SpringMVC源码总结（九）HandlerMethodArgumentResolver介绍

本文章主要介绍HandlerMethodArgumentResolver在SpringMVC中的使用，介绍几个HandlerMethodArgumentResolver具体的使用情况，然后说明HandlerMethodArgumentResolver的注册来源以及如何自定义注册。

首先具体看下请求映射到的handler的对应的映射函数的参数形式有哪些：

HandlerMethodArgumentResolver接口只有两个方法：

//判断是否支持要转换的参数类型
boolean supportsParameter(MethodParameter parameter);
//当支持后进行相应的转换
Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;

HandlerMethodArgumentResolver接口的抽象类：AbstractMessageConverterMethodArgumentResolver仅仅引入了HttpMessageConverter，即转换的工作有这些HttpMessageConverter来完成具体的转换和判断由子类来实现。
如下：

public abstract class AbstractMessageConverterMethodArgumentResolver implements HandlerMethodArgumentResolver {

	protected final Log logger = LogFactory.getLog(getClass());

	protected final List<HttpMessageConverter<?>> messageConverters;

	protected final List<MediaType> allSupportedMediaTypes;
     //略
}

AbstractMessageConverterMethodArgumentResolver 的抽象子类AbstractMessageConverterMethodProcessor仅仅是加入了对响应数据进行转换的支持。
也就是AbstractMessageConverterMethodProcessor的子类不仅可以用来转换请求数据，也可以用来转换响应数据。
AbstractMessageConverterMethodProcessor的子类HttpEntityMethodProcessor，支持请求和响应的转换，代码如下：

@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return HttpEntity.class.equals(parameter.getParameterType());
	}

	@Override
	public boolean supportsReturnType(MethodParameter returnType) {
		return HttpEntity.class.isAssignableFrom(returnType.getParameterType());
	}

使用场景如下：

@RequestMapping(value="/test/http",method=RequestMethod.POST)
	@ResponseBody
	public Map<String,Object> testHttp(HttpEntity<String> httpEntity) 
            //略 
        }


@RequestMapping(value="/test/httpEntity",method=RequestMethod.GET)
	public HttpEntity<String> testHttpEntity(){
		//略
	}

AbstractMessageConverterMethodProcessor的子类RequestResponseBodyMethodProcessor：支持@RequestBody和@ResponseBody,代码如下：

@Override
	public boolean supportsParameter(MethodParameter parameter) {
               //查找参数中是否含有@RequestBody注解
		return parameter.hasParameterAnnotation(RequestBody.class);
	}

	@Override
	public boolean supportsReturnType(MethodParameter returnType) {

//查找参数中是否含有@RequestBody注解或者controller类上是否含有@RequestBody
		return ((AnnotationUtils.findAnnotation(returnType.getContainingClass(), ResponseBody.class) != null) ||
				(returnType.getMethodAnnotation(ResponseBody.class) != null));
	}

使用场景如下：

@RequestMapping(value="/test/requestBody",method=RequestMethod.POST)
	@ResponseBody
	public Map<String,Object> testrequestBody(@RequestBody Map<String,Object> map1){
		Map<String,Object> map=new HashMap<String,Object>();
		map.put("name","lg");
		map.put("age",23);
		map.put("date",new Date());
		return map;
	}

HttpEntityMethodProcessor具体的解析参数的过程：

@Override
	public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory)
			throws IOException, HttpMediaTypeNotSupportedException {

		HttpInputMessage inputMessage = createInputMessage(webRequest);
		Type paramType = getHttpEntityType(parameter);

		Object body = readWithMessageConverters(webRequest, parameter, paramType);
		return new HttpEntity<Object>(body, inputMessage.getHeaders());
	}

就是通过HttpMessageConverter来进一步的判断是否支持HttpEntity<T>中我们想要的T类型以及是否支持相应的content-type，如public Map<String,Object> testHttp(HttpEntity<String> httpEntity) ，则会选择StringHttpMessageConverter来进行转换。具体的选择过程如下：

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage,
			MethodParameter methodParam, Type targetType) throws IOException, HttpMediaTypeNotSupportedException {

		MediaType contentType;
		try {
			contentType = inputMessage.getHeaders().getContentType();
		}
		catch (InvalidMediaTypeException ex) {
			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
		}
		if (contentType == null) {
			contentType = MediaType.APPLICATION_OCTET_STREAM;
		}

		Class<?> contextClass = methodParam.getContainingClass();

		for (HttpMessageConverter<?> converter : this.messageConverters) {
			if (converter instanceof GenericHttpMessageConverter) {
				GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
				if (genericConverter.canRead(targetType, contextClass, contentType)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Reading [" + targetType + "] as \"" +
								contentType + "\" using [" + converter + "]");
					}
					return genericConverter.read(targetType, contextClass, inputMessage);
				}
			}
			Class<T> targetClass = (Class<T>)
					ResolvableType.forMethodParameter(methodParam, targetType).resolve(Object.class);
			if (converter.canRead(targetClass, contentType)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Reading [" + targetClass.getName() + "] as \"" +
							contentType + "\" using [" + converter + "]");
				}
				return ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
			}
		}

		throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
	}

同理RequestResponseBodyMethodProcessor也会使用相应的HttpMessageConverter来进行转换。如public Map<String,Object> testrequestBody(@RequestBody Map<String,Object> map1)则会选择MappingJackson2HttpMessageConverter或者MappingJacksonHttpMessageConverter来完成转换。

再看看另一类的HandlerMethodArgumentResolver：
RequestParamMethodArgumentResolver支持的类型有，一种是含@RequestParam注解的参数，另一种就是简单类型，如Integer、String、Date、URI, URL,Locale等：
源代码如下：

public boolean supportsParameter(MethodParameter parameter) {
		Class<?> paramType = parameter.getParameterType();
		if (parameter.hasParameterAnnotation(RequestParam.class)) {
			if (Map.class.isAssignableFrom(paramType)) {
				String paramName = parameter.getParameterAnnotation(RequestParam.class).value();
				return StringUtils.hasText(paramName);
			}
			else {
				return true;
			}
		}
		else {
			if (parameter.hasParameterAnnotation(RequestPart.class)) {
				return false;
			}
			else if (MultipartFile.class.equals(paramType) || "javax.servlet.http.Part".equals(paramType.getName())) {
				return true;
			}
			else if (this.useDefaultResolution) {
				return BeanUtils.isSimpleProperty(paramType);
			}
			else {
				return false;
			}
		}
	}

BeanUtils.isSimpleProperty(paramType)判断是否是简单类型的具体内容如下：

/**
	 * Check if the given type represents a "simple" property:
	 * a primitive, a String or other CharSequence, a Number, a Date,
	 * a URI, a URL, a Locale, a Class, or a corresponding array.
	 * <p>Used to determine properties to check for a "simple" dependency-check.
	 * @param clazz the type to check
	 * @return whether the given type represents a "simple" property
	 * @see org.springframework.beans.factory.support.RootBeanDefinition#DEPENDENCY_CHECK_SIMPLE
	 * @see org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#checkDependencies
	 */
	public static boolean isSimpleProperty(Class<?> clazz) {
		Assert.notNull(clazz, "Class must not be null");
		return isSimpleValueType(clazz) || (clazz.isArray() && isSimpleValueType(clazz.getComponentType()));
	}


public static boolean isSimpleValueType(Class<?> clazz) {
		return ClassUtils.isPrimitiveOrWrapper(clazz) || clazz.isEnum() ||
				CharSequence.class.isAssignableFrom(clazz) ||
				Number.class.isAssignableFrom(clazz) ||
				Date.class.isAssignableFrom(clazz) ||
				clazz.equals(URI.class) || clazz.equals(URL.class) ||
				clazz.equals(Locale.class) || clazz.equals(Class.class);
	}

即当请求为 http://localhost:8080/test?name=abc时，处理函数若为test(String name)，则对name的解析就是采用RequestParamMethodArgumentResolver来解析的。

RequestHeaderMethodArgumentResolver:主要用来处理含有@RequestHeader注解的参数，但同时该参数又不是Map类型。如下：

@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return parameter.hasParameterAnnotation(RequestHeader.class)
				&& !Map.class.isAssignableFrom(parameter.getParameterType());
	}


	@Override
	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
		String[] headerValues = request.getHeaderValues(name);
		if (headerValues != null) {
			return (headerValues.length == 1 ? headerValues[0] : headerValues);
		}
		else {
			return null;
		}
	}

源代码已经说明的很明白了。
使用场景：

@RequestMapping(value="/test/requestHeader",method=RequestMethod.GET)
	@ResponseBody
	public Map<String,Object> testrequestHeader(@RequestHeader String  Accept){

若想获取所有的header信息：则使用另一个RequestHeaderMapMethodArgumentResolver，它则用来获取所有的header信息：

public class RequestHeaderMapMethodArgumentResolver implements HandlerMethodArgumentResolver {

//这里已经写明白了，要求参数必须含有@RequestHeader注解，并且是Map类型
	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return parameter.hasParameterAnnotation(RequestHeader.class)
				&& Map.class.isAssignableFrom(parameter.getParameterType());
	}

	@Override
	public Object resolveArgument(
			MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory)
			throws Exception {

		Class<?> paramType = parameter.getParameterType();

		if (MultiValueMap.class.isAssignableFrom(paramType)) {
			MultiValueMap<String, String> result;
			if (HttpHeaders.class.isAssignableFrom(paramType)) {
				result = new HttpHeaders();
			}
			else {
				result = new LinkedMultiValueMap<String, String>();
			}
			for (Iterator<String> iterator = webRequest.getHeaderNames(); iterator.hasNext();) {
				String headerName = iterator.next();
				for (String headerValue : webRequest.getHeaderValues(headerName)) {
					result.add(headerName, headerValue);
				}
			}
			return result;
		}
		else {
			Map<String, String> result = new LinkedHashMap<String, String>();
			for (Iterator<String> iterator = webRequest.getHeaderNames(); iterator.hasNext();) {
				String headerName = iterator.next();
				String headerValue = webRequest.getHeader(headerName);
				result.put(headerName, headerValue);
			}
			return result;
		}
	}
}

从上面的解析过程可以看出，参数类型可以是普通的Map类型，也可以是MultiValueMap或者进一步的HttpHeaders，他们与普通Map类型的区别是他们对value值后者们是以List形式存放，前者是以String形式存放。
使用场景：

@RequestMapping(value="/test/requestHeader",method=RequestMethod.GET)
	@ResponseBody
	public Map<String,Object> testrequestHeader(@RequestHeader Map<String,Object> map1){


public Map<String,Object> testrequestHeader(@RequestHeader MultiValueMap<String,Object> map1){

PathVariableMethodArgumentResolver:主要针对含有@PathVariable的参数，代码如下：

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



@Override
	@SuppressWarnings("unchecked")
	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
		Map<String, String> uriTemplateVars =
			(Map<String, String>) request.getAttribute(
					HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
		return (uriTemplateVars != null) ? uriTemplateVars.get(name) : null;
	}

对于支持的类型也说明的很详细。首先必须含有@PathVariable注解，其次如果是Map类型，必须要指定@PathVariable的值，即这个
ArgumentResolver只能获取一个uri变量。要想获取多个则要使用PathVariableMapMethodArgumentResolver：

@Override
	public boolean supportsParameter(MethodParameter parameter) {
		PathVariable annot = parameter.getParameterAnnotation(PathVariable.class);
		return ((annot != null) && (Map.class.isAssignableFrom(parameter.getParameterType()))
				&& (!StringUtils.hasText(annot.value())));
	}

public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

		@SuppressWarnings("unchecked")
		Map<String, String> uriTemplateVars =
				(Map<String, String>) webRequest.getAttribute(
						HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);

		if (!CollectionUtils.isEmpty(uriTemplateVars)) {
			return new LinkedHashMap<String, String>(uriTemplateVars);
		}
		else {
			return Collections.emptyMap();
		}
	}

它要求必须含有@PathVariable注解，并且必须是Map类型，并且@PathVariable注解的value没有值。同时我们可以从PathVariableMapMethodArgumentResolver和PathVariableMethodArgumentResolver上面看出，他们的取值都是从request的属性上进行获取的webRequest.getAttribute(
HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);也就是说，在解析完@RequestMapping匹配工作后，便将这些参数设置进request的属性上，属性名为HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE。其他的HandlerMethodArgumentResolver可以自行研究，这里不再说明。


至此，我们就要说明下HandlerMethodArgumentResolver的注册来源：
它的来源分为两部分，一部分spring默认的HandlerMethodArgumentResolver，另一部分就是我们自定义的HandlerMethodArgumentResolver。
还是先看mvc:annotation-driven中配置自定义的HandlerMethodArgumentResolver：

<mvc:annotation-driven >
		<mvc:argument-resolvers>
			<bean class="xxx"></bean>
		</mvc:argument-resolvers>
	</mvc:annotation-driven>  

在mvc:argument-resolvers标签下配置相应的自定义的HandlerMethodArgumentResolver。
然后在mvc:annotation-driven的注解驱动类AnnotationDrivenBeanDefinitionParser中会有这样的代码：

ManagedList<?> argumentResolvers = getArgumentResolvers(element, parserContext);

//略
if (argumentResolvers != null) {
			handlerAdapterDef.getPropertyValues().add("customArgumentResolvers", argumentResolvers);
		}

其中getArgumentResolvers就是获取我们自定义的HandlerMethodArgumentResolver

private ManagedList<?> getArgumentResolvers(Element element, ParserContext parserContext) {
		Element resolversElement = DomUtils.getChildElementByTagName(element, "argument-resolvers");
		if (resolversElement != null) {
			ManagedList<BeanDefinitionHolder> argumentResolvers = extractBeanSubElements(resolversElement, parserContext);
			return wrapWebArgumentResolverBeanDefs(argumentResolvers, parserContext);
		}
		return null;
	}

从上面的代码可以看出，获取我们自定义的HandlerMethodArgumentResolver然后把它设置进RequestMappingHandlerAdapter的customArgumentResolvers参数中，RequestMappingHandlerAdapter有两个与HandlerMethodArgumentResolver有关的参数：

private List<HandlerMethodArgumentResolver> customArgumentResolvers;
private HandlerMethodArgumentResolverComposite argumentResolvers;

HandlerMethodArgumentResolverComposite 也仅仅是内部存放一个List<HandlerMethodArgumentResolver>集合，同时本身又继承HandlerMethodArgumentResolver，所以它的实现都是靠内部的List<HandlerMethodArgumentResolver>集合来实现的。

private final List<HandlerMethodArgumentResolver> argumentResolvers =
			new LinkedList<HandlerMethodArgumentResolver>();

//使用了适合高并发的ConcurrentHashMap来进行缓存
	private final Map<MethodParameter, HandlerMethodArgumentResolver> argumentResolverCache =
			new ConcurrentHashMap<MethodParameter, HandlerMethodArgumentResolver>(256);


	/**
	 * Return a read-only list with the contained resolvers, or an empty list.
	 */
	public List<HandlerMethodArgumentResolver> getResolvers() {
		return Collections.unmodifiableList(this.argumentResolvers);
	}

	/**
	 * Whether the given {@linkplain MethodParameter method parameter} is supported by any registered
	 * {@link HandlerMethodArgumentResolver}.
	 */
	@Override
	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.
	 */
	@Override
	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);
	}

	/**
	 * 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;
	}

在RequestMappingHandlerAdapter完成参数设置后，会调用afterPropertiesSet方法

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

getDefaultArgumentResolvers方法完成了所有的HandlerMethodArgumentResolver的汇总，如下：

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()));
		resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters()));
		resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new RequestHeaderMapMethodArgumentResolver());
		resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));

		// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());
		resolvers.add(new HttpEntityMethodProcessor(getMessageConverters()));
		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
//获取我们自定义的HandlerMethodArgumentResolver
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

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

		return resolvers;
	}

不仅汇总了spring默认的，同时加进来我们自定义的HandlerMethodArgumentResolver。至此，HandlerMethodArgumentResolver的来龙去脉都说清楚了。然后就是我们自定义HandlerMethodArgumentResolver,下一篇文章再说。