SpringMVC重要接口（一）HandlerMethodArgumentResolver

在初学SpringMVC框架时，我就一直有一个疑问，为什么controller方法上竟然可以放这么多的参数，而且都能得到想要的对象，比如HttpServletRequest或HttpServletResponse， 各种注解@RequestParam、@RequestHeader、@RequestBody、@PathVariable、@ModelAttribute等。相信很多初学者都曾经感慨过。

这其实都是org.springframework.web.method.support.HandlerMethodArgumentResolver的功劳。

package org.springframework.web.method.support;

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.WebDataBinder;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;

/**
 * Strategy interface for resolving method parameters into argument values in
 * the context of a given request.
 *
 * @author Arjen Poutsma
 * @since 3.1
 * @see HandlerMethodReturnValueHandler
 */
public interface HandlerMethodArgumentResolver {

    /**
     * Whether the given {@linkplain MethodParameter method parameter} is
     * supported by this resolver.
     * @param parameter the method parameter to check
     * @return {@code true} if this resolver supports the supplied parameter;
     * {@code false} otherwise
     */
    boolean supportsParameter(MethodParameter parameter);

    /**
     * Resolves a method parameter into an argument value from a given request.
     * A {@link ModelAndViewContainer} provides access to the model for the
     * request. A {@link WebDataBinderFactory} provides a way to create
     * a {@link WebDataBinder} instance when needed for data binding and
     * type conversion purposes.
     * @param parameter the method parameter to resolve. This parameter must
     * have previously been passed to {@link #supportsParameter} which must
     * have returned {@code true}.
     * @param mavContainer the ModelAndViewContainer for the current request
     * @param webRequest the current request
     * @param binderFactory a factory for creating {@link WebDataBinder} instances
     * @return the resolved argument value, or {@code null}
     * @throws Exception in case of errors with the preparation of argument values
     */
    Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;

}

HandlerMethodArgumentResolver接口包含两个接口方法

• supportsParameter

通过该方法我们如果需要对某个参数进行处理 只要此处返回true即可， 
通过MethodParameter可以获取该方法参数上的一些信息， 如方法参数中的注解信息等

public class ServletRequestMethodArgumentResolver implements HandlerMethodArgumentResolver {

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        Class paramType = parameter.getParameterType();
        return (WebRequest.class.isAssignableFrom(paramType) ||
                ServletRequest.class.isAssignableFrom(paramType) ||
                MultipartRequest.class.isAssignableFrom(paramType) ||
                HttpSession.class.isAssignableFrom(paramType) ||
                Principal.class.isAssignableFrom(paramType) ||
                Locale.class == paramType ||
                TimeZone.class == paramType ||
                "java.time.ZoneId".equals(paramType.getName()) ||
                InputStream.class.isAssignableFrom(paramType) ||
                Reader.class.isAssignableFrom(paramType) ||
                HttpMethod.class == paramType);
    }

public class PathVariableMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver
        implements UriComponentsContributor {

    private static final TypeDescriptor STRING_TYPE_DESCRIPTOR = TypeDescriptor.valueOf(String.class);


    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        if (!parameter.hasParameterAnnotation(PathVariable.class)) {
            return false;
        }
        if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
            String paramName = parameter.getParameterAnnotation(PathVariable.class).value();
            return StringUtils.hasText(paramName);
        }
        return true;
    }

• resolveArgument 
  该方法就是对参数的解析，返回的Object会自动赋值到参数对象中。

SpringMVC自己的 
HandlerMethodArgumentResolver有哪些，并且会以什么样的顺序执行呢？

其实定义在RequestMappingHandlerAdapter里：

    /**
     * Return the list of argument resolvers to use including built-in resolvers
     * and custom resolvers provided via {@link #setCustomArgumentResolvers}.
     */
    private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
        List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();

        // Annotation-based argument resolution
        resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
        resolvers.add(new RequestParamMapMethodArgumentResolver());
        resolvers.add(new PathVariableMethodArgumentResolver());
        resolvers.add(new PathVariableMapMethodArgumentResolver());
        resolvers.add(new MatrixVariableMethodArgumentResolver());
        resolvers.add(new MatrixVariableMapMethodArgumentResolver());
        resolvers.add(new ServletModelAttributeMethodProcessor(false));
        resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
        resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
        resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
        resolvers.add(new RequestHeaderMapMethodArgumentResolver());
        resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
        resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
        resolvers.add(new SessionAttributeMethodArgumentResolver());
        resolvers.add(new RequestAttributeMethodArgumentResolver());

        // Type-based argument resolution
        resolvers.add(new ServletRequestMethodArgumentResolver());
        resolvers.add(new ServletResponseMethodArgumentResolver());
        resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
        resolvers.add(new RedirectAttributesMethodArgumentResolver());
        resolvers.add(new ModelMethodProcessor());
        resolvers.add(new MapMethodProcessor());
        resolvers.add(new ErrorsMethodArgumentResolver());
        resolvers.add(new SessionStatusMethodArgumentResolver());
        resolvers.add(new UriComponentsBuilderMethodArgumentResolver());

        // Custom arguments
        if (getCustomArgumentResolvers() != null) {
            resolvers.addAll(getCustomArgumentResolvers());
        }

        // Catch-all
        resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
        resolvers.add(new ServletModelAttributeMethodProcessor(true));

        return resolvers;
    }

HandlerMethodArgumentResolver子类	注解 or 类
org.springframework.web.method.annotation.RequestParamMethodArgumentResolver	@RequestParam
org.springframework.web.method.annotation.RequestParamMapMethodArgumentResolver	@RequestParam
org.springframework.web.servlet.mvc.method.annotation.PathVariableMethodArgumentResolver	@PathVariable
org.springframework.web.servlet.mvc.method.annotation.PathVariableMapMethodArgumentResolver	@PathVariable
org.springframework.web.servlet.mvc.method.annotation.MatrixVariableMethodArgumentResolver	@MatrixVariable
org.springframework.web.servlet.mvc.method.annotation.MatrixVariableMapMethodArgumentResolver	@MatrixVariable
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor	@ModelAttribute
org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor	@RequestBody
org.springframework.web.servlet.mvc.method.annotation.RequestPartMethodArgumentResolver	@RequestPart
org.springframework.web.method.annotation.RequestHeaderMethodArgumentResolver	@RequestHeader
org.springframework.web.method.annotation.RequestHeaderMapMethodArgumentResolver	@RequestHeader
org.springframework.web.servlet.mvc.method.annotation.ServletCookieValueMethodArgumentResolver	@CookieValue
org.springframework.web.method.annotation.ExpressionValueMethodArgumentResolver	@Value
org.springframework.web.servlet.mvc.method.annotation.SessionAttributeMethodArgumentResolver	@SessionAttribute
org.springframework.web.servlet.mvc.method.annotation.RequestAttributeMethodArgumentResolver	@RequestAttribute
org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver	HttpServletRequest
org.springframework.web.servlet.mvc.method.annotation.ServletResponseMethodArgumentResolver	HttpServletResponse
org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor	HttpEntity
org.springframework.web.servlet.mvc.method.annotation.RedirectAttributesMethodArgumentResolver	RedirectAttributes
org.springframework.web.method.annotation.ModelMethodProcessor	Model
org.springframework.web.method.annotation.MapMethodProcessor	Map
org.springframework.web.method.annotation.ErrorsMethodArgumentResolver	Errors
org.springframework.web.method.annotation.SessionStatusMethodArgumentResolver	SessionStatus
org.springframework.web.servlet.mvc.method.annotation.UriComponentsBuilderMethodArgumentResolver	UriComponentsBuilder

而 RequestMappingHandlerAdapter 被注册进去是使用了 注解，可参考我的这篇文章《SpringMVC 解读—— 》。

这里我们就不再自己重新写一个了，我们采用SpringMVC自带的一个 
org.springframework.data.web.PageableHandlerMethodArgumentResolver，

它将实现的功能如下：

绑定分页需要用到的参数，只是如何把这个Resolver注册进去呢，方式如下

      
         class="org.springframework.data.web.PageableHandlerMethodArgumentResolver"/>