RestTemplate post请求使用map传参 Controller 接收不到值的解决方案 postForObject方法源码解析.md

结论

post方法中若是使用map传参,须要使用MultiValueMap来传递

RestTemplate 的 postForObject 方法有四个参数

• String url => 顾名思义 这个参数是请求的url路径

• Object request => 请求的body 这个参数须要再controller类用 @RequestBody 注解接收

• Class responseType => 接收响应体的类型

• 第四个参数 postForObject 方法多种重构

  Map<String,?> uriVariables => uri 变量 顾名思义 这是放置变量的地方

  Object... uriVariables => 可变长 Object 类型 参数

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Object... uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Object[])uriVariables);
}

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Map)uriVariables);
}

@Nullable
public <T> T postForObject(URI url, @Nullable Object request, Class<T> responseType) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters());
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor);
}

首先咱们使用最简单的一种 可变长Object 参数 进行传值

@Service
public class HelloService {
 
    @Autowired
    RestTemplate restTemplate;
 
    public String helloService(String name,Integer age){
        return restTemplate.postForObject("http://SERVICE-HELLO/hello?name={name}&age={age}", null, String.class, name,age);
    }
}

须要再url上拼接参数并使用{参数名}占位符站位

而后将参数放到 第四个参数 可变长 Object 参数上 便可

Controller类代码

@RestController
public class DemoController {
    @Value("${server.port}")
    String port;
 
    @PostMapping("hello")
    public String home(String name,Integer age){
        return "hello " + name + " you age is " + age + " ,i am from port:" + port;
    }
}

测试成功

接下来咱们使用 Map传值 

map传值也很简单

public String helloService(String name,Integer age){
    Map<String,Object> map = new HashMap<>();
    map.put("name",name);
    map.put("age",age);
    return restTemplate.postForObject("http://SERVICE-HELLO/hello?name={name}&age={age}", null, String.class, map);
}

只须要将参数放入到map中便可

那有些人要问了 , 为何不能用 第二个 request 参数传值 , 实际上是能够的

我试过用HashMap 和 LinkedHashMap 都是接收不到的

因此咱们来看一下源码是怎么写的

首先进入到 postForObject 方法中 发现request 参数 传入了一个 httpEntityCallBack 方法中 , 那么接着追踪

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Map)uriVariables);
}

进入httpEntityCallBack方法中
httpEntityCallBack方法又调用了 RestTemplate的HttpEntityRequestCallback方法

public <T> RequestCallback httpEntityCallback(@Nullable Object requestBody, Type responseType) {
    return new RestTemplate.HttpEntityRequestCallback(requestBody, responseType);
}

进入HttpEntityRequestCallback

这里会出现一个分支 instanceof 类型断定 requestBody 参数是不是 HttpEntity类型

public HttpEntityRequestCallback(@Nullable Object requestBody, @Nullable Type responseType) {
    super(responseType);
    if (requestBody instanceof HttpEntity) {
        this.requestEntity = (HttpEntity)requestBody;
    } else if (requestBody != null) {
        this.requestEntity = new HttpEntity(requestBody);
    } else {
        this.requestEntity = HttpEntity.EMPTY;
    }

}

若是不是则 建立一个HttpEntity类将 requestBody 参数传入

那么咱们来看一下 HttpEntity 是怎么个构造

public HttpEntity(T body) {
    this(body, (MultiValueMap)null);
}

public HttpEntity(MultiValueMap<String, String> headers) {
    this((Object)null, headers);
}

这里能够看到 HttpEntity 有两个构造方法 一个是 传入 泛型的body 另外一个是传入 MultiValueMap<String,String> headers

那么 这个MultiValueMap 是个什么东东

百度一下 发现

MultiValueMap 能够让一个key对应多个value，感受是value产生了链表结构，能够很好的解决一些很差处理的字符串问题

那么咱们来用这个奇怪的map实验一下

首先进入 MultiValueMap 接口 找到他的实现类

实现类到idea中查找

MultiValueMap 的实现类应该是 LinkedMultiValueMap

那么咱们走起

public String helloService(String name,Integer age){
    MultiValueMap<String, Object> paramMap = new LinkedMultiValueMap<>();
    paramMap.add("name",name);
    paramMap.add("age", age);
    return restTemplate.postForObject("http://SERVICE-HELLO/hello",paramMap,String.class);
}

controller代码

public class DemoController {
 
    @Value("${server.port}")
    String port;
 
    @PostMapping("hello")
    public String home(String name,Integer age){
        return "MultiValueMap : hello " + name + " you age is " + age + " ,i am from port:" + port;
    }
}

测试成功

参考

原文：https://blog.csdn.net/weixin_40461281/article/details/83472648