SpringMVC源码总结（八）类型转换PropertyEditor的背后

2021-10-08 16:33:44


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

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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： 
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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： 
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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： 
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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？ 
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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： 
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/**
     * 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： 
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@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： 
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@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： 
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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： 
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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： 
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@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); 
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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 
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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： 
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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。
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