This is for blog - java17 upgrade and under sub section - Upgrading to Apache HttpClient 5

RestTemplateHttp5Upgrade.java

import org.apache.hc.client5.http.config.ConnectionConfig;
import org.apache.hc.client5.http.config.RequestConfig;
import org.apache.hc.client5.http.cookie.StandardCookieSpec;
import org.apache.hc.client5.http.impl.classic.CloseableHttpClient;
import org.apache.hc.client5.http.impl.classic.HttpClients;
import org.apache.hc.client5.http.impl.io.PoolingHttpClientConnectionManager;
import org.apache.hc.client5.http.impl.io.PoolingHttpClientConnectionManagerBuilder;
import org.apache.hc.client5.http.ssl.SSLConnectionSocketFactoryBuilder;
import org.apache.hc.client5.http.ssl.SSLConnectionSocketFactory;
import org.apache.hc.core5.http.io.SocketConfig;
import org.apache.hc.core5.http.ssl.TLS;
import org.apache.hc.core5.pool.PoolConcurrencyPolicy;
import org.apache.hc.core5.pool.PoolReusePolicy;
import org.apache.hc.core5.util.TimeValue;
import org.apache.hc.core5.util.Timeout;
import org.apache.http.HttpHost;
import org.apache.http.conn.ssl.TrustStrategy;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.ssl.SSLContexts;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;
import org.springframework.core.env.Environment;
import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.util.StringUtils;

import javax.net.ssl.SSLContext;
import java.io.IOException;
import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

@Configuration
public class RestTemplateConfiguration {

    private final static Logger logger = LoggerFactory.getLogger(RestTemplateConfiguration.class);

    @Autowired
    private Environment environment;

    @Autowired
    private ApplicationProperties appconfig;

    @Bean
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public ClientHttpRequestFactory clientHttpRequestFactory() throws KeyStoreException, NoSuchAlgorithmException, KeyManagementException, IOException {
        CloseableHttpClient httpClient = HttpClients.custom()
            .setConnectionManager(getConnectionManager())
            .setDefaultRequestConfig(RequestConfig.custom()
                .setCookieSpec(StandardCookieSpec.STRICT)
                .build())
            .build();

        HttpComponentsClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory();

        String proxyHost = environment.getProperty("proxy.host");
        String proxyPortStr = environment.getProperty("proxy.port");
        Integer proxyPort = null;
        if (!StringUtils.isEmpty(proxyPortStr)) {
            proxyPort = Integer.valueOf(proxyPortStr);
        }

        // Proxy
        if (!StringUtils.isEmpty(proxyHost) && proxyPort != null) {
            requestFactory = new HttpComponentsClientHttpRequestFactory(
                HttpClients.custom()
                    .setProxy(new org.apache.hc.core5.http.HttpHost(proxyHost, proxyPort))
                    .build());
            logger.info("Using proxy {}:{}", proxyHost, proxyPort);
        } else {
            requestFactory = new HttpComponentsClientHttpRequestFactory();
        }
        return requestFactory;
    }

    public PoolingHttpClientConnectionManager getConnectionManager() throws IOException {
        TrustStrategy acceptingTrustStrategy = (X509Certificate[] chain, String authType) -> true;

        SSLContext sslContext = null;
        try {
            sslContext = SSLContexts.custom().loadTrustMaterial(null, acceptingTrustStrategy).build();
        } catch (KeyManagementException | NoSuchAlgorithmException | KeyStoreException e) {
            throw new IOException(e);
        }

        SSLConnectionSocketFactory csf = new SSLConnectionSocketFactory(sslContext);
        PoolingHttpClientConnectionManager connectionManager = PoolingHttpClientConnectionManagerBuilder.create()
            .setSSLSocketFactory(SSLConnectionSocketFactoryBuilder.create()
                .setSslContext(org.apache.hc.core5.ssl.SSLContexts.createSystemDefault())
                .setTlsVersions(TLS.V_1_3)
                .build())
            .setSSLSocketFactory(csf)
            .setDefaultSocketConfig(SocketConfig.custom()
                .setSoTimeout(Timeout.ofMinutes(10))
                .build())
            .setPoolConcurrencyPolicy(PoolConcurrencyPolicy.STRICT)
            .setConnPoolPolicy(PoolReusePolicy.LIFO)
            .setDefaultConnectionConfig(ConnectionConfig.custom()
                .setSocketTimeout(Timeout.ofMilliseconds(appconfig.getConfig().getReadTimeout()))
                .setConnectTimeout(Timeout.ofMilliseconds(appconfig.getConfig().getConnectTimeout()))
                .setTimeToLive(TimeValue.ofMinutes(10))
                .build())
            .build();
        return connectionManager;
    }
}

RestTemplateLegacy.java

import org.apache.http.HttpHost;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.conn.ssl.TrustStrategy;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.ssl.SSLContexts;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;
import org.springframework.core.env.Environment;
import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.util.StringUtils;

import javax.net.ssl.SSLContext;
import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;

@Configuration
public class RestTemplateConfiguration {
  
    private final static Logger logger = LoggerFactory.getLogger(RestTemplateConfiguration.class);
    
    @Autowired
    private Environment environment;
    
    @Autowired
    private ApplicationProperties appconfig;
    
    @Bean
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public ClientHttpRequestFactory clientHttpRequestFactory() throws KeyStoreException, NoSuchAlgorithmException, KeyManagementException {
        HttpComponentsClientHttpRequestFactory requestFactory = null;
        
        String proxyHost = environment.getProperty("proxy.host");
        String proxyPortStr = environment.getProperty("proxy.port");
        Integer proxyPort = null;
        
        if (!StringUtils.isEmpty(proxyPortStr)) {
            proxyPort = Integer.valueOf(proxyPortStr);
        }
        
        // Proxy
        if (!StringUtils.isEmpty(proxyHost) && proxyPort != null) {
            requestFactory = new HttpComponentsClientHttpRequestFactory(
                HttpClients.custom()
                    .setProxy(new HttpHost(proxyHost, proxyPort))
                    .build());
            logger.info("Using proxy {}:{}", proxyHost, proxyPort);
        } else {
            requestFactory = new HttpComponentsClientHttpRequestFactory();
        }
        
        requestFactory.setConnectTimeout(appconfig.getConfig().getConnectTimeout());
        requestFactory.setReadTimeout(appconfig.getConfig().getReadTimeout());

        // SSL
        TrustStrategy acceptingTrustStrategy = (chain, authType) -> true;
        SSLContext sslContext = SSLContexts.custom()
            .loadTrustMaterial(null, acceptingTrustStrategy)
            .build();
        
        SSLConnectionSocketFactory csf = new SSLConnectionSocketFactory(sslContext);
        CloseableHttpClient httpClient = HttpClients.custom()
            .setSSLSocketFactory(csf)
            .build();
        
        requestFactory.setHttpClient(httpClient);

        return requestFactory;
    }
}