SpringFramework之RequestBodyAdvice

  • Z时代
  • 2024-01-10
  • 分类：综合

编程

    Spring版本5.1.4.release.

    本来RequestBodyAdvice和ResponseBodyAdvice是成对一起的，这里先分析RequestBodyAdvice.

    List-1

public interface RequestBodyAdvice {
	boolean supports(MethodParameter methodParameter, Type targetType,
			Class<? extends HttpMessageConverter<?>> converterType);
	HttpInputMessage beforeBodyRead(HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType) throws IOException;
	Object afterBodyRead(Object body, HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
	@Nullable
	Object handleEmptyBody(@Nullable Object body, HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
}

    如List-1所示, 看到supports方法就应该想到使用到了模板设计模式, 那么在哪使用到的呢?

    RequestBodyAdvice，只有在@RequestBody注解的方法时才有效，为什么呢? 

    一切的一切要从RequestResponseBodyMethodProcessor类上说起，如下图1，实现了HandlerMethodArgumentResolver

                                                                           图1

    RequestResponseBodyMethodProcessor类实现了HandlerMethodArgumentResolver接口, 使用到@RequestBody注解才会让RequestBodyAdvice生效就是这里导致的，如下List-2，只有supportsParameter返回true，调用调用resolveArgument方法.

    List-2

public interface HandlerMethodArgumentResolver {
    boolean supportsParameter(MethodParameter var1);
    @Nullable
    Object resolveArgument(MethodParameter var1, @Nullable ModelAndViewContainer var2, NativeWebRequest var3, @Nullable WebDataBinderFactory var4) throws Exception;
}

    List-3

RequestResponseBodyMethodProcessor{
  ...
  @Override
  public boolean supportsParameter(MethodParameter parameter) {
    return parameter.hasParameterAnnotation(RequestBody.class);
  }
  @Override
  public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
      NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
    ...
    Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
    ...
    return adaptArgumentIfNecessary(arg, parameter);
  }
...
}

    如上List-3，RequestResponseBodyMethodProcessor的supportsParameter方法中判断方法上是否有RequestBody注解，所以如果方法上没有此注解，那么RequestResponseBodyMethodProcessor的resolveArgument方法就不会被调用，而调用RequestBodyAdice的代码就在resolveArgument方法的readWithMessageConverters中。

    RequestResponseBodyMethodProcessor.readWithMessageConverters()会间接调用父类AbstractMessageConverterMethodArgumentResolver的readWithMessageConverters方法，如下List-4

    List-4

AbstractMessageConverterMethodArgumentResolver{
...
	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
    　　...
		EmptyBodyCheckingHttpInputMessage message;
		try {
			message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
      　　　　//1
			for (HttpMessageConverter<?> converter : this.messageConverters) {
				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
				GenericHttpMessageConverter<?> genericConverter =
						(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
				if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
						(targetClass != null && converter.canRead(targetClass, contentType))) {
					if (message.hasBody()) {
            　　　　　　　//2
						HttpInputMessage msgToUse = getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
						body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
								((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
						//3
            　　　　　　　body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
					}
					else {
            　　　　　　　//4
						body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
					}
					break;
				}
			}
		}
    ...
		return body;
	}
...
}

  1. 遍历HttpMessageConverter，找到能处理解析请求参数的Converter

  2. 在读取body前，调用RequestBodyAdvice的beforeBodyRead

  3. 在读取body后，调用RequestBodyAdvice的afterBodyRead

  4. 如果请求的消息体为空，则调用RequestBodyAdvice的handleEmptyBody

    AbstractMessageConverterMethodArgumentResolver中getAdvice方法返回的是RequestResponseBodyAdviceChain，看类名就知道使用到了链式设计模式。如下List-5，方法before/afterBodyRead中遍历requstBodyAdvice，逐个调用RequestBodyAdvice的supports方法，如果返回true则再调用before/afterBodyAdvice.

    List-5

class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyAdvice<Object> {
	private final List<Object> requestBodyAdvice = new ArrayList<>(4);
	private final List<Object> responseBodyAdvice = new ArrayList<>(4);
  ...
	@Override
	public HttpInputMessage beforeBodyRead(HttpInputMessage request, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType) throws IOException {
		for (RequestBodyAdvice advice : getMatchingAdvice(parameter, RequestBodyAdvice.class)) {
			if (advice.supports(parameter, targetType, converterType)) {
				request = advice.beforeBodyRead(request, parameter, targetType, converterType);
			}
		}
		return request;
	}
	@Override
	public Object afterBodyRead(Object body, HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType) {
		for (RequestBodyAdvice advice : getMatchingAdvice(parameter, RequestBodyAdvice.class)) {
			if (advice.supports(parameter, targetType, converterType)) {
				body = advice.afterBodyRead(body, inputMessage, parameter, targetType, converterType);
			}
		}
		return body;
	}
	@Override
	@Nullable
	public Object handleEmptyBody(@Nullable Object body, HttpInputMessage inputMessage, MethodParameter parameter,
			Type targetType, Class<? extends HttpMessageConverter<?>> converterType) {
		for (RequestBodyAdvice advice : getMatchingAdvice(parameter, RequestBodyAdvice.class)) {
			if (advice.supports(parameter, targetType, converterType)) {
				body = advice.handleEmptyBody(body, inputMessage, parameter, targetType, converterType);
			}
		}
		return body;
	}
...
}

    到此，我们知道了RequestBodyAdvice可以修改spring解析出的参数，而后再传入到controller类的方法上，后续我会讲解如何使用RequestBodyAdvice特性来处理业务问题.

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