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/。

Java开发小技巧（五）：HttpClient工具类