Springboot版本2.0.5.release。

    前一篇博客讲解了如何用自定义HandlerMethodArgumentResolver进行参数解析，现在来看看springmvc是如何实现这个的。

    来看下RequestMappingHandlerAdapter，

                                                               图1

    HandlerMethodArgumentResolver是个接口，只有俩个方法，supportsParameter返回true标示支持解析，之后调用resolveArgument将结果作为参数传入，如下List-1所示。

List-1

public interface HandlerMethodArgumentResolver {
    boolean supportsParameter(MethodParameter var1);

    @Nullable
    Object resolveArgument(MethodParameter var1, @Nullable ModelAndViewContainer var2, NativeWebRequest var3, @Nullable WebDataBinderFactory var4) throws Exception;
}

    RequestMappingHandlerAdapter中有个属性argumentResolvers，是HandlerMethodArgumentResolverComposite，由类名称可以看出使用了组合模式。RequestMappingHandlerAdapter的getDefaultArgumentResolvers，这些是默认的参数处理器。

List-2

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;
}

    如下图2所示：

                                                              图2

    DispatcherServlet的doDispatch方法中，间接调用ServletInvocableHandlerMethod的invokeAndHandle方法，之后会逐个调用HandlerMethodArgumentResolver。所以我们自定义的HandlerMethodArgumentResolver在图2中的步骤9和步骤10处处理。如下List-3所示，遍历所有的参数，对每个参数调用HandlerMethodArgumentResolverComposite的support和resolve方法，HandlerMethodArgumentResolverComposite再委托给各个HandlerMethodArgumentResolver处理。

List-3

private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    MethodParameter[] parameters = getMethodParameters();
    Object[] args = new Object[parameters.length];
    for (int i = 0; i < parameters.length; i++) {
        MethodParameter parameter = parameters[i];
        parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
        args[i] = resolveProvidedArgument(parameter, providedArgs);
        if (args[i] != null) {
            continue;
        }
        if (this.argumentResolvers.supportsParameter(parameter)) {
            try {
                args[i] = this.argumentResolvers.resolveArgument(
                        parameter, mavContainer, request, this.dataBinderFactory);
                continue;
            }
            catch (Exception ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                }
                throw ex;
            }
        }
        if (args[i] == null) {
            throw new IllegalStateException("Could not resolve method parameter at index " +
                    parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
                    ": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
        }
    }
    return args;
}

    值得注意的是图2中的步骤12，HandlerMethodReturnValueHandler可以改变controller返还的值。

Reference

1. 源码https://github.com/spring-projects/spring-boot/tree/v2.0.5.RELEASE

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