public void configure(HttpSecurity http) throws Exception
  {
    http
       .addFilterBefore(corsFilter(), ChannelProcessingFilter.class)
       .logout()
       .deleteCookies("JESSIONID")
       .logoutUrl("/api/logout")
       .logoutSuccessHandler(logoutSuccessHandler())
       .and()
       .formLogin()
       .loginPage("/login")
       .loginProcessingUrl("/api/login")
       .failureHandler(authenticationFailureHandler())
       .successHandler(authenticationSuccessHandler())
       .and()
       .csrf()
       .disable()
       .exceptionHandling()
       .authenticationEntryPoint(authenticationEntryPoint())
       .accessDeniedHandler(accessDeniedHandler());
  }

  /**
   * @return Custom {@link AuthenticationFailureHandler} to send suitable response to REST clients in the event of a
   *         failed authentication attempt.
   */
  @Bean
  public AuthenticationFailureHandler authenticationFailureHandler()
  {
    return new RestAuthenticationFailureHandler();
  }

  /**
   * @return Custom {@link AuthenticationSuccessHandler} to send suitable response to REST clients in the event of a
   *         successful authentication attempt.
   */
  @Bean
  public AuthenticationSuccessHandler authenticationSuccessHandler()
  {
    return new RestAuthenticationSuccessHandler();
  }

  /**
   * @return Custom {@link AccessDeniedHandler} to send suitable response to REST clients in the event of an attempt to
   *         access resources to which the user has insufficient privileges.
   */
  @Bean
  public AccessDeniedHandler accessDeniedHandler()
  {
    return new RestAccessDeniedHandler();
  }
}

In case if you're using @EnableResourceServer, you may also find convenient to extend ResourceServerConfigurerAdapter instead of WebSecurityConfigurerAdapter in your @Configuration class. By doing this, you may simply register a custom AuthenticationEntryPoint by overriding configure(ResourceServerSecurityConfigurer resources) and using resources.authenticationEntryPoint(customAuthEntryPoint()) inside the method.

Something like this:

@Configuration
@EnableResourceServer
public class CommonSecurityConfig extends ResourceServerConfigurerAdapter {

    @Override
    public void configure(ResourceServerSecurityConfigurer resources) throws Exception {
        resources.authenticationEntryPoint(customAuthEntryPoint());
    }

    @Bean
    public AuthenticationEntryPoint customAuthEntryPoint(){
        return new AuthFailureHandler();
    }
}

There's also a nice OAuth2AuthenticationEntryPoint that can be extended (since it's not final) and partially re-used while implementing a custom AuthenticationEntryPoint. In particular, it adds "WWW-Authenticate" headers with error-related details.