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", "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("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", "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("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", "123456"));
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", "123456");
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);

关于HttpClient的详细介绍看这里：HttpClient入门

本文为作者kMacro原创，转载请注明来源：https://zkhdev.github.io/2018/10/12/java_dev5/。