前言

大多數Java應用程式都會通過HTTP協議來呼叫介面訪問各種網路資源，JDK也提供了相應的HTTP工具包，但是使用起來不夠方便靈活，所以我們可以利用Apache的HttpClient來封裝一個具有訪問HTTP協議基本功能的高效工具類，為後續開發使用提供方便。

文章要點：

• HttpClient使用流程
• 工具類封裝
• 使用例項

HttpClient使用流程

1、匯入Maven依賴

<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、建立HttpClient例項

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

3、建立請求方法的例項

GET請求使用HttpGet，POST請求使用HttpPost，並傳入請求的URL

// POST請求
HttpPost post = new HttpPost(url);
// GET請求，URL中帶請求引數
HttpGet get = new HttpGet(url);

4、新增請求引數

普通形式

List<NameValuePair> list = new ArrayList<>();
list.add(new BasicNameValuePair("username", "admin"));
list.add(new BasicNameValuePair("password", "123456"));

// GET請求方式
// 由於GET請求的引數是拼裝在URL後方，所以需要構建一個完整的URL，再建立HttpGet例項
URIBuilder uriBuilder = new URIBuilder("http://www.baidu.com");
uriBuilder.setParameters(list);
HttpGet get = new HttpGet(uriBuilder.build());

// POST請求方式
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、傳送請求

呼叫HttpClient例項的execute方法傳送請求，返回一個HttpResponse物件

HttpResponse response = client.execute(post);

6、獲取結果

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

7、釋放連線

post.releaseConnection();

工具類封裝

HttpClient工具類程式碼（根據相應使用場景進行封裝）：

public class HttpClientUtil {
    // 傳送GET請求
    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();
        }
    }

    // 傳送POST請求（普通表單形式）
    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);
    }

    // 傳送POST請求（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);
    }

    // 傳送POST請求
    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();
        }
    }
}

使用例項

GET請求

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

POST請求

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請求（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);

關於HttpClient的詳細介紹看這裡：HttpClient入門