Configuring HttpClient with Spring RestTemplate

In the Spring RestTemplate example, we learned to access REST APIs inside a Spring application. In this tutorial, we are extending the RestTemplate configuration to use Apache HttpClient 4.

The purpose of this tutorial is to give you a pre-cooked recipe for a little head-start and save you from writing all bits and pieces, which really takes lots of time.

1. HttpClient Configuration

In HttpClientConfig class, we are configuring mainly two things –

• PoolingHttpClientConnectionManager – As the name suggests, it is a connection pool manager. Here, connections are pooled on a per-route basis. A request for a route that the manager already has persistent connections for available in the pool will be serviced by leasing a connection from the pool rather than creating a brand new connection.
• ConnectionKeepAliveStrategy helps in setting time which decides how long a connection can remain idle before being reused. And set a idleConnectionMonitor thread that periodically checks all connections and frees up which have not been used and idle time has elapsed.

The real HTTP client to use is CloseableHttpClient bean. It is what RestTemplate will use to get the connection to API endpoints.

import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.util.concurrent.TimeUnit;
 
import org.apache.http.HeaderElement;
import org.apache.http.HeaderElementIterator;
import org.apache.http.HttpResponse;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.config.Registry;
import org.apache.http.config.RegistryBuilder;
import org.apache.http.conn.ConnectionKeepAliveStrategy;
import org.apache.http.conn.socket.ConnectionSocketFactory;
import org.apache.http.conn.socket.PlainConnectionSocketFactory;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
 
import org.apache.http.conn.ssl.TrustSelfSignedStrategy;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.message.BasicHeaderElementIterator;
import org.apache.http.protocol.HTTP;
import org.apache.http.protocol.HttpContext;
import org.apache.http.ssl.SSLContextBuilder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.EnableScheduling;
import org.springframework.scheduling.annotation.Scheduled;
 
/**
 * - Supports both HTTP and HTTPS
 * - Uses a connection pool to re-use connections and save overhead of creating connections.
 * - Has a custom connection keep-alive strategy (to apply a default keep-alive if one isn't specified)
 * - Starts an idle connection monitor to continuously clean up stale connections.
 */
@Configuration
@EnableScheduling
public class HttpClientConfig {
 
    private static final Logger LOGGER = LoggerFactory.getLogger(HttpClientConfig.class);
 
    // Determines the timeout in milliseconds until a connection is established.
    private static final int CONNECT_TIMEOUT = 30000;
     
    // The timeout when requesting a connection from the connection manager.
    private static final int REQUEST_TIMEOUT = 30000;
     
    // The timeout for waiting for data
    private static final int SOCKET_TIMEOUT = 60000;
 
    private static final int MAX_TOTAL_CONNECTIONS = 50;
    private static final int DEFAULT_KEEP_ALIVE_TIME_MILLIS = 20 * 1000;
    private static final int CLOSE_IDLE_CONNECTION_WAIT_TIME_SECS = 30;
 
    @Bean
    public PoolingHttpClientConnectionManager poolingConnectionManager() {
        SSLContextBuilder builder = new SSLContextBuilder();
        try {
            builder.loadTrustMaterial(null, new TrustSelfSignedStrategy());
        } catch (NoSuchAlgorithmException | KeyStoreException e) {
            LOGGER.error("Pooling Connection Manager Initialisation failure because of " + e.getMessage(), e);
        }
 
        SSLConnectionSocketFactory sslsf = null;
        try {
            sslsf = new SSLConnectionSocketFactory(builder.build());
        } catch (KeyManagementException | NoSuchAlgorithmException e) {
            LOGGER.error("Pooling Connection Manager Initialisation failure because of " + e.getMessage(), e);
        }
 
        Registry<ConnectionSocketFactory> socketFactoryRegistry = RegistryBuilder
                .<ConnectionSocketFactory>create().register("https", sslsf)
                .register("http", new PlainConnectionSocketFactory())
                .build();
 
        PoolingHttpClientConnectionManager poolingConnectionManager = new PoolingHttpClientConnectionManager(socketFactoryRegistry);
        poolingConnectionManager.setMaxTotal(MAX_TOTAL_CONNECTIONS);
        return poolingConnectionManager;
    }
 
    @Bean
    public ConnectionKeepAliveStrategy connectionKeepAliveStrategy() {
        return new ConnectionKeepAliveStrategy() {
            @Override
            public long getKeepAliveDuration(HttpResponse response, HttpContext context) {
                HeaderElementIterator it = new BasicHeaderElementIterator
                        (response.headerIterator(HTTP.CONN_KEEP_ALIVE));
                while (it.hasNext()) {
                    HeaderElement he = it.nextElement();
                    String param = he.getName();
                    String value = he.getValue();
 
                    if (value != null && param.equalsIgnoreCase("timeout")) {
                        return Long.parseLong(value) * 1000;
                    }
                }
                return DEFAULT_KEEP_ALIVE_TIME_MILLIS;
            }
        };
    }
 
    @Bean
    public CloseableHttpClient httpClient() {
        RequestConfig requestConfig = RequestConfig.custom()
                .setConnectionRequestTimeout(REQUEST_TIMEOUT)
                .setConnectTimeout(CONNECT_TIMEOUT)
                .setSocketTimeout(SOCKET_TIMEOUT).build();
 
        return HttpClients.custom()
                .setDefaultRequestConfig(requestConfig)
                .setConnectionManager(poolingConnectionManager())
                .setKeepAliveStrategy(connectionKeepAliveStrategy())
                .build();
    }
     
    @Bean
    public Runnable idleConnectionMonitor(final PoolingHttpClientConnectionManager connectionManager) {
        return new Runnable() {
            @Override
            @Scheduled(fixedDelay = 10000)
            public void run() {
                try {
                    if (connectionManager != null) {
                        LOGGER.trace("run IdleConnectionMonitor - Closing expired and idle connections...");
                        connectionManager.closeExpiredConnections();
                        connectionManager.closeIdleConnections(CLOSE_IDLE_CONNECTION_WAIT_TIME_SECS, TimeUnit.SECONDS);
                    } else {
                        LOGGER.trace("run IdleConnectionMonitor - Http Client Connection manager is not initialised");
                    }
                } catch (Exception e) {
                    LOGGER.error("run IdleConnectionMonitor - Exception occurred. msg={}, e={}", e.getMessage(), e);
                }
            }
        };
    }
}

2. Spring RestTemplate Configuration

Here we are configuring RestTemplate bean which we will finally use to invoke REST APIs. As mentioned above, it uses CloseableHttpClient bean instance to build ClientHttpRequestFactory, which is used to create RestTemplate.

• HttpComponentsClientHttpRequestFactory is ClientHttpRequestFactory implementation that uses Apache HttpComponents HttpClient to create requests.
• We have used @Scheduled annotation in httpClient configuration. To support this, we have to add support for the scheduled execution of the thread. For that, we have used bean ThreadPoolTaskScheduler which internally utilizes ScheduledThreadPoolExecutor to schedule commands to run after a given delay or to execute periodically.

import org.apache.http.impl.client.CloseableHttpClient;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.scheduling.TaskScheduler;
import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
import org.springframework.web.client.RestTemplate;

@Configuration
public class RestTemplateConfig {

	@Autowired
	CloseableHttpClient httpClient;

	@Bean
	public RestTemplate restTemplate() {
		RestTemplate restTemplate = new RestTemplate(clientHttpRequestFactory());
		return restTemplate;
	}

	@Bean
	public HttpComponentsClientHttpRequestFactory clientHttpRequestFactory() {
		HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory();
		clientHttpRequestFactory.setHttpClient(httpClient);
		return clientHttpRequestFactory;
	}

	@Bean
	public TaskScheduler taskScheduler() {
		ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
		scheduler.setThreadNamePrefix("poolScheduler");
		scheduler.setPoolSize(50);
		return scheduler;
	}
}

3. Demo

To use the above configured RestTemplate, simply inject it into the controller or any other bean.

@RestController
public class EmployeeController {

	@Autowired
	RestTemplate restTemplate;

	public void getEmployees() {
		final String uri = "http://localhost:8080/employees";

		String result = restTemplate.getForObject(uri, String.class);

		Assert.assertEquals(true, result.indexOf("Lokesh") > 0);
	}
}

4. Maven Dependencies

Primarily, you will be required to have two dependencies i.e. httpclient and spring-web. We are using the spring boot application, so the pom file looks like this:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.apache.httpcomponents</groupId>
        <artifactId>httpclient</artifactId>
    </dependency>
</dependencies>

Happy Learning !!

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