使用HttpClient4来构建Spring RestTemplate

Spring RestTemplate简单说明

现在REST服务已经很普及了，在我们的程序中，经常会需要调用REST API，这时候会有很多选择，原始一点的JDK自带的，再进一步点使用HttpClient，或者说如果我们使用Jersey这种框架的话，也会自带rest client。但是我们项目使用的SpringMVC，所以直接使用RestTemplate。使用RestTemplate比直接使用Httpclient简单很多，同时也可以借助httpclient来实现RestTemplate。

通过使用RestTemplate仅仅只需要写几行代码，就可以完成直接使用httpclient很多行代码的事情,具体见：https://spring.io/blog/2009/03/27/rest-in-spring-3-resttemplate

RestTemplate有三个构造函数：

1. 默认构造函数，默认使用SimpleClientHttpRequestFactory，使用JDK自带的java.net包进行网络传输。
2. public RestTemplate(ClientHttpRequestFactory requestFactory)。传入一个ClientHttpRequestFactory，ClientHttpRequestFactory在Spring中的实现有很多个，如HttpComponentsClientHttpRequestFactory，Netty4ClientHttpRequestFactory等，具体的可以看代码，这里只介绍HttpComponentsClientHttpRequestFactory，需要用到 HttpClient4.
3. public RestTemplate(List<HttpMessageConverter<?>> messageConverters),使用SpringMvc的应该对HttpMessageConverter很熟悉了，RestTemplate默认会给我们设置好常用的HttpMessageConverter，我一般很少使用到这个构造方法。

HttpComponentsClientHttpRequestFactory

这里主要讨论的是通过第二个构造方法来使用HttpClient4 来进行网络传输。下面我们来看下HttpComponentsClientHttpRequestFactory这个类。先看看他的构造方法

	/**
	 * Create a new instance of the {@code HttpComponentsClientHttpRequestFactory}
	 * with a default {@link HttpClient}.
	 */
	public HttpComponentsClientHttpRequestFactory() {
		this(HttpClients.createSystem());
	}

	/**
	 * Create a new instance of the {@code HttpComponentsClientHttpRequestFactory}
	 * with the given {@link HttpClient} instance.
	 * <p>As of Spring Framework 4.0, the given client is expected to be of type
	 * {@link CloseableHttpClient} (requiring HttpClient 4.3+).
	 * @param httpClient the HttpClient instance to use for this request factory
	 */
	public HttpComponentsClientHttpRequestFactory(HttpClient httpClient) {
		Assert.notNull(httpClient, "'httpClient' must not be null");
		Assert.isInstanceOf(CloseableHttpClient.class, httpClient, "'httpClient' is not of type CloseableHttpClient");
		this.httpClient = (CloseableHttpClient) httpClient;
	}

如果我们不指定一个HttpClient的话，会默认帮我们创建一个，如果我们程序调用比较频繁的话，为了提高性能，会考虑使用PoolingHttpClientConnectionManager来构建HttpClient，这时候就会使用到第二个。如何使用PoolingHttpClientConnectionManager来创建HttpClient呢？可以参考几个文章：http://www.yeetrack.com/?p=782 以及我前面的一篇：http://www.cnblogs.com/hupengcool/p/4554525.html ，这里就不具体讲了，可以网上搜索相关资料。
下面写写代码来描述下怎么通过HttpComponentsClientHttpRequestFactory来创建RestTemplate,为了方便创建HttpClient的代码我就直接使用我前面文章中的代码:

import org.apache.http.config.Registry;
import org.apache.http.config.RegistryBuilder;
import org.apache.http.conn.socket.ConnectionSocketFactory;
import org.apache.http.conn.socket.PlainConnectionSocketFactory;
import org.apache.http.conn.ssl.NoopHostnameVerifier;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.ssl.SSLContextBuilder;
import org.apache.http.ssl.TrustStrategy;

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

/**
 * Created by Administrator on 2015/6/8.
 */
public class HttpClientUtils {

    public static CloseableHttpClient acceptsUntrustedCertsHttpClient() throws KeyStoreException, NoSuchAlgorithmException, KeyManagementException {
        HttpClientBuilder b = HttpClientBuilder.create();

        // setup a Trust Strategy that allows all certificates.
        //
        SSLContext sslContext = new SSLContextBuilder().loadTrustMaterial(null, new TrustStrategy() {
            public boolean isTrusted(X509Certificate[] arg0, String arg1) throws CertificateException {
                return true;
            }
        }).build();
        b.setSSLContext(sslContext);

        // don't check Hostnames, either.
        //      -- use SSLConnectionSocketFactory.getDefaultHostnameVerifier(), if you don't want to weaken
        HostnameVerifier hostnameVerifier = NoopHostnameVerifier.INSTANCE;

        // here's the special part:
        //      -- need to create an SSL Socket Factory, to use our weakened "trust strategy";
        //      -- and create a Registry, to register it.
        //
        SSLConnectionSocketFactory sslSocketFactory = new SSLConnectionSocketFactory(sslContext, hostnameVerifier);
        Registry<ConnectionSocketFactory> socketFactoryRegistry = RegistryBuilder.<ConnectionSocketFactory>create()
                .register("http", PlainConnectionSocketFactory.getSocketFactory())
                .register("https", sslSocketFactory)
                .build();

        // now, we create connection-manager using our Registry.
        //      -- allows multi-threaded use
        PoolingHttpClientConnectionManager connMgr = new PoolingHttpClientConnectionManager( socketFactoryRegistry);
        connMgr.setMaxTotal(200);
        connMgr.setDefaultMaxPerRoute(100);
        b.setConnectionManager( connMgr);

        // finally, build the HttpClient;
        //      -- done!
        CloseableHttpClient client = b.build();

        return client;
    }

}

创建并使用RestTemplate

        CloseableHttpClient httpClient = HttpClientUtils.acceptsUntrustedCertsHttpClient();
        HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
        RestTemplate restTemplate = new RestTemplate(clientHttpRequestFactory);
        String result = restTemplate.getForObject("http://www.baidu.com",String.class);
        System.out.println(result);

那么问题来了，我们既然是使用Spring，那肯定希望把他RestTemplate配置成Spring bean来使用，HttpClient是线程安全的，他可以在程序中共享，创建一个成Spring bean刚好。下面是xml配置。

   <bean id="httpClient" class="com.hupengcool.util.HttpClientUtils" factory-method="acceptsUntrustedCertsHttpClient"/>

    <bean id="clientHttpRequestFactory" class="org.springframework.http.client.HttpComponentsClientHttpRequestFactory">
        <constructor-arg ref="httpClient"/>
    </bean>

    <bean id="restTemplate" class=" org.springframework.web.client.RestTemplate">
        <constructor-arg ref="clientHttpRequestFactory" />
    </bean>

开始使用Spring RestTemplate吧。。。。。
PS:项目中除了Spring相关jar包外。需要添加HttpClient4.5，jackson 2.x的jar包。