跟服务器交互，更多的是发送数据，然后接收到服务器返回的数据，一般我们利用http-client中的实体(Entity)，具体在org.apache.http.entity包下面的类来封装我们的请求，从服务器上接收的也是实体，这个在上一篇的response.getEntity()可知。

HttpClient根据内容的出处来区分3种实体：

• 流式(Stream)：内容从流(Stream)中接收，或者在运行中产生(generated on the fly)。

• 自我包含(self-contained)：内容在内存中或通过独立的连接或其他实体中获得。

• 包装(wrapping)：内容是从其他实体(Entity)中获得的。

只有自我包含的实体是可以重复的，重复意味着可以被多次读取，如多次调用EntityUtils.toString(entity)。具体来讲有如：StringEntity、ByteArrayEntity……

废话不多说，上代码，看看如何向服务器发送请求。

import java.util.ArrayList;
import java.util.List;

import org.apache.http.HttpEntity;
import org.apache.http.NameValuePair;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

/**
 * 使用httpclient-4.5.2发送请求参数
 * @author chmod400
 * 2016.3.24
 */
public class RequestParameterDemo {

    public static void main(String[] args) {
        try {
            String url = "http://localhost:9090";
            // 使用默认配置创建httpclient的实例
            CloseableHttpClient client = HttpClients.createDefault();
            
            HttpPost post = new HttpPost(url);
            
            /**
             * 设置参数，常用的有StringEntity,UrlEncodedFormEntity,MultipartEntity
             * 具体看org.apache.http.entity包
             */
            List<NameValuePair> params = new ArrayList<>();
            params.add(new BasicNameValuePair("username", "张三"));
            params.add(new BasicNameValuePair("password", "123456"));
            UrlEncodedFormEntity e = new UrlEncodedFormEntity(params, "UTF-8");
            post.setEntity(e);
            
            CloseableHttpResponse response = client.execute(post);
            
            // 服务器返回码
            int status_code = response.getStatusLine().getStatusCode();
            System.out.println(status_code);
            
            // 服务器返回内容
            String respStr = null;
            HttpEntity entity = response.getEntity();
            if(entity != null) {
                respStr = EntityUtils.toString(entity, "UTF-8");
            }
            System.out.println("respStr = " + respStr);
            // 释放资源
            EntityUtils.consume(entity);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

代码模拟了客户端向服务器发送一个表单数据，最常用的场景就是发送报文数据/登陆动作了。

这段代码应该不需要过多的解释。

来欣赏一下官方代码是如何完成一个登陆动作的：

import java.net.URI;
import java.util.List;

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.HttpUriRequest;
import org.apache.http.client.methods.RequestBuilder;
import org.apache.http.cookie.Cookie;
import org.apache.http.impl.client.BasicCookieStore;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

/**
 * A example that demonstrates how HttpClient APIs can be used to perform
 * form-based logon.
 */
public class ClientFormLogin {

    public static void main(String[] args) throws Exception {
        BasicCookieStore cookieStore = new BasicCookieStore();
        CloseableHttpClient httpclient = HttpClients.custom().setDefaultCookieStore(cookieStore).build();
        try {
            HttpGet httpget = new HttpGet("https://someportal/");
            CloseableHttpResponse response1 = httpclient.execute(httpget);
            try {
                HttpEntity entity = response1.getEntity();

                System.out.println("Login form get: " + response1.getStatusLine());
                EntityUtils.consume(entity);

                System.out.println("Initial set of cookies:");
                List<Cookie> cookies = cookieStore.getCookies();
                if (cookies.isEmpty()) {
                    System.out.println("None");
                } else {
                    for (int i = 0; i < cookies.size(); i++) {
                        System.out.println("- " + cookies.get(i).toString());
                    }
                }
            } finally {
                response1.close();
            }

            HttpUriRequest login = RequestBuilder.post().setUri(new URI("https://someportal/"))
                    .addParameter("IDToken1", "username").addParameter("IDToken2", "password").build();
            CloseableHttpResponse response2 = httpclient.execute(login);
            try {
                HttpEntity entity = response2.getEntity();

                System.out.println("Login form get: " + response2.getStatusLine());
                EntityUtils.consume(entity);

                System.out.println("Post logon cookies:");
                List<Cookie> cookies = cookieStore.getCookies();
                if (cookies.isEmpty()) {
                    System.out.println("None");
                } else {
                    for (int i = 0; i < cookies.size(); i++) {
                        System.out.println("- " + cookies.get(i).toString());
                    }
                }
            } finally {
                response2.close();
            }
        } finally {
            httpclient.close();
        }
    }
}