[Spring] 요청 매핑 핸들러 어뎁터 구조

그렇다면 HTTP 메시지 컨버터는 스프링 MVC 어디쯤에서 사용되는 것일까?

 

SpringMVC 구조

SpringMVC 구조

핵심은 애노테이션 기반의 컨트롤러, 그러니까 @RequestMapping 을 처리하는 핸들러 어댑터인

RequestMappingHandlerAdapter(요청 매핑 핸들러 어뎁터)에 있다.

 

RequestMappingHandlerAdapter 동작 방식

RequestMappingHandlerAdapter 동작 방식

 

ArgumentResolver

생각해보면, 애노테이션 기반의 컨트롤러는 매우 다양한 파라미터를 사용할 수 있었다.

HttpServletRequest, Model은 물론이고, @RequestParam, @ModelAttribute 같은 애노테이션 그리고 @RequestBody, HttpEntity 같은 HTTP 메시지를 처리하는 부분까지 큰 유연함을 보여주었다.

이렇게 파라미터를 유연하게 처리할 수 있는 이유가 바로 ArgumentResolver 덕분이다.

 

애노테이션 기반 컨트롤러를 처리하는 RequestMappingHandlerAdapter는

바로 이 ArgumentResolver를 호출해서 컨트롤러(핸들러)가 필요로 하는 다양한 파라미터의 값(객체)을 생성한다.

그리고 이렇게 파라미터의 값이 모두 준비되면 컨트롤러를 호출하면서 값을 넘겨준다.

 

스프링은 30개가 넘는 ArgumentResolver를 기본으로 제공한다.

 

※ 참고 

가능한 파라미터 목록은 다음 공식 메뉴얼에서 확인할 수 있다.

Method Arguments :: Spring Framework

Method Arguments :: Spring Framework

 

Method Arguments :: Spring Framework

 

정확히는 HandlerMethodArgumentResolver 인데 줄여서  ArgumentResolver 라고 부른다.

public interface HandlerMethodArgumentResolver {
    boolean supportsParameter(MethodParameter parameter);
    
    @Nullable
    Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
                           NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;
}

 

동작 방식

ArgumentResolver의 supportsParameter()를 호출해서 해당 파라미터를 지원하는지 체크하고,

지원하면 resolverArgument()를 호출해서 실제 객체를 생성한다.

그리고 이렇게 생성된 객체가 컨트롤러 호출 시 넘어가는 것이다.

 

그리고 직접 커스텀해서 인터페이스를 확장하여 원하는 ArgumentResolver를 만들 수도 있다.

 

ReturnVlueHandler

HandlerMethodReturnValueHandler를 줄여서 ReturnValueHandler 라고 부른다.

ArgumentResolver와 비슷한데, 이것은 응답 값을 변환하고 처리한다.

 

컨트롤러에서 String으로 뷰 이름을 반환해도, 동작하는 이유가 바로 ReturnValueHandler 덕분이다.

 

스프링은 10여개가 넘는 ReturnValueHandler를 지원한다.

예) ModelAndView, @ResponseBody, HttpEntity, String

 

※ 참고

가능한 응답 값 목록은 다음 공식 메뉴얼에서 확인할 수 있다.

Return Values :: Spring Framework

Return Values :: Spring Framework

Return Values :: Spring Framework

 

HTTP 메시지 컨버터

 

HTTP 메시지 컨버터 위치

HTTP 메시지 컨버터 위치

 

HTTP 메시지 컨버터는 어디쯤 위치해있을까?

 

HTTP 메시지 컨버터를 사용하는 @RequestBody도 컨트롤러가 필요로 하는 파라미터의 값에 사용된다.

@ResponseBody 의 경우도 컨트롤러의 반환 값을 이용한다.

 

요청의 경우 @RequestBody를 처리하는 ArgumentResolver가 있고, HttpEntity를 처리하는 ArgumentResolver가 있다.

이 ArgumentResolver들이 HTTP 메시지 컨버터를 사용해서 필요한 객체를 생성하는 것이다.

 

응답의 경우 @ResponseBody와 HttpEntity를 처리하는 ReturnValueHandler가 있다.

그리고 여기에서 HTTP 메시지 컨버터를 호출해서 응답 결과를 만든다.

 

스프링 MVC는 @RequestBody, @ResponseBody 가 있으면

RequestResponseBodyMethodProcessor(ArgumentResolver)

HttpEntity가 있으면 HttpEntityMethodProcessor(ArgumentResolver)를 사용한다,.

 

※ 참고

HttpMessageConverter

/*
 * Copyright 2002-2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.converter;

import java.io.IOException;
import java.util.Collections;
import java.util.List;

import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.lang.Nullable;

/**
 * Strategy interface for converting from and to HTTP requests and responses.
 *
 * @author Arjen Poutsma
 * @author Juergen Hoeller
 * @author Rossen Stoyanchev
 * @since 3.0
 * @param <T> the converted object type
 */
public interface HttpMessageConverter<T> {

	/**
	 * Indicates whether the given class can be read by this converter.
	 * @param clazz the class to test for readability
	 * @param mediaType the media type to read (can be {@code null} if not specified);
	 * typically the value of a {@code Content-Type} header.
	 * @return {@code true} if readable; {@code false} otherwise
	 */
	boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);

	/**
	 * Indicates whether the given class can be written by this converter.
	 * @param clazz the class to test for writability
	 * @param mediaType the media type to write (can be {@code null} if not specified);
	 * typically the value of an {@code Accept} header.
	 * @return {@code true} if writable; {@code false} otherwise
	 */
	boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);

	/**
	 * Return the list of media types supported by this converter. The list may
	 * not apply to every possible target element type and calls to this method
	 * should typically be guarded via {@link #canWrite(Class, MediaType)
	 * canWrite(clazz, null}. The list may also exclude MIME types supported
	 * only for a specific class. Alternatively, use
	 * {@link #getSupportedMediaTypes(Class)} for a more precise list.
	 * @return the list of supported media types
	 */
	List<MediaType> getSupportedMediaTypes();

	/**
	 * Return the list of media types supported by this converter for the given
	 * class. The list may differ from {@link #getSupportedMediaTypes()} if the
	 * converter does not support the given Class or if it supports it only for
	 * a subset of media types.
	 * @param clazz the type of class to check
	 * @return the list of media types supported for the given class
	 * @since 5.3.4
	 */
	default List<MediaType> getSupportedMediaTypes(Class<?> clazz) {
		return (canRead(clazz, null) || canWrite(clazz, null) ?
				getSupportedMediaTypes() : Collections.emptyList());
	}

	/**
	 * Read an object of the given type from the given input message, and returns it.
	 * @param clazz the type of object to return. This type must have previously been passed to the
	 * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
	 * @param inputMessage the HTTP input message to read from
	 * @return the converted object
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotReadableException in case of conversion errors
	 */
	T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

	/**
	 * Write a given object to the given output message.
	 * @param t the object to write to the output message. The type of this object must have previously been
	 * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
	 * @param contentType the content type to use when writing. May be {@code null} to indicate that the
	 * default content type of the converter must be used. If not {@code null}, this media type must have
	 * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
	 * returned {@code true}.
	 * @param outputMessage the message to write to
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotWritableException in case of conversion errors
	 */
	void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}

 

확장

스프링은 다음을 모두 인터페이스로 제공한다. 따라서 필요하면 언ㄴ제든지 기능을 확장할 수 있다.

1) HandlerMethodArgumentResolver

2) HandlerMethodReturnValueHandler

3) HttpMessageConverter

 

스프링이 필요한 대부분의 기능을 제공하기 때문에 실제 기능을 확장할 일이 많지는 않다.

기능 확장은 WebMvcConfigurer 를 상속 받아서 스프링 빈으로 등록하면 된다.

 

WebMvcConfigurer 

/*
 * Copyright 2002-2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet.config.annotation;

import java.util.List;

import org.springframework.core.convert.converter.Converter;
import org.springframework.format.Formatter;
import org.springframework.format.FormatterRegistry;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.lang.Nullable;
import org.springframework.validation.MessageCodesResolver;
import org.springframework.validation.Validator;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
import org.springframework.web.servlet.DispatcherServlet;
import org.springframework.web.servlet.HandlerExceptionResolver;
import org.springframework.web.servlet.HandlerMapping;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;

/**
 * Defines callback methods to customize the Java-based configuration for
 * Spring MVC enabled via {@code @EnableWebMvc}.
 *
 * <p>{@code @EnableWebMvc}-annotated configuration classes may implement
 * this interface to be called back and given a chance to customize the
 * default configuration.
 *
 * @author Rossen Stoyanchev
 * @author Keith Donald
 * @author David Syer
 * @since 3.1
 */
public interface WebMvcConfigurer {

	/**
	 * Help with configuring {@link HandlerMapping} path matching options such as
	 * whether to use parsed {@code PathPatterns} or String pattern matching
	 * with {@code PathMatcher}, whether to match trailing slashes, and more.
	 * @since 4.0.3
	 * @see PathMatchConfigurer
	 */
	default void configurePathMatch(PathMatchConfigurer configurer) {
	}

	/**
	 * Configure content negotiation options.
	 */
	default void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
	}

	/**
	 * Configure asynchronous request handling options.
	 */
	default void configureAsyncSupport(AsyncSupportConfigurer configurer) {
	}

	/**
	 * Configure a handler to delegate unhandled requests by forwarding to the
	 * Servlet container's "default" servlet. A common use case for this is when
	 * the {@link DispatcherServlet} is mapped to "/" thus overriding the
	 * Servlet container's default handling of static resources.
	 */
	default void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
	}

	/**
	 * Add {@link Converter Converters} and {@link Formatter Formatters} in addition to the ones
	 * registered by default.
	 */
	default void addFormatters(FormatterRegistry registry) {
	}

	/**
	 * Add Spring MVC lifecycle interceptors for pre- and post-processing of
	 * controller method invocations and resource handler requests.
	 * Interceptors can be registered to apply to all requests or be limited
	 * to a subset of URL patterns.
	 */
	default void addInterceptors(InterceptorRegistry registry) {
	}

	/**
	 * Add handlers to serve static resources such as images, js, and, css
	 * files from specific locations under web application root, the classpath,
	 * and others.
	 * @see ResourceHandlerRegistry
	 */
	default void addResourceHandlers(ResourceHandlerRegistry registry) {
	}

	/**
	 * Configure "global" cross-origin request processing. The configured CORS
	 * mappings apply to annotated controllers, functional endpoints, and static
	 * resources.
	 * <p>Annotated controllers can further declare more fine-grained config via
	 * {@link org.springframework.web.bind.annotation.CrossOrigin @CrossOrigin}.
	 * In such cases "global" CORS configuration declared here is
	 * {@link org.springframework.web.cors.CorsConfiguration#combine(CorsConfiguration) combined}
	 * with local CORS configuration defined on a controller method.
	 * @since 4.2
	 * @see CorsRegistry
	 * @see CorsConfiguration#combine(CorsConfiguration)
	 */
	default void addCorsMappings(CorsRegistry registry) {
	}

	/**
	 * Configure simple automated controllers pre-configured with the response
	 * status code and/or a view to render the response body. This is useful in
	 * cases where there is no need for custom controller logic -- e.g. render a
	 * home page, perform simple site URL redirects, return a 404 status with
	 * HTML content, a 204 with no content, and more.
	 * @see ViewControllerRegistry
	 */
	default void addViewControllers(ViewControllerRegistry registry) {
	}

	/**
	 * Configure view resolvers to translate String-based view names returned from
	 * controllers into concrete {@link org.springframework.web.servlet.View}
	 * implementations to perform rendering with.
	 * @since 4.1
	 */
	default void configureViewResolvers(ViewResolverRegistry registry) {
	}

	/**
	 * Add resolvers to support custom controller method argument types.
	 * <p>This does not override the built-in support for resolving handler
	 * method arguments. To customize the built-in support for argument
	 * resolution, configure {@link RequestMappingHandlerAdapter} directly.
	 * @param resolvers initially an empty list
	 */
	default void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
	}

	/**
	 * Add handlers to support custom controller method return value types.
	 * <p>Using this option does not override the built-in support for handling
	 * return values. To customize the built-in support for handling return
	 * values, configure RequestMappingHandlerAdapter directly.
	 * @param handlers initially an empty list
	 */
	default void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> handlers) {
	}

	/**
	 * Configure the {@link HttpMessageConverter HttpMessageConverter}s for
	 * reading from the request body and for writing to the response body.
	 * <p>By default, all built-in converters are configured as long as the
	 * corresponding 3rd party libraries such Jackson JSON, JAXB2, and others
	 * are present on the classpath.
	 * <p><strong>Note</strong> use of this method turns off default converter
	 * registration. Alternatively, use
	 * {@link #extendMessageConverters(java.util.List)} to modify that default
	 * list of converters.
	 * @param converters initially an empty list of converters
	 */
	default void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
	}

	/**
	 * Extend or modify the list of converters after it has been, either
	 * {@link #configureMessageConverters(List) configured} or initialized with
	 * a default list.
	 * <p>Note that the order of converter registration is important. Especially
	 * in cases where clients accept {@link org.springframework.http.MediaType#ALL}
	 * the converters configured earlier will be preferred.
	 * @param converters the list of configured converters to be extended
	 * @since 4.1.3
	 */
	default void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
	}

	/**
	 * Configure exception resolvers.
	 * <p>The given list starts out empty. If it is left empty, the framework
	 * configures a default set of resolvers, see
	 * {@link WebMvcConfigurationSupport#addDefaultHandlerExceptionResolvers(List, org.springframework.web.accept.ContentNegotiationManager)}.
	 * Or if any exception resolvers are added to the list, then the application
	 * effectively takes over and must provide, fully initialized, exception
	 * resolvers.
	 * <p>Alternatively you can use
	 * {@link #extendHandlerExceptionResolvers(List)} which allows you to extend
	 * or modify the list of exception resolvers configured by default.
	 * @param resolvers initially an empty list
	 * @see #extendHandlerExceptionResolvers(List)
	 * @see WebMvcConfigurationSupport#addDefaultHandlerExceptionResolvers(List, org.springframework.web.accept.ContentNegotiationManager)
	 */
	default void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) {
	}

	/**
	 * Extending or modify the list of exception resolvers configured by default.
	 * This can be useful for inserting a custom exception resolver without
	 * interfering with default ones.
	 * @param resolvers the list of configured resolvers to extend
	 * @since 4.3
	 * @see WebMvcConfigurationSupport#addDefaultHandlerExceptionResolvers(List, org.springframework.web.accept.ContentNegotiationManager)
	 */
	default void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) {
	}

	/**
	 * Provide a custom {@link Validator} instead of the one created by default.
	 * The default implementation, assuming JSR-303 is on the classpath, is:
	 * {@link org.springframework.validation.beanvalidation.OptionalValidatorFactoryBean}.
	 * Leave the return value as {@code null} to keep the default.
	 */
	@Nullable
	default Validator getValidator() {
		return null;
	}

	/**
	 * Provide a custom {@link MessageCodesResolver} for building message codes
	 * from data binding and validation error codes. Leave the return value as
	 * {@code null} to keep the default.
	 */
	@Nullable
	default MessageCodesResolver getMessageCodesResolver() {
		return null;
	}

}