Spring RestTemplate簡單說明

現在REST服務已經很普及了，在我們的程式中，經常會需要呼叫REST API，這時候會有很多選擇，原始一點的JDK自帶的，再進一步點使用HttpClient，或者說如果我們使用Jersey這種框架的話，也會自帶rest client。但是我們專案使用的SpringMVC，所以直接使用RestTemplate。使用RestTemplate比直接使用Httpclient簡單很多，同時也可以藉助httpclient來實現RestTemplate。

通過使用RestTemplate僅僅只需要寫幾行程式碼，就可以完成直接使用httpclient很多行程式碼的事情,具體見：https://spring.io/blog/2009/03/27/rest-in-spring-3-resttemplate

RestTemplate有三個建構函式：

1. 預設建構函式，預設使用SimpleClientHttpRequestFactory，使用JDK自帶的java.net包進行網路傳輸。
2. public RestTemplate(ClientHttpRequestFactory requestFactory)。傳入一個ClientHttpRequestFactory，ClientHttpRequestFactory在Spring中的實現有很多個，如HttpComponentsClientHttpRequestFactory，Netty4ClientHttpRequestFactory等，具體的可以看程式碼，這裡只介紹HttpComponentsClientHttpRequestFactory，需要用到 HttpClient4.
3. public RestTemplate(List<HttpMessageConverter<?>> messageConverters),使用SpringMvc的應該對HttpMessageConverter很熟悉了，RestTemplate預設會給我們設定好常用的HttpMessageConverter，我一般很少使用到這個構造方法。

HttpComponentsClientHttpRequestFactory

這裡主要討論的是通過第二個構造方法來使用HttpClient4 來進行網路傳輸。下面我們來看下HttpComponentsClientHttpRequestFactory這個類。先看看他的構造方法

    /**
     * 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;
    }

如果我們不指定一個HttpClient的話，會預設幫我們建立一個，如果我們程式呼叫比較頻繁的話，為了提高效能，會考慮使用PoolingHttpClientConnectionManager來構建HttpClient，這時候就會使用到第二個。如何使用PoolingHttpClientConnectionManager來建立HttpClient呢？可以參考幾個文章：http://www.yeetrack.com/?p=782 以及我前面的一篇：http://www.cnblogs.com/hupengcool/p/4554525.html ，這裡就不具體講了，可以網上搜索相關資料。
下面寫寫程式碼來描述下怎麼通過HttpComponentsClientHttpRequestFactory來建立RestTemplate,為了方便建立HttpClient的程式碼我就直接使用我前面文章中的程式碼:

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;
    }

}

建立並使用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);

那麼問題來了，我們既然是使用Spring，那肯定希望把他RestTemplate配置成Spring bean來使用，HttpClient是執行緒安全的，他可以在程式中共享，建立一個成Spring bean剛好。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>

開始使用Spring RestTemplate吧。。。。。
PS:專案中除了Spring相關jar包外。需要新增HttpClient4.5，jackson 2.x的jar包。