上一篇SpringBoot 2.1 | 第三篇：RestTemplate请求HTTP(1)简单运用了RestTemplate,
本篇主要讲解RestTemplate的主要请求方法, getForObject, getForEntity, exchange(方法列举只用了Get)。Method包括GET、POST、PUT、DELETE。参数传递，解析等。

maven-jar-plugin。在讲述使用之前，想要理解SpringMVC的几个常用注解：

1. @Controller：修饰class，用来创建处理http请求的对象
2. @RestController：Spring4之后加入的注解，原来在@Controller中返回json需要@ResponseBody来配合，如果直接用@RestController替代@Controller就不需要再配置@ResponseBody，默认返回json格式。
3. @RequestMapping：配置url映射
4. @PostMapping: 这个是@RequestMapping+POST方法的简写
5. @RequestHeader: 请求Header参数
6. @PathVariable: URL路径参数，比如/user/{id}中的id参数
7. @RequestParam: URL请求参数，比如/user?id=1中的id参数
8. @RequestBody: 请求Body参数

参照教程【SpringBoot 2.1 | 第一篇：构建第一个SpringBoot工程】新建一个Spring Boot项目，名字叫demoresttemplatemethod, 在目录src/main/java/resources 下找到配置文件application.properties，重命名为application.yml。

请求都需要用到restTemplate对象，用@Bean的方式注入，用同一个工厂对象统一管理ClientHttpRequestFactory。
新建类：
com.zgpeace.demoresttemplatemethod.configure.RestTemplateConfig

package com.zgpeace.demoresttemplatemethod.configure;import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.client.SimpleClientHttpRequestFactory;
import org.springframework.web.client.RestTemplate;@Configuration
public class RestTemplateConfig {@Beanpublic RestTemplate restTemplate(ClientHttpRequestFactory factory) {return new RestTemplate(factory);}@Beanpublic ClientHttpRequestFactory simpleClientHttpRequestFactory() {SimpleClientHttpRequestFactory factory = new SimpleClientHttpRequestFactory();factory.setReadTimeout(5000);//msfactory.setConnectTimeout(15000);//msreturn factory;}
}

Springboot教程？结果实体User用于数据传递，新建类
com.zgpeace.demoresttemplatemethod.model.User

package com.zgpeace.demoresttemplatemethod.model;public class User {private Integer id;private String methodName;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getMethodName() {return methodName;}public void setMethodName(String methodName) {this.methodName = methodName;}@Overridepublic String toString() {return "User{" +"id=" + id +", methodName='" + methodName + '\'' +'}';}
}

REST，即Representational State Transfer的缩写，对这个词组的翻译是表现层状态转化。

RESTful是一种软件设计风格，就是目前最流行的一种互联网软件架构。它结构清晰、符合标准、易于理解、扩展方便，所以正得到越来越多网站的采用。

Spring MVC。这里为了演示，会将数据保存到内存Map中，实际使用肯定是保存到数据库中。

创建Restful被调用类：
com.zgpeace.demoresttemplatemethod.web.UserController
调用方法包括GET、POST、PUT、DELETE, 以及带参数的例子。
说明：PUT是幂等性方法，也就是请求多次跟一次的效果一样，就像微信转账一样，一次转账弱网情况下可能会重试，但是结果不会因为多次尝试而不一样, 一般用户更新。而POST不是幂等性的方法，也就是多次重试，会有多个结果，一般用于创建。

package com.zgpeace.demoresttemplatemethod.web;import com.zgpeace.demoresttemplatemethod.model.User;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;@RestController
public class UserController {@RequestMapping(value = "/testGet", method = RequestMethod.GET)public User testGet() {User user = new User();user.setId(1);user.setMethodName("get");return user;}@RequestMapping(value = "/testPost", method = RequestMethod.POST)public User testPost() {User user = new User();user.setId(1);user.setMethodName("post");return user;}@RequestMapping(value = "/testPostParam", method = RequestMethod.POST)public String testPostParam(@RequestParam("id") String id, @RequestParam("methodName") String methodName) {System.out.println("Post id: " + id);System.out.println("Post methodName: " + methodName);return "post id{" + id + "} success";}@RequestMapping(value = "/testPut", method = RequestMethod.PUT)public String testPut(@RequestParam("id") String id, @RequestParam("methodName") String methodName) {System.out.println("put id: " + id);System.out.println("put methodName: " + methodName);return "put id{" + id + "} success";}@RequestMapping(value = "/testDel", method = RequestMethod.DELETE)public String testDel(@RequestParam("id") String id) {System.out.println("del id: " + id);return "del id{" + id + "} success";}}

例子：@RequestParam(value = "name", required = true, defaultValue = "defaultName"

@RequestParam 支持下面四种参数
name 绑定本次参数的名称，要跟URL上面的一样
value 跟name一样的作用，是name属性的一个别名
defaultValue 如果本次请求没有携带这个参数，或者参数为空，那么就会启用默认值
required 这个参数是不是必须的

调用的方法包括：getForObject, getForEntity, exchange(方法列举只用了Get，Method包括GET、POST、PUT、DELETE).
新建类com.zgpeace.demoresttemplatemethod.web.UserRequestController

package com.zgpeace.demoresttemplatemethod.web;import com.zgpeace.demoresttemplatemethod.model.User;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.*;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;import java.net.URI;
import java.net.URISyntaxException;@RestController
public class UserRequestController {@Autowiredprivate RestTemplate restTemplate;private static String PROTOCOL = "http";private static String HOST = "localhost";private static String PORT = "8080";private static String PRE_URL = PROTOCOL + "://" + HOST + ":" + PORT + "/";private static String GET_URL = PRE_URL + "testGet";private static String POST_URL = PRE_URL + "testPost";private static String POST_PARAM_URL = PRE_URL +  "testPostParam";private static String PUT_URL = PRE_URL + "testPut";private static String DEL_URL = PRE_URL +  "testDel";@GetMapping("/requestTestGet")public String requestTestGet() throws URISyntaxException {// 1. getForObject()User user1 = restTemplate.getForObject(GET_URL, User.class);System.out.println("get user1: " + user1);// 2. getForEntity()ResponseEntity<User> responseEntity1 = restTemplate.getForEntity(GET_URL, User.class);HttpStatus statusCode = responseEntity1.getStatusCode();HttpHeaders header = responseEntity1.getHeaders();User user2 = responseEntity1.getBody();System.out.println("get user2: " + user2);System.out.println("get statusCode: " + statusCode);System.out.println("get header: " + header);// 3. exchange()RequestEntity requestEntity = RequestEntity.get(new URI(GET_URL)).build();ResponseEntity<User> responseEntity2 = restTemplate.exchange(requestEntity, User.class);User user3 = responseEntity2.getBody();System.out.println("get user3: " + user3);return "requestTestGet";}@GetMapping("/requestTestPost")public String requestTestPost() throws URISyntaxException {HttpHeaders headers = new HttpHeaders();String data = new String();HttpEntity<String> formEntity = new HttpEntity<String>(data, headers);// 1. postForObject()User user1 = restTemplate.postForObject(POST_URL, formEntity, User.class);System.out.println("post user1: " + user1);// 2. postForEntity()ResponseEntity<User> responseEntity1 = restTemplate.postForEntity(POST_URL, formEntity, User.class);HttpStatus statusCode = responseEntity1.getStatusCode();HttpHeaders header = responseEntity1.getHeaders();User user2 = responseEntity1.getBody();System.out.println("post user2: " + user2);System.out.println("post statusCode: " + statusCode);System.out.println("post header: " + header);// 3. exchange()RequestEntity requestEntity = RequestEntity.post(new URI(POST_URL)).body(formEntity);ResponseEntity<User> responseEntity2 = restTemplate.exchange(requestEntity, User.class);User user3 = responseEntity2.getBody();System.out.println("post user3: " + user3);return "requestTestPost";}@GetMapping("/requestTestPostParam")public String requestTestPostParam() {HttpHeaders headers = new HttpHeaders();MultiValueMap<String, String> map = new LinkedMultiValueMap<String, String>();map.add("id", "100");map.add("methodName", "requestTestPostParam");HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<MultiValueMap<String, String>>(map, headers);String data = restTemplate.postForObject(POST_PARAM_URL, request, String.class);System.out.println("requestTestPostParam data: " + data);System.out.println("requestTestPostParam success");return "requestTestPostParam";}@GetMapping("requestTestPut")public String requestTestPut() {MultiValueMap<String, String> map = new LinkedMultiValueMap<String, String>();map.add("id", "101");map.add("methodName", "requestTestPut");restTemplate.put(PUT_URL, map);System.out.println("requestTestPut success");return "requestTestPut";}@GetMapping("requestTestDel")public String requestTestDel() {HttpHeaders headers = new HttpHeaders();//  请勿轻易改变此提交方式，大部分的情况下，提交方式都是表单提交headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);//  封装参数，千万不要替换为Map与HashMap，否则参数无法传递MultiValueMap<String, String> map = new LinkedMultiValueMap<String, String>();map.add("id", "101");map.add("methodName", "requestTestDel");HttpEntity<MultiValueMap<String, String>> requestEntity = new HttpEntity<MultiValueMap<String, String>>(map, headers);// 方法一ResponseEntity<String> resp = restTemplate.exchange(DEL_URL , HttpMethod.DELETE, requestEntity, String.class, 227);System.out.println("requestTestDel response: " + resp.getBody());// 方法二
//        restTemplate.delete(DEL_URL + "?id={id}", 102);System.out.println("requestTestDel success");return "requestTestDel";}}

1. HttpEntity的结构
   HttpEntity是对HTTP请求的封装，包含两部分，header与body，header用于设置请求头，而body则用于设置请求体，所以其的构造器如下：

//  value为请求体
//  header为请求头
HttpEntity<String> requestEntity = new HttpEntity<String>(value, headers);

1. 后端处理前端提交的数据时，既可以使用Form解析，也可以使用JSON解析Payload字符串。

Form解析可以直接从Request对象中获取请求参数，这样对象转换与处理相对容易，但在大片JSON数据需要提交时，可能会出现大量的数据拆分与处理工作，另外针对集合类型的处理，也是其比较孱弱的地方。

而Payload的优势是一次可以提交大量JSON字符串，但无法从Request从获取参数，也会受限于JSON解析的深度（尤其是有多层对象级联的情况，最底层的对象几乎无法转换为具体类型）。

 HttpHeaders headers = new HttpHeaders();//  form表单提交headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);//  payload提交headers.setContentType(MediaType.APPLICATION_JSON);

1. 用exchange方法提交
   exchange可以执行所有HTTP的方法(GET、POST、PUT、DELETE、HEAD).
   HttpEntity封装参数的时候必须用MultiValueMap，千万不要替换为Map与HashMap，否则参数无法传递。

 //  封装参数，MultiValueMap千万不要替换为Map与HashMap，否则参数无法传递MultiValueMap<String, String> map = new LinkedMultiValueMap<String, String>();map.add("id", "101");map.add("methodName", "requestTestDel");HttpEntity<MultiValueMap<String, String>> requestEntity = new HttpEntity<MultiValueMap<String, String>>(map, headers);

requestTestGet运行输出

get user1: User{id=1, methodName='get'}
get user2: User{id=1, methodName='get'}
get statusCode: 200 OK
get header: {Content-Type=[application/json;charset=UTF-8], Transfer-Encoding=[chunked], Date=[Thu, 27 Dec 2018 10:28:22 GMT]}
get user3: User{id=1, methodName='get'}

requestTestPost运行输出

post user1: User{id=1, methodName='post'}
post user2: User{id=1, methodName='post'}
post statusCode: 200 OK
post header: {Content-Type=[application/json;charset=UTF-8], Transfer-Encoding=[chunked], Date=[Thu, 27 Dec 2018 10:29:50 GMT]}
post user3: User{id=1, methodName='post'}

requestTestPostParam运行输出

PostParam id: 100
PostParam methodName: requestTestPostParam
requestTestPostParam data: post id{100} success
requestTestPostParam success

requestTestPut运行输出

put id: 101
put methodName: requestTestPut
requestTestPut success

requestTestDel运行输出

requestTestDel before
del id: 101
requestTestDel response: del id{101} success
requestTestDel success

恭喜你！ 已经完成RestTemplate中的常用方法用法。

代码下载：
https://github.com/zgpeace/Spring-Boot2.1/tree/master/demoresttemplatemethod