使用HttpClient库，可以发送请求或通过传递参数登录表单。

按照以下步骤登录表单。

第1步 - 创建一个HttpClient对象
HttpClients类的createDefault()方法返回类CloseableHttpClient的对象，该对象是HttpClient接口的基本实现。使用此方法创建一个HttpClient对象 -

CloseableHttpClient httpClient = HttpClients.createDefault();

第2步 - 创建RequestBuilder对象

RequestBuilder类用于通过向其添加参数来构建请求。如果请求类型是：PUT或POST，则它将参数作为URL编码实体添加到请求中

使用post()方法创建RequestBuilder对象(类型为POST)。

//Building the post request object
RequestBuilder reqbuilder = RequestBuilder.post();

第3步 - 将Uri和参数设置为RequestBuilder。
使用RequestBuilder类的setUri()和addParameter()方法将URI和参数设置为RequestBuilder对象。

//Set URI and parameters
RequestBuilder reqbuilder = reqbuilder.setUri("http://httpbin.org/post");
reqbuilder = reqbuilder1.addParameter("Name", "username").addParameter("password", "password");

第4步 - 构建HttpUriRequest对象
设置所需参数后，使用build()方法构建HttpUriRequest对象。

//Building the HttpUriRequest object
HttpUriRequest httppost = reqbuilder2.build();

第5步 - 执行请求
CloseableHttpClient对象的execute()方法接受HttpUriRequest(接口)对象(即：HttpGet，HttpPost，HttpPut，HttpHead等)并返回响应对象。

通过将它传递给execute()方法来执行前面步骤中创建的HttpUriRequest。

//Execute the request
HttpResponse httpresponse = httpclient.execute(httppost);

示例

以下示例演示了如何通过发送登录凭据登录到表单。在这里，我们向表单发送了两个参数 - username和password，并尝试打印消息实体和请求的状态。

import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.client.methods.RequestBuilder;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
import java.io.IOException;
import java.net.URISyntaxException;

public class FormLoginExample {

   public static void main(String args[]) throws Exception {

      //Creating CloseableHttpClient object
      CloseableHttpClient httpclient = HttpClients.createDefault();

      //Creating the RequestBuilder object
      RequestBuilder reqbuilder = RequestBuilder.post();

      //Setting URI and parameters
      RequestBuilder reqbuilder1 = reqbuilder.setUri("http://httpbin.org/post");
      RequestBuilder reqbuilder2 = reqbuilder1.addParameter("Name", 
         "username").addParameter("password", "password");

      //Building the HttpUriRequest object
      HttpUriRequest httppost = reqbuilder2.build();

      //Executing the request
      HttpResponse httpresponse = httpclient.execute(httppost);

      //Printing the status and the contents of the response
      System.out.println(EntityUtils.toString(httpresponse.getEntity()));
      System.out.println(httpresponse.getStatusLine());
   }
}

执行上面示例代码，得到以下结果:

{
 "args": {},
 "data": "",
 "files": {},
 "form": {
 "Name": "username",
 "password": "password"
 },
 "headers": {
 "Accept-Encoding": "gzip,deflate",
 "Connection": "close",
 "Content-Length": "31",
 "Content-Type": "application/x-www-form-urlencoded; charset = UTF-8",
 "Host": "httpbin.org",
 "User-Agent": "Apache-HttpClient/4.5.6 (Java/1.8.0_91)"
 },
 "json": null,
 "origin": "217.216.145.180",
 "url": "http://httpbin.org/post"
}
HTTP/1.1 200 OK

Cookie表单登录

如果表单存储cookie，而不是创建默认的CloseableHttpClient对象。通过实例化BasicCookieStore类来创建CookieStore对象。

//Creating a BasicCookieStore object
BasicCookieStore cookieStore = new BasicCookieStore();

使用HttpClients类的custom()方法创建HttpClientBuilder。

//Creating an HttpClientBuilder object
HttpClientBuilder clientbuilder = HttpClients.custom();

使用setDefaultCookieStore()方法将cookie存储设置为客户端构建器。

//Setting default cookie store to the client builder object
Clientbuilder = clientbuilder.setDefaultCookieStore(cookieStore);

使用build()方法构建CloseableHttpClient对象。

//Building the CloseableHttpClient object
CloseableHttpClient httpclient = clientbuilder1.build();

通过传递执行请求来构建上面指定的HttpUriRequest对象。如果页面存储cookie，则传递的参数将添加到cookie存储中。

可以打印CookieStore对象的内容，在其中查看参数(以及以前存储的页面)。

要打印cookie，请使用getCookies()方法从CookieStore对象获取所有cookie。此方法返回List对象。使用Iterator打印列表对象内容，如下所示 -

//Printing the cookies
List list = cookieStore.getCookies();

System.out.println("list of cookies");
Iterator it = list.iterator();
if(it.hasNext()){
   System.out.println(it.next());
}