RestTemplate

RestTemplete是一个同步Web Http客户端请求模板工具

常用API

Method group	Description
getForObject	Retrieves a representation via GET.
getForEntity	Retrieves a ResponseEntity (that is, status, headers, and body) by using GET.
headForHeaders	Retrieves all headers for a resource by using HEAD.
postForLocation	Creates a new resource by using POST and returns the Location header from the response.
postForObject	Creates a new resource by using POST and returns the representation from the response.
postForEntity	Creates a new resource by using POST and returns the representation from the response.
put	Creates or updates a resource by using PUT.
patchForObject	Updates a resource by using PATCH and returns the representation from the response. Note that the JDK HttpURLConnection does not support PATCH, but Apache HttpComponents and others do.
delete	Deletes the resources at the specified URI by using DELETE.
optionsForAllow	Retrieves allowed HTTP methods for a resource by using ALLOW.
exchange	More generalized (and less opinionated) version of the preceding methods that provides extra flexibility when needed. It accepts a RequestEntity (including HTTP method, URL, headers, and body as input) and returns a ResponseEntity.These methods allow the use of ParameterizedTypeReference instead of Class to specify a response type with generics.
execute	The most generalized way to perform a request, with full control over request preparation and response extraction through callback interfaces.

初始化

常见的Http客户端工具有

• JDK HttpURLConnenction
• Apache HttpComponents
• Okttp
• Netty

而RestTemplete都是基于以上客户端工具的进一步封装

初始化：

• 默认构造器使用的是java.net.HttpURLConnection

RestTemplate restTemplate = new RestTemplate();

• 可以在构造器中创建实现ClientHttpRequestFactory接口的实现类

比如想用Apache HttpComponents 作为底层执行引擎

        RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory());

配置类：

package com.example.demo.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;

@Configuration
public class RestConfig {

    @Bean
    public RestTemplate restTemplate(){
        return new RestTemplate();
    }
}

使用

Get请求

• getForObject获得Get请求到的返回值

  所需要的相关参数

使用案例：

package com.example.demo.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

import java.util.Collections;
import java.util.HashMap;

@RestController
public class GetController {

    @Autowired
    RestTemplate restTemplate;

    @RequestMapping(value = "/get/object",method = RequestMethod.GET)
    public String getForObject(){
        String url = "http://localhost:8089/test01/{username}/{userage}";

        HashMap<String,String> params = new HashMap();
        params.put("username","zhangsan");
        params.put("userage","12");

        //1.采用hashMap进行封装参数
        return restTemplate.getForObject(url,String.class,params);

        //2.直接传参数
       // return restTemplate.getForObject(url,String.class,"zhangsan","13");
    }
}

//RestTemplate请求的接口
@RequestMapping(value = "test01/{username}/{userage}",method = RequestMethod.GET)
public String test01(@PathVariable("username") String username,
                     @PathVariable("userage") String userage){
    return "姓名："+username+" "+"年龄："+userage;
}

• getForEntity获取详细报文包括状态码，返回体，返回头等

  所需要的相关参数

使用案例：

@RequestMapping(value = "/get/entity",method = RequestMethod.GET)
public String getForEntity(){
    String url = "http://localhost:8089/test01/{username}/{userage}";

    HashMap<String,String> params = new HashMap();
    params.put("username","zhangsan");
    params.put("userage","12");

    ResponseEntity<String> forEntity = restTemplate.getForEntity(url, String.class, params);
    HttpStatus statusCode = forEntity.getStatusCode();
    System.out.println("状态码值："+statusCode);
    int statusCodeValue = forEntity.getStatusCodeValue();
    System.out.println("状态码："+statusCodeValue);
    HttpHeaders headers = forEntity.getHeaders();
    System.out.println("返回头："+headers);
    String body = forEntity.getBody();
    System.out.println("返回体："+body);
    return body;
}

返回结果：

2022-02-10 22:25:53.161  INFO 20552 --- [nio-8089-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
2022-02-10 22:25:53.161  INFO 20552 --- [nio-8089-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
2022-02-10 22:25:53.162  INFO 20552 --- [nio-8089-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 1 ms
状态码值：200 OK
状态码：200
返回头：[Content-Type:"text/plain;charset=UTF-8", Content-Length:"29", Date:"Thu, 10 Feb 2022 14:25:53 GMT", Keep-Alive:"timeout=60", Connection:"keep-alive"]
返回体：姓名：zhangsan 年龄：12

Post请求

• postForObject获得Post请求传来的返回值

所需参数与之前的GetForObject有一定的出入，传入值与转换类型泛型类位置是颠倒的；

情景一：服务商接受参数没有@RequestBody

@PostMapping("test03")
public String test03(UserDto userDto){
    return "姓名："+userDto.getUsername()+" "+"ID："+userDto.getUserId();
}

@GetMapping("post/object")
public String postForObject(){
    //如果服务商直接接受参数或者包装类的话
    //必须要使用MultiValueMap，否则无法完成传输  ******特别注意******
    String url = "http://localhost:8089/test03/";
    MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
    map.add("userId","1000");
    map.add("username","Lisi");
    return restTemplate.postForObject(url, map, String.class);
}

情景二：服务商请求参数含有@RequestBody

@RequestHeader HttpHeaders headers 非必须，但是可以获取请求头相关参数。

@PostMapping("test02")
public String test02(@RequestBody UserDto userDto, @RequestHeader HttpHeaders headers){
    System.out.println(headers.getAccept());
    return "姓名："+userDto.getUsername()+" "+"ID："+userDto.getUserId();
}

• 方式一：HashMap传递参数

@GetMapping("post/object1")
public String postForObject1(){
    //如果服务商采用RequestBody进行接收
    //必须要使用HashMap，否则无法完成传输
    String url = "http://localhost:8089/test02/";
    HashMap<String, String> map = new HashMap<>();
    map.put("userId","1000");
    map.put("username","Lisi");
    return restTemplate.postForObject(url, map, String.class);

}

• 方式二：通过包装HttpEntity进行与服务端交互（推荐）

@GetMapping("post/object2")
public String postForObject2(){
    //如果服务商采用RequestBody进行接收
    //同时也可以采用这种方式（推荐）
    //申明请求头
    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.setContentType(MediaType.APPLICATION_JSON);

    String url = "http://localhost:8089/test02/";

    UserDto userDto = new UserDto();
    userDto.setUserId("10086");
    userDto.setUsername("王五");

    //HttpEntity包装
    HttpEntity<UserDto> userDtoHttpEntity = new HttpEntity<>(userDto, httpHeaders);

    return restTemplate.postForObject(url, userDtoHttpEntity, String.class);

}

• postForEntity获取详细信息

两种情况与postForObject一致

@GetMapping("post/entity")
public String postForEntity(){
    //申明请求头
    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.setContentType(MediaType.APPLICATION_JSON);

    String url = "http://localhost:8089/test02/";

    UserDto userDto = new UserDto();
    userDto.setUserId("10086");
    userDto.setUsername("王五");

    //HttpEntity包装
    HttpEntity<UserDto> userDtoHttpEntity = new HttpEntity<>(userDto, httpHeaders);

    ResponseEntity<String> stringResponseEntity = restTemplate.postForEntity(url, userDtoHttpEntity, String.class);
    String body = stringResponseEntity.getBody();
    return body;

}

exchange方式

这种方式可以看成是对Get与Post整个方法的一个整合，只需要在restTemplate.exchange()中指定是Get或者是Post请求即可。

package com.example.demo.controller;

import com.example.demo.dto.UserDto;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.*;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

@RestController
public class ExchangeController {

    @Autowired
    RestTemplate restTemplate;

    @GetMapping("post/change")
    public String postForEntity(){
        //申明请求头
        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.setContentType(MediaType.APPLICATION_JSON);

        String url = "http://localhost:8089/test02/";

        UserDto userDto = new UserDto();
        userDto.setUserId("10086");
        userDto.setUsername("王五");

        //HttpEntity包装
        HttpEntity<UserDto> userDtoHttpEntity = new HttpEntity<>(userDto, httpHeaders);

        ResponseEntity<String> stringResponseEntity = restTemplate.exchange(url, HttpMethod.POST,userDtoHttpEntity, String.class);
        String body = stringResponseEntity.getBody();
        return body;

    }

}