首先来看下RequestMappingHandlerAdapter这个类的源码

/** * An {@link AbstractHandlerMethodAdapter} that supports {@link HandlerMethod}s * with the signature -- method argument and return types, defined in * {@code @RequestMapping}. * * 
Support for custom argument and return value types can be added via * {@link #setCustomArgumentResolvers} and {@link #setCustomReturnValueHandlers}. * Or alternatively to re-configure all argument and return value types use * {@link #setArgumentResolvers} and {@link #setReturnValueHandlers(List)}. * * @author Rossen Stoyanchev * @since 3.1 * @see HandlerMethodArgumentResolver * @see HandlerMethodReturnValueHandler */public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter		implements BeanFactoryAware, InitializingBean {	private List
     
       customArgumentResolvers;	private HandlerMethodArgumentResolverComposite argumentResolvers;	private HandlerMethodArgumentResolverComposite initBinderArgumentResolvers;	private List
      
        customReturnValueHandlers;	private HandlerMethodReturnValueHandlerComposite returnValueHandlers;	private List
       
         modelAndViewResolvers;	private ContentNegotiationManager contentNegotiationManager = new ContentNegotiationManager();	private List
        
         > messageConverters;	private WebBindingInitializer webBindingInitializer;	private AsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor("MvcAsync");	private Long asyncRequestTimeout;............................................
        
       
      
     

从注释中可以了解到通过设置setCustomArgumentResolvers and setCustomReturnValueHandlers可以实现自定义出参和入参的相关规则

由于RequestMappingHandlerAdapter实现了InitializingBean接口所以在初始化的时候会调用afterPropertiesSet()方法

public void afterPropertiesSet() {		if (this.argumentResolvers == null) {			List
    
      resolvers = getDefaultArgumentResolvers();			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);		}		if (this.initBinderArgumentResolvers == null) {			List
     
       resolvers = getDefaultInitBinderArgumentResolvers();			this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);		}		if (this.returnValueHandlers == null) {			List
      
        handlers = getDefaultReturnValueHandlers();			this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);		}		initControllerAdviceCache();	}
      
     
    

RequestMappingHandlerAdapter方法中有两个方法，getDefaultArgumentResolvers和getDefaultReturnValueHandlers方法，当然我们这边自定义是通过RequestMappingHandlerAdapter的customArgumentResolvers和customReturnValueHandlers属性来实现的，如果通过argumentResolvers和returnValueHandlers属性来直接设置的话，会将spring默认的请求入参和出参覆盖掉

getDefaultArgumentResolvers()方法可以看到我们自定义的入参处理类是被放在中后位置的，同样的getDefaultReturnValueHandlers()方法也是这样

/**	 * Return the list of return value handlers to use including built-in and	 * custom handlers provided via {@link #setReturnValueHandlers}.	 */	private List
    
      getDefaultReturnValueHandlers() {		List
     
       handlers = new ArrayList
      
       ();		// Single-purpose return value types		handlers.add(new ModelAndViewMethodReturnValueHandler());		handlers.add(new ModelMethodProcessor());		handlers.add(new ViewMethodReturnValueHandler());		handlers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.contentNegotiationManager));		handlers.add(new CallableMethodReturnValueHandler());		handlers.add(new DeferredResultMethodReturnValueHandler());		handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));		// Annotation-based return value types		handlers.add(new ModelAttributeMethodProcessor(false));		handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.contentNegotiationManager));		// Multi-purpose return value types		handlers.add(new ViewNameMethodReturnValueHandler());		handlers.add(new MapMethodProcessor());		// Custom return value types		if (getCustomReturnValueHandlers() != null) {			handlers.addAll(getCustomReturnValueHandlers());		}		// Catch-all		if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {			handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));		}		else {			handlers.add(new ModelAttributeMethodProcessor(true));		}		return handlers;	}
      
     
    

此处需要注意的是返回值的处理，由于我们自定义的是处于偏后的位置，如果其他处理类已经返回了，那么我们这边是不会再进入我们自定义的处理类中的

我们可以看到HandlerMethodArgumentResolverComposite 类

public class HandlerMethodArgumentResolverComposite implements HandlerMethodArgumentResolver {      ............................/**	 * Whether the given {@linkplain MethodParameter method parameter} is supported by any registered	 * {@link HandlerMethodArgumentResolver}.	 */	public boolean supportsParameter(MethodParameter parameter) {		return getArgumentResolver(parameter) != null;	}	/**	 * Iterate over registered {@link HandlerMethodArgumentResolver}s and invoke the one that supports it.	 * @exception IllegalStateException if no suitable {@link HandlerMethodArgumentResolver} is found.	 */	public Object resolveArgument(			MethodParameter parameter, ModelAndViewContainer mavContainer,			NativeWebRequest webRequest, WebDataBinderFactory binderFactory)			throws Exception {		HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);		Assert.notNull(resolver, "Unknown parameter type [" + parameter.getParameterType().getName() + "]");		return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);	}......................................

实现了HandlerMethodArgumentResolver接口

而在其中

/**	 * Find a registered {@link HandlerMethodArgumentResolver} that supports the given method parameter.	 */	private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {		HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);		if (result == null) {			for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {				if (logger.isTraceEnabled()) {					logger.trace("Testing if argument resolver [" + methodArgumentResolver + "] supports [" +							parameter.getGenericParameterType() + "]");				}				if (methodArgumentResolver.supportsParameter(parameter)) {					result = methodArgumentResolver;					this.argumentResolverCache.put(parameter, result);					break;				}			}		}		return result;	}

这里的argumentResolvers就是前文指定添加的

public void afterPropertiesSet() {		if (this.argumentResolvers == null) {			List
    
      resolvers = getDefaultArgumentResolvers();			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);		}		if (this.initBinderArgumentResolvers == null) {			List
     
       resolvers = getDefaultInitBinderArgumentResolvers();			this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);		}		if (this.returnValueHandlers == null) {			List
      
        handlers = getDefaultReturnValueHandlers();                       /*********就是这里************/			this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);                       /*********就是这里************/		}		initControllerAdviceCache();	}