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 = HttpClient build er.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", "123456"));
// 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("");
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", "123456");
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("",parametersBody);
 

POST请求

List<NameValuePair> parametersBody = new ArrayList();
parametersBody.add(new BasicNameValuePair("username", "admin"));
parametersBody.add(new BasicNameValuePair("password", "123456"));
String result = HttpClientUtil.postForm("",parametersBody);
 

POST请求（JSON形式）

Map<String,String> map = new HashMap<>();
map.put("username", "admin");
map.put("password", "123456");
Gson gson = new Gson();
String json = gson.toJson(map, new TypeToken<Map<String, String>>() {}.getType());
String result = HttpClientUtil.postJSON("", json);