SpringMVC源码总结（八）类型转换PropertyEditor的背后

PropertyEditor是Spring最初采用的转换策略。将会转移到Converter上。本文章主要对@InitBinder注解背后代码层面的运行过程做介绍。所以最好先熟悉它的用法然后来看通代码流程。


先看实例，controller代码如下：

@Controller
public class FormAction{
	
// 这样的方法里，一般是用来注册一些PropertyEditor
	@InitBinder  
	public void initBinder(WebDataBinder binder) throws Exception {  
	    DateFormat df = new SimpleDateFormat("yyyy---MM---dd HH:mm:ss");  
	    CustomDateEditor dateEditor = new CustomDateEditor(df, true);  
	    binder.registerCustomEditor(Date.class, dateEditor);      
	}   
	
	
	@RequestMapping(value="/test/json",method=RequestMethod.GET)
	@ResponseBody
	public Map<String,Object> getFormData(Date date){
		Map<String,Object> map=new HashMap<String,Object>();
		map.put("name","lg");
		map.put("age",23);
		map.put("date",new Date());
		return map;
	}
}

xml文件仅仅开启mvc:ananotation-driven:

        <mvc:annotation-driven />  

然后访问  http://localhost:8080/test/json?date=2014---08---3 03:34:23，便看到成功的获取到了数据。接下来源代码代码分析这一过程：

由于使用了@RequestMapping所以会选择RequestMappingHandlerAdapter来调度执行相应的方法，如下：

/**
	 * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
	 * if view resolution is required.
	 */
	private ModelAndView invokeHandleMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

		ServletWebRequest webRequest = new ServletWebRequest(request, response);
//我们关注的重点重点重点重点重点重点重点重点
		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
		ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

		ModelAndViewContainer mavContainer = new ModelAndViewContainer();
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
		modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
		mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

		AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
		asyncWebRequest.setTimeout(this.asyncRequestTimeout);

		final WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.setTaskExecutor(this.taskExecutor);
		asyncManager.setAsyncWebRequest(asyncWebRequest);
		asyncManager.registerCallableInterceptors(this.callableInterceptors);
		asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

		if (asyncManager.hasConcurrentResult()) {
			Object result = asyncManager.getConcurrentResult();
			mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
			asyncManager.clearConcurrentResult();

			if (logger.isDebugEnabled()) {
				logger.debug("Found concurrent result value [" + result + "]");
			}
			requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
		}

		requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

		if (asyncManager.isConcurrentHandlingStarted()) {
			return null;
		}

		return getModelAndView(mavContainer, modelFactory, webRequest);
	}

这里面就是整个执行过程。首先绑定请求参数到方法的参数上，然后执行方法，接下来根据方法返回的类型来选择合适的HandlerMethodReturnValueHandler来进行处理，最后要么走view路线，要么直接写入response的body中返回。

我们此时关注的重点是：如何绑定请求参数到方法的参数上的呢？
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
针对每次对该handlerMethod请求产生一个绑定工厂，由这个工厂来完成数据的绑定。
这里的handlerMethod包含了 controller对象FormAction和、test/json映射到的方法即getFormData。
然后详细看下getDataBinderFactory的实现：

private WebDataBinderFactory getDataBinderFactory(HandlerMethod handlerMethod) throws Exception {
//这里的handlerType便是controller的类型FormAction
		Class<?> handlerType = handlerMethod.getBeanType();
		Set<Method> methods = this.initBinderCache.get(handlerType);
		if (methods == null) {
//关注点1：找出FormAction类的所有的含有@InitBinder的方法（方法的返回类型必须为void），找到后同时缓存起来
			methods = HandlerMethodSelector.selectMethods(handlerType, INIT_BINDER_METHODS);
			this.initBinderCache.put(handlerType, methods);
		}
		List<InvocableHandlerMethod> initBinderMethods = new ArrayList<InvocableHandlerMethod>();
		// Global methods first
//关注点2：再寻找出全局的初始化Binder的方法
		for (Entry<ControllerAdviceBean, Set<Method>> entry : this.initBinderAdviceCache .entrySet()) {
			if (entry.getKey().isApplicableToBeanType(handlerType)) {
				Object bean = entry.getKey().resolveBean();
				for (Method method : entry.getValue()) {
					initBinderMethods.add(createInitBinderMethod(bean, method));
				}
			}
		}
		for (Method method : methods) {
			Object bean = handlerMethod.getBean();
			initBinderMethods.add(createInitBinderMethod(bean, method));
		}
//关注点3：找到了所有的与该handlerMethod有关的初始化binder的方法，保存起来
		return createDataBinderFactory(initBinderMethods);
	}

上面稍微做了些注释，然后看下详细的内容：
关注点1：就是使用过滤，过滤类为：INIT_BINDER_METHODS，如下

/**
	 * MethodFilter that matches {@link InitBinder @InitBinder} methods.
	 */
	public static final MethodFilter INIT_BINDER_METHODS = new MethodFilter() {

		@Override
		public boolean matches(Method method) {
			return AnnotationUtils.findAnnotation(method, InitBinder.class) != null;
		}
	};

这个过滤类就是在handlerType即FormAction中过滤那些含有@InitBinder注解的方法。找到了之后就缓存起来，供下次使用。key为：handlerType，value为找到的方法。存至initBinderCache中。

关注点2：从initBinderAdviceCache中获取所有支持这个handlerType的method。这一块有待继续研究，这个initBinderAdviceCache是如何初始化来的等等。针对目前的工程来说，initBinderAdviceCache是为空的。

关注点3：遍历所有找到的和handlerType有关的method,然后封装成InvocableHandlerMethod，如下：

for (Method method : methods) {
			Object bean = handlerMethod.getBean();
			initBinderMethods.add(createInitBinderMethod(bean, method));
		}

private InvocableHandlerMethod createInitBinderMethod(Object bean, Method method) {
		InvocableHandlerMethod binderMethod = new InvocableHandlerMethod(bean, method);
		binderMethod.setHandlerMethodArgumentResolvers(this.initBinderArgumentResolvers);
		binderMethod.setDataBinderFactory(new DefaultDataBinderFactory(this.webBindingInitializer));
		binderMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
		return binderMethod;
	}

在封装的过程中，同时设置一些RequestMappingHandlerAdapter的一些参数进去initBinderArgumentResolvers、webBindingInitializer、parameterNameDiscoverer。
封装完所有的方法后，创建出最终的WebDataBinderFactory。如下：

protected InitBinderDataBinderFactory createDataBinderFactory(List<InvocableHandlerMethod> binderMethods)
			throws Exception {

		return new ServletRequestDataBinderFactory(binderMethods, getWebBindingInitializer());
	}

getWebBindingInitializer()也是RequestMappingHandlerAdapter的webBindingInitializer参数。

至此绑定数据的工厂完成了，包含了这个handlerType的所有的PropertyEditor。这是准备工作，然后就是等待执行这个我们自己的方法getFormData执行时来完成参数的绑定过程。

绑定参数过程即getFormData的执行过程如下：

ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);
略
requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

其中的requestMappingMethod经过了进一步的包装，已经包含刚才已经创建的绑定工厂。
执行过程如下：

public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

		Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
		if (logger.isTraceEnabled()) {
			StringBuilder sb = new StringBuilder("Invoking [");
			sb.append(getBeanType().getSimpleName()).append(".");
			sb.append(getMethod().getName()).append("] method with arguments ");
			sb.append(Arrays.asList(args));
			logger.trace(sb.toString());
		}
		Object returnValue = invoke(args);
		if (logger.isTraceEnabled()) {
			logger.trace("Method [" + getMethod().getName() + "] returned [" + returnValue + "]");
		}
		return returnValue;
	}

分两大步，绑定参数和执行方法体。最重要的就是如何来绑定参数呢？

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);
			GenericTypeResolver.resolveParameterType(parameter, getBean().getClass());
			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.isTraceEnabled()) {
						logger.trace(getArgumentResolutionErrorMessage("Error resolving argument", i), ex);
					}
					throw ex;
				}
			}
			if (args[i] == null) {
				String msg = getArgumentResolutionErrorMessage("No suitable resolver for argument", i);
				throw new IllegalStateException(msg);
			}
		}
		return args;
	}

绑定参数又引出来另一个重要名词：HandlerMethodArgumentResolver。args[i] = this.argumentResolvers.resolveArgument(
parameter, mavContainer, request, this.dataBinderFactory);的具体内容如下：

/**
	 * 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);
	}

遍历所有已注册的HandlerMethodArgumentResolver，然后找出一个适合的来进行参数绑定，对于本工程来说，getFormData(Date date)的参数date默认是request params级别的，所以使用RequestParamMethodArgumentResolver来处理这一过程。处理过程如下：

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

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

		Object arg = resolveName(namedValueInfo.name, parameter, webRequest);
		if (arg == null) {
			if (namedValueInfo.defaultValue != null) {
				arg = resolveDefaultValue(namedValueInfo.defaultValue);
			}
			else if (namedValueInfo.required) {
				handleMissingValue(namedValueInfo.name, parameter);
			}
			arg = handleNullValue(namedValueInfo.name, arg, paramType);
		}
		else if ("".equals(arg) && (namedValueInfo.defaultValue != null)) {
			arg = resolveDefaultValue(namedValueInfo.defaultValue);
		}

		if (binderFactory != null) {
			WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
			arg = binder.convertIfNecessary(arg, paramType, parameter);
		}

		handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);

		return arg;
	}

NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);获取参数信息，就是按照@RequestParam的3个属性来收集的，即defaultValue=null、required=false、name=date,
Object arg = resolveName(namedValueInfo.name, parameter, webRequest);然后就是获取原始数据,获取过程如下：

@Override
	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest webRequest) throws Exception {
		Object arg;

		HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
		MultipartHttpServletRequest multipartRequest =
			WebUtils.getNativeRequest(servletRequest, MultipartHttpServletRequest.class);

		if (MultipartFile.class.equals(parameter.getParameterType())) {
			assertIsMultipartRequest(servletRequest);
			Assert.notNull(multipartRequest, "Expected MultipartHttpServletRequest: is a MultipartResolver configured?");
			arg = multipartRequest.getFile(name);
		}
		else if (isMultipartFileCollection(parameter)) {
			assertIsMultipartRequest(servletRequest);
			Assert.notNull(multipartRequest, "Expected MultipartHttpServletRequest: is a MultipartResolver configured?");
			arg = multipartRequest.getFiles(name);
		}
		else if(isMultipartFileArray(parameter)) {
			assertIsMultipartRequest(servletRequest);
			Assert.notNull(multipartRequest, "Expected MultipartHttpServletRequest: is a MultipartResolver configured?");
			arg = multipartRequest.getFiles(name).toArray(new MultipartFile[0]);
		}
		else if ("javax.servlet.http.Part".equals(parameter.getParameterType().getName())) {
			assertIsMultipartRequest(servletRequest);
			arg = servletRequest.getPart(name);
		}
		else if (isPartCollection(parameter)) {
			assertIsMultipartRequest(servletRequest);
			arg = new ArrayList<Object>(servletRequest.getParts());
		}
		else if (isPartArray(parameter)) {
			assertIsMultipartRequest(servletRequest);
			arg = RequestPartResolver.resolvePart(servletRequest);
		}
		else {
			arg = null;
			if (multipartRequest != null) {
				List<MultipartFile> files = multipartRequest.getFiles(name);
				if (!files.isEmpty()) {
					arg = (files.size() == 1 ? files.get(0) : files);
				}
			}
			if (arg == null) {
//对于本工程，我们的重点在这里这里这里这里这里这里
				String[] paramValues = webRequest.getParameterValues(name);
				if (paramValues != null) {
					arg = paramValues.length == 1 ? paramValues[0] : paramValues;
				}
			}
		}

		return arg;
	}

通过webRequest.getParameterValues(name)来获取原始的字符串。这里便有涉及到了容器如tomcat的处理过程，这一获取参数的过程在本系列的第五篇文章tomcat的获取参数中进行了详细的源码介绍，那一篇主要是介绍乱码的。本文章不再介绍，接着说，这样就可以获取到我们请求的原始字符串"2014---08---3 03:34:23"，接下来便是执行转换绑定的过程：

if (binderFactory != null) {
			WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
			arg = binder.convertIfNecessary(arg, paramType, parameter);
		}

这一过程就是要寻找我们已经注册的所有的PropertyEditor来进行转换，如果还没有找到，则使用另一套转换流程，使用conversionService来进行转换。我们慢慢来看这一过程，有了binderFactory便可以创建出WebDataBinder，具体的创建过程如下：

public final WebDataBinder createBinder(NativeWebRequest webRequest, Object target, String objectName)
			throws Exception {
		WebDataBinder dataBinder = createBinderInstance(target, objectName, webRequest);
		if (this.initializer != null) {
			this.initializer.initBinder(dataBinder, webRequest);
		}
		initBinder(dataBinder, webRequest);
		return dataBinder;
	}

先创建出WebDataBinder，然后使用initializer的initBinder方法来初始化一些PropertyEditor，initializer的类型为我们常见的ConfigurableWebBindingInitializer即在mvc:annotation-driven时默认注册的最终设置为RequestMappingHandlerAdapter的webBindingInitializer属性值。this.initializer.initBinder(dataBinder, webRequest);过程如下：

@Override
	public void initBinder(WebDataBinder binder, WebRequest request) {
		binder.setAutoGrowNestedPaths(this.autoGrowNestedPaths);
		if (this.directFieldAccess) {
			binder.initDirectFieldAccess();
		}
		if (this.messageCodesResolver != null) {
			binder.setMessageCodesResolver(this.messageCodesResolver);
		}
		if (this.bindingErrorProcessor != null) {
			binder.setBindingErrorProcessor(this.bindingErrorProcessor);
		}
		if (this.validator != null && binder.getTarget() != null &&
				this.validator.supports(binder.getTarget().getClass())) {
			binder.setValidator(this.validator);
		}
		if (this.conversionService != null) {
			binder.setConversionService(this.conversionService);
		}
		if (this.propertyEditorRegistrars != null) {
			for (PropertyEditorRegistrar propertyEditorRegistrar : this.propertyEditorRegistrars) {
				propertyEditorRegistrar.registerCustomEditors(binder);
			}
		}
	}

即设置一些我们conversionService、messageCodesResolver、validator 等，这些参数即我们在mvc:annotation中进行设置的，若无设置，采用默认的。
继续执行initBinder(dataBinder, webRequest);

public void initBinder(WebDataBinder binder, NativeWebRequest request) throws Exception {
		for (InvocableHandlerMethod binderMethod : this.binderMethods) {
			if (isBinderMethodApplicable(binderMethod, binder)) {
				Object returnValue = binderMethod.invokeForRequest(request, null, binder);
				if (returnValue != null) {
					throw new IllegalStateException("@InitBinder methods should return void: " + binderMethod);
				}
			}
		}
	}

执行那些适合我们已经创建的WebDataBinder，怎样才叫适合的呢？看isBinderMethodApplicable(binderMethod, binder)方法

protected boolean isBinderMethodApplicable(HandlerMethod initBinderMethod, WebDataBinder binder) {
		InitBinder annot = initBinderMethod.getMethodAnnotation(InitBinder.class);
		Collection<String> names = Arrays.asList(annot.value());
		return (names.size() == 0 || names.contains(binder.getObjectName()));
	}

当initBinderMethod上的@InitBinder注解指定了value，该value可以是多个，当它包含了我们的方法的参数date,则这个initBinderMethod就会被执行。当@InitBinder注解没有指定value，则也会被执行。所以为了不用执行一些不必要的initBinderMethod，我们最好为这些initBinderMethod上的@InitBinder加上value限定。对于我们写的initBinder便因此开始执行了。
由binderFactory创建出来的WebDataBinder就此完成，然后才是详细的转换过程：

public <T> T convertIfNecessary(String propertyName, Object oldValue, Object newValue,
			Class<T> requiredType, TypeDescriptor typeDescriptor) throws IllegalArgumentException {

		Object convertedValue = newValue;

		// Custom editor for this type?
		PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName);

		ConversionFailedException firstAttemptEx = null;

		// No custom editor but custom ConversionService specified?
		ConversionService conversionService = this.propertyEditorRegistry.getConversionService();

            //略
}

这里首先使用已注册的PropertyEditor，当仍然没有找到时才使用ConversionService。对于本工程来说，由于已经手动注册了对于Date的转换的PropertyEditor即CustomDateEditor，然后便会执行CustomDateEditor的具体的转换过程。至此，大体过程就算是完了。