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The RestTemplate is the core class for client-side access to RESTful services. It is conceptually similar to other template classes in Spring, such as JdbcTemplate and JmsTemplate and other template classes found in other Spring portfolio projects.

前言

这里总结一下，怎么用代码发起HTTP请求：Post、Get等。之前类似的请求都是用Postman。RestTemplate网上教程也是很多，但是编程就是要多实战，可能受制于版本等各种客观因素，同样的教程实战可能会有不同的结果。

对于实现HTTP请求有多种类库，目前用过RestTemplate和HttpComponents，以Post方法为例，创建以下服务：

    @RequestMapping(value = "/selectPost", method = RequestMethod.POST)
    public ModelMap selectPost(String username, String password) {
        List<User> userList = userService.selectPost(username, password);

        return result(Constant.SUCCESS_CODE, Constant.SUCCESS_MSG, userList);
    }

RestTemplate

RestTemplate支持以下6六种HTTP请求方法。这些请求方法是定义在RestOperations接口中的，RestTemplate提供实现。

Spring

代码示例

    @Test
    public void selectPost1() {
        //参数
        String url = "http://localhost:9091/user/selectPost";
        String username = "admin";
        String password = "1212";

        MultiValueMap<String, Object> map = new LinkedMultiValueMap<>();
        map.add("username", username);
        map.add("password", password);
        //设置响应头
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
        //封装参数响应头
        HttpEntity<MultiValueMap<String, Object>> entity = new HttpEntity<>(map, headers);


        RestTemplate template = new RestTemplate();
        //执行请求，并接受返回结果
        ResponseEntity<String> result = template.exchange(url, HttpMethod.POST, entity, String.class);
        JSONObject jsonObject = JSONObject.fromString(result.getBody());
        System.out.println("===========selectPost1===============");

        System.out.println(jsonObject);

    }

结果

返回字符串

{"msg":"SUCCESS","result":[{"password":"1212","createTime":"2017-38-14 07:38:05","updateUser":"admin","createUser":"admin","updateTime":"2017-38-14 07:38:05","id":3,"username":"admin"},{"password":"1212","createTime":"2017-39-14 07:39:47","updateUser":"admin","createUser":"admin","updateTime":"2017-39-14 07:39:47","id":4,"username":"admin"}],"code":"200"}

返回对象

通常返回一个字符串在项目中用处不大，如果能够直接返回相应的实体，那就更好了。

创建返回值对应的对象

public class UserVo {
    private String code;
    private String msg;
    private List<User> result;
    }

以对象的类型接受返回值

UserVo result = template.postForObject(url, entity, UserVo.class);

返回结果：

UserVo{code='200', msg='SUCCESS', result=[User{id=3, username='admin', password='1212', createUser='admin', createTime=Sat Jan 14 15:38:05 CST 2017, updateUser='admin', updateTime=Sat Jan 14 15:38:05 CST 2017}, User{id=4, username='admin', password='1212', createUser='admin', createTime=Sat Jan 14 15:39:47 CST 2017, updateUser='admin', updateTime=Sat Jan 14 15:39:47 CST 2017}]}

HttpComponents

HttpComponents是Apache下的一个项目，目前最新的版本是4.5.3。
需要说明一下，4.3.6中提供的HttpPost是非线程安全的。

Spring

代码示例

    @Test
    public void selectPost3() {
        //参数
        String url = "http://localhost:9091/user/selectPost";
        String username = "admin";
        String password = "1212";

        try {
            //初始化
            CloseableHttpClient httpClient = HttpClients.createDefault();
            CloseableHttpResponse httpResponse = null;
            HttpPost httpPost = new HttpPost(url);

            //请求参数
            List<NameValuePair> nameValuePairs = new ArrayList<>();
            nameValuePairs.add(new BasicNameValuePair("username", username));
            nameValuePairs.add(new BasicNameValuePair("password", password));

            //编码 封装参数
            UrlEncodedFormEntity formEntity = new UrlEncodedFormEntity(nameValuePairs, "utf-8");
            httpPost.setEntity(formEntity);
            httpPost.addHeader("Content-Type", "application/x-www-form-urlencoded");

            //执行请求
            httpResponse = httpClient.execute(httpPost);

            //返回结果
            org.apache.http.HttpEntity respHttpEntity = httpResponse.getEntity();
            String resultStr = EntityUtils.toString(respHttpEntity, "utf-8");

            System.out.println(resultStr);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

结果

{"code":"200","msg":"SUCCESS","result":[{"id":3,"username":"admin","password":"1212","createUser":"admin","createTime":"2017-38-14 07:38:05","updateUser":"admin","updateTime":"2017-38-14 07:38:05"},{"id":4,"username":"admin","password":"1212","createUser":"admin","createTime":"2017-39-14 07:39:47","updateUser":"admin","updateTime":"2017-39-14 07:39:47"}]}

扩展

HttpComponents 结构：

HttpComponents Core 是一组低级的HTTP传输组件，支持2中I/O模型:基于经典Java I/O的阻塞I/O和非阻塞I/O。
HttpComponents Client 是一个基于HttpCore的符合HTTP/1.1标准的HTTP代理实现。
HttpComponents AsyncClient 是一个基于HttpCore NIO的符合HTTP/1.1标准的HTTP代理实现和HttpClient组件。