Run RestTemplate with an external resource on HTTPS

I'm trying to consume a public endpoint that is available in both http and SSL, https://jsonplaceholder.typicode.com/posts/1 which returns a very simple json.

I have configured my RestTemplate as follows:

@Configuration
public class RestTemplateConfig {

    @Value("${https.remote.proxy.host}")
    private String PROXY_URI;

    @Value("${https.remote.proxy.port}")
    private int PROXY_PORT;

    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {

        SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
        Proxy proxy = new Proxy(Proxy.Type.HTTP,
        new InetSocketAddress(PROXY_URI, PROXY_PORT));
        requestFactory.setProxy(proxy);

        return new RestTemplate(requestFactory);
    }
}

Also I did the following, I went to https://jsonplaceholder.typicode.com/posts/1 in chrome to download the certificate, by clicking on the padlock symbol ⟼ valid ⟼ Details ⟼ Copy to file as jsonplaceholder.cer and with the KeyStore Explorer I copied this certificate in the cacerts of JAVA (JAVA_HOME / jre / lib / security).

When I do the following:

@Autowired
RestTemplate restTemplate;

@RequestMapping(value="test", method = RequestMethod.GET)
    public Post getPost() {
    Post post = restTemplate.getForObject("http://jsonplaceholder.typicode.com/posts/1", Post.class);
    return post;
} 

I get a successful answer:

{
    "userId": "1",
    "id": 1,
    "title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
    "body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}

But when I do the same thing about https I get the following error:

{
    "timestamp": 1520596009201,
    "status": 500,
    "error": "Internal Server Error",
    "exception": "org.springframework.web.client.ResourceAccessException",
    "message": "I/O error on GET request for \"https://jsonplaceholder.typicode.com/posts/1\": sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target; nested exception is javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target",
    "path": "/api/v1/sepa/test/"
}

That comes from Java:

sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141) ~[na:1.8.0_161]
at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:126) ~[na:1.8.0_161]
at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280) ~[na:1.8.0_161]
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:392) ~[na:1.8.0_161]

What am I forgetting? I do this test with the intention of testing the RestTemplate, but later I'll have to connect to an API from a provider.