前言

大多数Java应用程序都会通过HTTP协议来调用接口访问各种网络资源，JDK也提供了相应的HTTP工具包，但是使用起来不够方便灵活，所以我们可以利用Apache的HttpClient来封装一个具有访问HTTP协议基本功能的高效工具类，为后续开发使用提供方便。

文章要点：

• HttpClient使用流程
• 工具类封装
• 使用实例

HttpClient使用流程

1、导入Maven依赖

<dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.5.5</version> </dependency> <dependency> <groupId>commons-codec</groupId> <artifactId>commons-codec</artifactId> <version>1.11</version> </dependency> <dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.8.5</version> </dependency> 

2、创建HttpClient实例

HttpClient client = HttpClientBuilder.create().build(); 

3、创建请求方法的实例

GET请求使用HttpGet，POST请求使用HttpPost，并传入请求的URL

// POST请求 HttpPost post = new HttpPost(url); // GET请求，URL中带请求参数 HttpGet get = new HttpGet(url); 

4、添加请求参数

普通形式

List<NameValuePair> list = new ArrayList<>(); list.add(new BasicNameValuePair("username", "admin")); list.add(new BasicNameValuePair("password", "")); // GET请求方式 // 由于GET请求的参数是拼装在URL后方，所以需要构建一个完整的URL，再创建HttpGet实例 URIBuilder uriBuilder = new URIBuilder("http://www.baidu.com"); uriBuilder.setParameters(list); HttpGet get = new HttpGet(uriBuilder.build()); // POST请求方式 post.setEntity(new UrlEncodedFormEntity(list, Charsets.UTF_8)); 

JSON形式

Map<String,String> map = new HashMap<>(); map.put("username", "admin"); map.put("password", ""); Gson gson = new Gson(); String json = gson.toJson(map, new TypeToken<Map<String, String>>() {}.getType()); post.setEntity(new StringEntity(json, Charsets.UTF_8)); post.addHeader("Content-Type", "application/json"); 

5、发送请求

调用HttpClient实例的execute方法发送请求，返回一个HttpResponse对象

HttpResponse response = client.execute(post); 

6、获取结果

String result = EntityUtils.toString(response.getEntity()); 

7、释放连接

post.releaseConnection(); 

工具类封装

HttpClient工具类代码（根据相应使用场景进行封装）：

public class HttpClientUtil { // 发送GET请求 public static String getRequest(String path, List<NameValuePair> parametersBody) throws RestApiException, URISyntaxException { URIBuilder uriBuilder = new URIBuilder(path); uriBuilder.setParameters(parametersBody); HttpGet get = new HttpGet(uriBuilder.build()); HttpClient client = HttpClientBuilder.create().build(); try { HttpResponse response = client.execute(get); int code = response.getStatusLine().getStatusCode(); if (code >= 400) throw new RuntimeException((new StringBuilder()).append("Could not access protected resource. Server returned http code: ").append(code).toString()); return EntityUtils.toString(response.getEntity()); } catch (ClientProtocolException e) { throw new RestApiException("postRequest -- Client protocol exception!", e); } catch (IOException e) { throw new RestApiException("postRequest -- IO error!", e); } finally { get.releaseConnection(); } } // 发送POST请求（普通表单形式） public static String postForm(String path, List<NameValuePair> parametersBody) throws RestApiException { HttpEntity entity = new UrlEncodedFormEntity(parametersBody, Charsets.UTF_8); return postRequest(path, "application/x-www-form-urlencoded", entity); } // 发送POST请求（JSON形式） public static String postJSON(String path, String json) throws RestApiException { StringEntity entity = new StringEntity(json, Charsets.UTF_8); return postRequest(path, "application/json", entity); } // 发送POST请求 public static String postRequest(String path, String mediaType, HttpEntity entity) throws RestApiException { logger.debug("[postRequest] resourceUrl: {}", path); HttpPost post = new HttpPost(path); post.addHeader("Content-Type", mediaType); post.addHeader("Accept", "application/json"); post.setEntity(entity); try { HttpClient client = HttpClientBuilder.create().build(); HttpResponse response = client.execute(post); int code = response.getStatusLine().getStatusCode(); if (code >= 400) throw new RestApiException(EntityUtils.toString(response.getEntity())); return EntityUtils.toString(response.getEntity()); } catch (ClientProtocolException e) { throw new RestApiException("postRequest -- Client protocol exception!", e); } catch (IOException e) { throw new RestApiException("postRequest -- IO error!", e); } finally { post.releaseConnection(); } } } 

使用实例

GET请求

List<NameValuePair> parametersBody = new ArrayList(); parametersBody.add(new BasicNameValuePair("userId", "admin")); String result = HttpClientUtil.getRequest("http://www.test.com/user",parametersBody); 

POST请求

List<NameValuePair> parametersBody = new ArrayList(); parametersBody.add(new BasicNameValuePair("username", "admin")); parametersBody.add(new BasicNameValuePair("password", "")); String result = HttpClientUtil.postForm("http://www.test.com/login",parametersBody); 

POST请求（JSON形式）

Map<String,String> map = new HashMap<>(); map.put("username", "admin"); map.put("password", ""); Gson gson = new Gson(); String json = gson.toJson(map, new TypeToken<Map<String, String>>() {}.getType()); String result = HttpClientUtil.postJSON("http://www.test.com/login", json);