Spring的ResponseBodyAdvice:执行顺序？

Question

我想实现一个自定义ResponseBodyAdvice,它只是查找Page<?>,然后将总元素的数量添加到响应头.

@ControllerAdvice
public class PageResponseAdvice implements ResponseBodyAdvice<Object> {

    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return Page.class.isAssignableFrom(returnType.getParameterType());
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
        ((Page<?>) body).getTotalElements();
        ...
    }

}

问题是:

• 为什么body(在beforeBodyWrite方法内)类型MappingJacksonValue？

• 有没有更好的方法来实现这一目标？/我使用了错误的拦截器吗？

我不想处理包装类,但我只想Page在beforeBodyWrite方法中使用普通的未修改对象.

---

编辑:

我现在只是extend AbstractMappingJacksonResponseBodyAdvice.这很好用,但感觉不对.也许有人还有更好的主意.

这是以下代码Page -> Content-Range Header:

@ControllerAdvice
public class PageResponseAdvice extends AbstractMappingJacksonResponseBodyAdvice {
    @Override public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return super.supports(returnType, converterType) && Page.class.isAssignableFrom(returnType.getParameterType());
    }

    @Override protected void beforeBodyWriteInternal(MappingJacksonValue bodyContainer, MediaType contentType, MethodParameter returnType, ServerHttpRequest request, ServerHttpResponse response) {
        Page<?> page = ((Page<?>) bodyContainer.getValue());
        Long from = null;
        Long to = null;
        if(page.getTotalElements() > 0 && page.getNumberOfElements() > 0) {
            from = Integer.valueOf(page.getNumber()).longValue()*page.getSize();
            to = from + page.getNumberOfElements() - 1;
        }
        response.getHeaders().add(
                HttpHeaders.CONTENT_RANGE,
                ContentRangeEncoder.encode(
                        "items",
                        from,
                        to,
                        page.getTotalElements()
                )
        );
        response.getHeaders().add(
                HttpHeaders.ACCEPT_RANGES,
                "items"
        );
    }
}

(如果有人有兴趣的话.这是ContentRangeEncoder):

public class ContentRangeEncoder {
    private static final Pattern TYPE_PATTERN = Pattern.compile("[a-zA-Z0-9]+");
    private static final Predicate<String> TYPE_PATTERN_PREDICATE = TYPE_PATTERN.asPredicate();

    public static <T extends Number & Comparable<T>> String encode(String unit, T from, T to, T length) {
        StringBuilder sb = new StringBuilder();
        if(unit != null) {
            Assert.isTrue(TYPE_PATTERN_PREDICATE.test(unit));
            sb.append(unit).append(" ");
        }

        if(from == null && to == null) {
            sb.append("*");
        } else {
            Assert.notNull(from);
            Assert.notNull(to);
            Assert.isTrue(from.compareTo(to) <= 0);
            sb.append(from).append("-").append(to);
        }
        sb.append("/");
        if(length == null) {
            sb.append("*");
        } else {
            Assert.isTrue(to == null || length.compareTo(to) > 0);
            sb.append(length);
        }
        return sb.toString();
    }
}

Answer 1

至于为什么主体的类型是MappingJacksonValue，这取决于您的配置和特定处理程序方法的返回值。

作为旁注，WebMvcConfigurationSupport添加JsonViewResponseBodyAdvice到RequestMappingHandlerAdapter，如果这个ResponseBodyAdvicebean 在您的 bean 之前被调用，那就可以解释为什么主体是MappingJacksonValue。主体从一个 bean 传递ResponseBodyAdvice到下一个，每个 bean 以它认为合适的任何“形式”返回主体。

以下是“AbstractMappingJacksonResponseBodyAdvice”的摘录，它是“AbstractMappingJacksonResponseBodyAdvice”的超类，JsonViewResponseBodyAdvice用于查看主体会发生什么情况：

public final Object beforeBodyWrite(@Nullable Object body,
    MethodParameter returnType, ..., ..., ..., ...) {

    //...

    MappingJacksonValue container = getOrCreateContainer(body);

    //...

    return container;
}