RequestMapping中consumes 和produces的作用

环境与参考资料

环境：spring 5.2.1 release,springboot 2.2.1.RELEASE
参考资料：HTTP 响应代码 - HTTP _ MDN

源码

先看源码

/**
	 * Narrows the primary mapping by media types that can be consumed by the
	 * mapped handler. Consists of one or more media types one of which must
	 * match to the request {@code Content-Type} header. Examples:
	 * <pre class="code">
	 * consumes = "text/plain"
	 * consumes = {"text/plain", "application/*"}
	 * consumes = MediaType.TEXT_PLAIN_VALUE
	 * </pre>
	 * Expressions can be negated by using the "!" operator, as in
	 * "!text/plain", which matches all requests with a {@code Content-Type}
	 * other than "text/plain".
	 * <p><b>Supported at the type level as well as at the method level!</b>
	 * If specified at both levels, the method level consumes condition overrides
	 * the type level condition.
	 * @see org.springframework.http.MediaType
	 * @see javax.servlet.http.HttpServletRequest#getContentType()
	 */
	String[] consumes() default {};

	/**
	 * Narrows the primary mapping by media types that can be produced by the
	 * mapped handler. Consists of one or more media types one of which must
	 * be chosen via content negotiation against the "acceptable" media types
	 * of the request. Typically those are extracted from the {@code "Accept"}
	 * header but may be derived from query parameters, or other. Examples:
	 * <pre class="code">
	 * produces = "text/plain"
	 * produces = {"text/plain", "application/*"}
	 * produces = MediaType.TEXT_PLAIN_VALUE
	 * produces = "text/plain;charset=UTF-8"
	 * </pre>
	 * <p>If a declared media type contains a parameter (e.g. "charset=UTF-8",
	 * "type=feed", type="entry") and if a compatible media type from the request
	 * has that parameter too, then the parameter values must match. Otherwise
	 * if the media type from the request does not contain the parameter, it is
	 * assumed the client accepts any value.
	 * <p>Expressions can be negated by using the "!" operator, as in "!text/plain",
	 * which matches all requests with a {@code Accept} other than "text/plain".
	 * <p><b>Supported at the type level as well as at the method level!</b>
	 * If specified at both levels, the method level produces condition overrides
	 * the type level condition.
	 * @see org.springframework.http.MediaType
	 * @see org.springframework.http.MediaType
	 */
	String[] produces() default {};

源码中主要是这两句话
consumer：Narrows the primary mapping by media types that can be consumed by the mapped handler.
produces: Narrows the primary mapping by media types that can be produced by the mapped handler.

这两者都是和MediaType相关，而MediaType让我想到了 http协议request header中 content-type 和 Accept 字段的值。

consumer 和 produces 翻译就是消费者和生产者。这让我联想到生产者消费者模型，Http请求也是一个IO流的操作，肯定有输入和输出了，consumer和produces用在某个方法上时，应该指的就是该方法只能处理 consumer类型的input，produces类型的output了。

那么这些字段相互的作用应该是这样的

如果 request header的content-type和方法中的consumer不匹配的话，那么该方法将无法处理。
如果 request header的accept和方法中的produces不匹配的话，那么该方法将无法处理。

Demo

处理方法

@PostMapping(value = "/hello",consumes = {"application/json"},produces = {"application/xml"})
@ResponseBody
public User hello(@RequestBody User user) {
      System.out.println(user);
      return user;
}

User类

import lombok.Data;

@Data
public class User {
    private String name;
    private Integer age;
}

request content-type为 application/x-www-form-urlencoded，accept不设置

后台无打印
response status 406
此情况是读取input数据就有问题了。

request content-type为 application/json,accept为不设置

request body

{
    "name":"2",
    "age":"1"

}

后台打印 User{name='2', age=1}
response status 406
response body

{
    "timestamp": "2020-12-18T03:43:35.664+0000",
    "status": 406,
    "error": "Not Acceptable",
    "message": "Could not find acceptable representation",
    "path": "/hello"
}

此情况是读取input数据没有问题，写output出现问题。

request content-type为 application/json,accept为application/xml

request body

{
    "name":"2",
    "age":"1"

}

后台打印 User{name='2', age=1}
response status 406
response body 为空

此情况是读取input数据没有问题，写output出现问题。

解决方法

在UserBean 添加XmlRootElement注解。

import lombok.Data;
import javax.xml.bind.annotation.XmlRootElement;
@Data
@XmlRootElement
public class User {
    private String name;
    private Integer age;
}

再次请求
response status 200
response body

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<user>
    <age>1</age>
    <name>2</name>
</user>

hello方法删除produces

request header为 application/json的请求返回json数据。
request header为 application/xml的请求返回xml数据。

总结

后端使用consumers和produces来指定当前方法处理 content-type和accept为什么的请求，前端使用accept告诉后端应该返回什么样的数据。