I'm trying to create a demo app using spring security, I need to protect my Rest API with SSL certificate. These are the steps I did to create the example: 1- Generated a JKS file using keytool command:
keytool -genkey -keyalg RSA -alias tomcat -keystore selfsigned.jks -validity 365 -keysize 2048
2- Converted the resulted jks to pkcs12 format:
keytool -genkey -keyalg RSA -alias tomcat -keystore selfsigned.jks -validity 365 -keysize 2048
3- Created a server-side application as follow:
@EnableWebSecurity
public class X509AuthenticationServer extends WebSecurityConfigurerAdapter {
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
                .authorizeRequests()
                .anyRequest()
                .authenticated()
                .and()
                .x509()
                .subjectPrincipalRegex("OU=My self signed certificate");
    }
}
@Controller
@RequestMapping("/get")
public class UserController {
    @PostMapping(value = "/user")
    public String user(@RequestBody UserInfo userInfo) {
        return "Welcome: " + userInfo.getUsername();
    }
}
server.ssl.key-store=/Users/mymacuser/devhome/certpoc/selfsigned.jks
server.ssl.key-store-password=marwan
server.ssl.key-alias=tomcat
server.ssl.key-password=marwan
server.ssl.enabled=true
server.port=8443
spring.security.user.name=Admin
spring.security.user.password=admin
spring.security.require-ssl=true
4- Created a client-side application as following:
@Configuration
public class BeanConfig {
    @Bean("httpsClient")
    public RestTemplate getSSLRestTemplate() throws IOException, CertificateException, NoSuchAlgorithmException, KeyStoreException, UnrecoverableKeyException, KeyManagementException {
        char[] storePassword = "marwan".toCharArray();
        File keyStore =  new File("/Users/mymacuser/devhome/certpoc/selfsigned.jks");
        SSLContext sslContext = new SSLContextBuilder()
                .loadTrustMaterial(keyStore, storePassword)
                .loadKeyMaterial(keyStore, storePassword, storePassword)
                .build();
        SSLConnectionSocketFactory socketFactory = new SSLConnectionSocketFactory(sslContext, NoopHostnameVerifier.INSTANCE);
        CloseableHttpClient client = HttpClients.custom().setSSLSocketFactory(socketFactory).build();
        HttpComponentsClientHttpRequestFactory factory = new HttpComponentsClientHttpRequestFactory(client);
        RestTemplate restTemplate = new RestTemplate(factory);
        // restTemplate.setMessageConverters(List.of(new Jaxb2RootElementHttpMessageConverter()));
        return restTemplate;
    }
@RestController
@RequestMapping("/get")
public class ClientController {
    @Autowired
    @Qualifier("httpsClient")
    private RestTemplate httpsClient;
    @RequestMapping("/user")
    public String getUserFromServer() throws URISyntaxException {
        ResponseEntity<String> s = httpsClient.exchange("https://localhost:8443/get/user", HttpMethod.POST, new HttpEntity<UserInfo>(new UserInfo("marwan")), String.class);
        return s.getBody();
    }
}
As you can notice, My client-side is a (MITM) man in the middle service that I call from a POSTMAN or curl request.
When I invoke my request to the MITM service, I get as a response:
{
    "timestamp": "2020-02-17T09:16:20.399+0000",
    "status": 500,
    "error": "Internal Server Error",
    "message": "403 : [{\"timestamp\":\"2020-02-17T09:16:20.325+0000\",\"status\":403,\"error\":\"Forbidden\",\"message\":\"Forbidden\",\"path\":\"/get/user\"}]",
    "path": "/get/user"
}
I tried printing the network logs for both MITM and server-side, and both show they are exchanging the correct certificate