ClientCustomSSL.java

package com.flatmappable.models;

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.ssl.SSLContexts;
import org.apache.http.util.EntityUtils;

import javax.net.ssl.SSLContext;
import java.io.FileInputStream;
import java.nio.charset.StandardCharsets;
import java.security.KeyStore;
import java.util.Arrays;

public class ClientCustomSSL {

    public static void main(String[] args) throws Exception {

        KeyStore ks = KeyStore.getInstance("JKS");
        ks.load(new FileInputStream("/path_to_keystore/keystore"), "changeme".toCharArray());
        SSLContext sslcontext = SSLContexts.custom().loadKeyMaterial(ks, "changeme".toCharArray()).build();

        try (CloseableHttpClient httpclient = HttpClients.custom()
                .setSSLContext(sslcontext)
                .build()) {

            HttpGet httpget = new HttpGet("https://api.certify.demo.ubirch.com");

            try (CloseableHttpResponse response = httpclient.execute(httpget)) {
                HttpEntity entity = response.getEntity();

                System.out.println("----------------------------------------");
                System.out.println(response.getStatusLine());
                System.out.println(EntityUtils.toString(entity));
            }

            HttpPost httpPost = new HttpPost("https://api.certify.demo.ubirch.com/api/certify/v2/issue");

            String s = """
                    {
                      "nam":{
                        "fn":"MüllèČÇ",
                        "gn":"Eriká"
                      },
                      "dob":"1964-08-12",
                      "t":[
                        {
                          "id":"8888888",
                          "tg":"840539006",
                          "tt":"LP6464-4",
                          "nm":"Roche LightCycler qPCR",
                          "tr":"260415000",
                          "sc":"2021-04-13T14:20:00+00:00",
                          "dr":"2021-04-13T20:00:01+00:00",
                          "tc":"Hauptbahnhof Köln"
                        }
                      ]
                    }""";
            httpPost.setEntity(new StringEntity(s, ContentType.create("application/vnd.dgc.v1.3+json", StandardCharsets.UTF_8)));
            httpPost.setHeader("Accept", "application/cbor+base45");

            try (CloseableHttpResponse response1 = httpclient.execute(httpPost)) {
                HttpEntity entity = response1.getEntity();

                System.out.println("----------------------------------------");
                System.out.println(response1.getStatusLine());
                Arrays.stream(response1.getAllHeaders()).forEach(a -> System.out.println(a.getName() + " - " + a.getValue()));
                System.out.println(EntityUtils.toString(entity));
            }

        }
    }

}

Note that in some cases it is recommended to create pkcs12 keystore for both compatibility and security reasons. In that case you can load the keystore like this:

 KeyStore ks = KeyStore.getInstance("pkcs12");
        ks.load(new FileInputStream("/path_to_keystore/keystore"), "changeme".toCharArray());
        SSLContext sslcontext = SSLContexts.custom().loadKeyMaterial(ks, "changeme".toCharArray()).build();