浅析ResponseBodyAdvice接口的理解和实际应用

一、官方解释

官方对ResponseBodyAdvice接口类，大致描述是这样的

允许执行{@code ResponseBody} 或 {@code ResponseEntity} 控制器方法之后，但在使用{@code HttpMessageConverter}编写主体之前自定义响应。

实现可以直接用{@code RequestMappingHandlerAdapter} 和 {@code ExceptionHandlerExceptionResolver}注释，在这种情况下，它们将被两者自动检测。

/**
 * Allows customizing the response after the execution of an {@code @ResponseBody}
 * or a {@code ResponseEntity} controller method but before the body is written
 * with an {@code HttpMessageConverter}.
 *
 * Implementations may be registered directly with
 * {@code RequestMappingHandlerAdapter} and {@code ExceptionHandlerExceptionResolver}
 * or more likely annotated with {@code @ControllerAdvice} in which case they
 * will be auto-detected by both.*/
public interface ResponseBodyAdvice<T> {
    /**
     * Whether this component supports the given controller method return type
     * and the selected {@code HttpMessageConverter} type.
     * @param returnType the return type
     * @param converterType the selected converter type
     * @return {@code true} if {@link #beforeBodyWrite} should be invoked;
     * {@code false} otherwise
     */
    boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType);

    /**
     * Invoked after an {@code HttpMessageConverter} is selected and just before
     * its write method is invoked.
     * @param body the body to be written
     * @param returnType the return type of the controller method
     * @param selectedContentType the content type selected through content negotiation
     * @param selectedConverterType the converter type selected to write to the response
     * @param request the current request
     * @param response the current response
     * @return the body that was passed in or a modified (possibly new) instance
     */
    T beforeBodyWrite(T body, MethodParameter returnType, MediaType selectedContentType,
            Class<? extends HttpMessageConverter<?>> selectedConverterType,
            ServerHttpRequest request, ServerHttpResponse response);

}

二、方法分析

ResponseBodyAdvice接口是在Controller执行return之后，在Response返回给客户端之前，执行的对response的一些处理，可以实现对response数据的一些统一封装或者加密等操作。

该接口一共有两个方法：

1. supports —— 判断是否需要执行beforeBodyWrite方法，true为执行，false不执行 —— 通过supports方法，我们可以选择哪些类或哪些方法要对response进行处理，其余的则不处理。
2. beforeBodyWrite —— 对response处理的具体执行方法。

三、在Spring项目开发中的应用场景

对Controller层的返回值进行修改或增强处理。比如返回值5，需要封装成

{
    "code": 0,
    "data": 5,
    "msg": "success"
}

格式返回给前端。

1. Controller层

@RestController //此注解包含@ResponseBody注解
@RequestMapping("/mp")
public class ResponseBodyAdviceController {
    @RequestMapping(value = "/hello", method = RequestMethod.GET)
    public int hello() {
        return 5;
    } 
}

2. 实现ResponseBodyAdvice接口的切面类

/**
*此注解针对controller层的类做增强功能，即对加了@RestController注解的类进行处理
*/
@ControllerAdvice(annotations = RestController.class)
public class RestResultWrapper implements ResponseBodyAdvice<Object> {
    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return true;
    }
 
    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request,
                                  ServerHttpResponse response) {
 
            //定义一个统一的返回类
            RestResult responseResult = new RestResult( "0", body, "success");
             //如果handler处理类的返回类型是String（即控制层的返回值类型），为了保证一致性，这里需要将ResponseResult转回去
            if(body instanceof String) {
                return JSON.toJSONString(responseResult);
            }
           //封装后的数据返回到前端页面
            return JSONObject.toJSON(responseResult);       
    } 
}

3. 返回公共类的创建

// 统一返回Rest风格的数据结构
public class RestResult<T>  implements Serializable {
    private String code = "2000";
    // 成功时返回的数据，失败时返回具体的异常信息
    private T data;
    // 请求失败返回的提示信息，给前端进行页面展示的信息
    private String message ; 

    public RestResult() {
    }
 
    @Override
    public String toString() {
        return "RestResult{" +
                "code='" + code + '\'' +
                ", data=" + data +
                ", message=" + message +
                '}';
    }
 
    public RestResult(String code, T data, String message) {
        this.code = code;
        this.data = data;
        this.message = message;
    }
}