本篇使用的Spring版本为5.2.2.RELEASE
九大组件
SpringMVC几乎所有的功能都由九大组件来完成，所以明白九大组件的作用，对于学习SpringMVC来说非常重要。
/** 文件上传解析器 */
private MultipartResolver multipartResolver;
/** 区域解析器，用于国际化 */
private LocaleResolver localeResolver;
/** 主题解析器 */
private ThemeResolver themeResolver;
/** Handler映射信息 */
private List handlerMappings;
/** Handler适配器*/
private List handlerAdapters;
/** Handler执行异常解析器 */
private List handlerExceptionResolvers;
/** 请求到视图的转换器 */
private RequestToViewNameTranslator viewNameTranslator;
/** SpringMVC允许重定向时携带参数，存在session中，用完就销毁，所以叫FlashMap */
private FlashMapManager flashMapManager;
/** 视图解析器 */
private List viewResolvers;
复制代码 - HandlerMapping：Handler映射信息，根据请求携带的url信息查找处理器（Handler）。每个请求都需要对应的Handler处理。
- HandlerAdapter：Handler适配器，SpringMVC没有直接调用处理器（Handler），而是通过HandlerAdapter来调用，主要是为了统一Handler的调用方式
- ViewResolver：视图解析器，用来将字符串类型的视图名称解析为View类型的视图。ViewResolver需要找到渲染所用的模板和所用的技术（也就是视图的类型）进行渲染，具体的渲染过程则交由不同的视图自己完成。
- MultipartResolver：文件上传解析器，主要用来处理文件上传请求
- HandlerExceptionResolver：Handler执行异常解析器，用来对异常进行统一处理
- RequestToViewNameTranslator：请求到视图的转换器
- LocaleResolver：区域解析器，用于支持国际化
- FlashMapManager：SpringMVC允许重定向时携带参数，存在session中，用完就销毁，所以叫FlashMap
- ThemeResolver：主题解析器，用于支持不同的主题
九大组件中最重的的前三个，HandlerMapping、HandlerAdapter和ViewResolver，因为这是阅读源码时，避不开的三个组件。
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调试准备
搭建一个基本的Spring web项目即可
Controller部分
@Controller
public class IndexController {
@RequestMapping("/index/home")
public String home(String id, Student student, @RequestParam("code") String code) {
System.out.println(student.getName());
return "index";
}
@ResponseBody
@RequestMapping("/index/list")
public String list() {
return "success";
}
}
复制代码Entity部分
public class Student {
private String name;
private Integer gender;
// getter、setter
}
复制代码还是那句话，Spring源码非常庞大，不能只见树木不见森林，需要有针对性的阅读，所以本篇只需要关注主体流程即可。
核心方法
我们都知道，SpringMVC有一个用来分发请求的前端控制器DispatcherServlet，其中用来处理请求的方法就是doService，该方法定义如下
doService
/**
* Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
* for the actual dispatching.
*/
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
logRequest(request);
// Keep a snapshot of the request attributes in case of an include,
// to be able to restore the original attributes after the include.
Map attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<>();
Enumeration attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
// Make framework objects available to handlers and view objects.
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
if (this.flashMapManager != null) {
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
}
try {
// 真正执行的方法
doDispatch(request, response);
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
复制代码 doDispatch
doDispatch是doService中真正用来处理请求的方法
/**
* 实际处理请求的方法
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// 校验是否是文件上传请求
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
// 为当前请求找到一个合适的处理器（Handler）
// 返回值是一个HandlerExecutionChain，也就是处理器执行链
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
// 根据HandlerExecutionChain携带的Handler找到合适的HandlerAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
// 处理GET请求的缓存
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
// 执行拦截器的preHandle方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
// 利用HandlerAdapter来执行Handler里对应的处理方法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
// 如果没有设置视图，则应用默认的视图名
applyDefaultViewName(processedRequest, mv);
// 执行拦截器的postHandle方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
// 根据ModelAndView对象解析视图
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
复制代码该方法就是SpringMVC处理请求的整体流程，其中涉及到几个重要的方法。
getHandler
该方法定义如下
/**
* Return the HandlerExecutionChain for this request.
* 为这个request返回一个HandlerExecutionChain
*/
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
for (HandlerMapping mapping : this.handlerMappings) {
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
复制代码调试信息如下
根据调试信息可以看出，getHandler方法主要是从List handlerMappings集合中遍历查找一个合适的处理器（Handler），返回的结果是一个HandlerExecutionChain。然后再根据HandlerExecutionChain里携带的Handler去获取HandlerAdapter。
getHandlerAdapter
getHandlerAdapter方法定义如下
/**
* Return the HandlerAdapter for this handler object.
* @param handler the handler object to find an adapter for
* @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
*/
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
for (HandlerAdapter adapter : this.handlerAdapters) {
if (adapter.supports(handler)) {
return adapter;
}
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
复制代码调试信息如下
同样getHandlerAdapter方法主要是从List handlerAdapters集合中遍历查找一个合适的处理器适配器（HandlerAdapter），返回的结果是一个HandlerAdapter。
可以看到此处HandlerAdapter真正的实现类是RequestMappingHandlerAdapter。
processDispatchResult
processDispatchResult方法主要根据方法执行完成后封装的ModelAndView，转发到对应页面，定义如下
/**
* Handle the result of handler selection and handler invocation, which is
* either a ModelAndView or an Exception to be resolved to a ModelAndView.
*/
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
// 主要调用该方法渲染视图
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No view rendering, null ModelAndView returned.");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
if (mappedHandler != null) {
// Exception (if any) is already handled..
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
复制代码render
render方法定义如下
/**
* Render the given ModelAndView.
* This is the last stage in handling a request. It may involve resolving the view by name.
* @param mv the ModelAndView to render
* @param request current HTTP servlet request
* @param response current HTTP servlet response
* @throws ServletException if view is missing or cannot be resolved
* @throws Exception if there's a problem rendering the view
*/
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale =
(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
response.setLocale(locale);
View view;
String viewName = mv.getViewName();
if (viewName != null) {
// We need to resolve the view name.
// 根据给定的视图名称，解析获取View对象
view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
"' in servlet with name '" + getServletName() + "'");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
"View object in servlet with name '" + getServletName() + "'");
}
}
// Delegate to the View object for rendering.
if (logger.isTraceEnabled()) {
logger.trace("Rendering view [" + view + "] ");
}
try {
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "]", ex);
}
throw ex;
}
}
复制代码
resolveViewName
resolveViewName方法定义如下
@Nullable
protected View resolveViewName(String viewName, @Nullable Map model,
Locale locale, HttpServletRequest request) throws Exception {
if (this.viewResolvers != null) {
for (ViewResolver viewResolver : this.viewResolvers) {
View view = viewResolver.resolveViewName(viewName, locale);
if (view != null) {
return view;
}
}
}
return null;
}
复制代码 调试信息如下
根据调试信息可以看到真正解析视图的ViewResolver的是InternalResourceViewResolver类，也就是我们经常配置的一项类型
复制代码至此我们就得到了SpringMVC处理请求的完整逻辑
SpringMVC处理请求的整个流程已经梳理清楚了。
但是，有两个重要的问题没有解决，那就是：参数绑定和返回值处理。
因为在编写Controller里面的方法的时候，各种类型的参数都有，SpringMVC是怎么处理不同类型的参数的呢？ > SpringMVC处理请求完成后，一定会返回ModelAndView吗，如果加了@ResponseBody注解呢？
参数绑定
在整个流程中，还有一个最重要的方法，那就是真正执行handler的方法，参数的绑定和返回值的处理都在这个方法里，也就是
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
复制代码handle
handle方法的作用是根据请求参数，执行真正的处理方法，并且返回合适的ModelAndView对象，也有可能返回null。该方法定义如下 在AbstractHandlerMethodAdapter类中
/**
* This implementation expects the handler to be an {@link HandlerMethod}.
*/
@Override
@Nullable
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return handleInternal(request, response, (HandlerMethod) handler);
}
复制代码可以看到这个方法实现只有一行代码
handleInternal
继续深入handleInternal方法
@Override
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ModelAndView mav;
// 校验指定的请求以获取受支持的方法类型（GET、POST等）和所需的session
checkRequest(request);
// Execute invokeHandlerMethod in synchronized block if required.
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No HttpSession available -> no mutex necessary
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No synchronization on session demanded at all...
// 真正执行handler的方法
mav = invokeHandlerMethod(request, response, handlerMethod);
}
if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
}
else {
prepareResponse(response);
}
}
return mav;
}
复制代码invokeHandlerMethod
继续深入invokeHandlerMethod方法
/**
* Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
* if view resolution is required.
* 执行@RequestMapping标注的handler方法，如果需要解析视图就准备一个ModelAndView
*/
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ServletWebRequest webRequest = new ServletWebRequest(request, response);
try {
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
// HandlerMethod接口封装执行方法的信息，提供对方法参数，方法返回值，方法注释等的便捷访问。
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
// ModelAndViewContainer可以看做ModelAndView的上下文容器，关联着Model和View的信息
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);
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();
LogFormatUtils.traceDebug(logger, traceOn -> {
String formatted = LogFormatUtils.formatValue(result, !traceOn);
return "Resume with async result [" + formatted + "]";
});
invocableMethod = invocableMethod.wrapConcurrentResult(result);
}
// 真正执行Handler的方法
invocableMethod.invokeAndHandle(webRequest, mavContainer);
if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}
// 获取ModelAndeView对象
return getModelAndView(mavContainer, modelFactory, webRequest);
}
finally {
webRequest.requestCompleted();
}
}
复制代码invokeAndHandle
invokeAndHandle方法的作用是执行并处理真正响应请求的方法，该方法定义如下
/**
* Invoke the method and handle the return value through one of the
* configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
* @param webRequest the current request
* @param mavContainer the ModelAndViewContainer for this request
* @param providedArgs "given" arguments matched by type (not resolved)
*/
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 执行handler的方法
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
disableContentCachingIfNecessary(webRequest);
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
try {
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(formatErrorForReturnValue(returnValue), ex);
}
throw ex;
}
}
复制代码invokeForRequest
/**
* Invoke the method after resolving its argument values in the context of the given request.
* Argument values are commonly resolved through
* {@link HandlerMethodArgumentResolver HandlerMethodArgumentResolvers}.
* The {@code providedArgs} parameter however may supply argument values to be used directly,
* i.e. without argument resolution. Examples of provided argument values include a
* {@link WebDataBinder}, a {@link SessionStatus}, or a thrown exception instance.
* Provided argument values are checked before argument resolvers.
*
Delegates to {@link #getMethodArgumentValues} and calls {@link #doInvoke} with the
* resolved arguments.
* @param request the current request
* @param mavContainer the ModelAndViewContainer for this request
* @param providedArgs "given" arguments matched by type, not resolved
* @return the raw value returned by the invoked method
* @throws Exception raised if no suitable argument resolver can be found,
* or if the method raised an exception
* @see #getMethodArgumentValues
* @see #doInvoke
*/
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 获取参数
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Arguments: " + Arrays.toString(args));
}
// 执行
return doInvoke(args);
}
复制代码
真正的执行无非就是通过反射invoke，所以更重要的是参数是如何绑定的，详情就在getMethodArgumentValues方法
getMethodArgumentValues
getMethodArgumentValues方法用于从request请求中获取真正的参数，返回的是Object数组，该方法定义如下
/**
* Get the method argument values for the current request, checking the provided
* argument values and falling back to the configured argument resolvers.
* The resulting array will be passed into {@link #doInvoke}.
* @since 5.1.2
*/
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 获取方法上所有的参数
MethodParameter[] parameters = getMethodParameters();
if (ObjectUtils.isEmpty(parameters)) {
return EMPTY_ARGS;
}
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = findProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (!this.resolvers.supportsParameter(parameter)) {
throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
}
try {
args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
}
catch (Exception ex) {
// Leave stack trace for later, exception may actually be resolved and handled...
if (logger.isDebugEnabled()) {
String exMsg = ex.getMessage();
if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
logger.debug(formatArgumentError(parameter, exMsg));
}
}
throw ex;
}
}
return args;
}
复制代码
根据调试信息可以看到，用来处理请求参数的类是HandlerMethodArgumentResolver接口的实现类HandlerMethodArgumentResolverComposite，此时正在处理的参数是一个Student对象，并且已经把值注绑定了，也就是说真正执行绑定的是方法resolveArgument
resolveArgument
resolveArgument是真正执行绑定的的方法
/**
* Iterate over registered
* {@link HandlerMethodArgumentResolver HandlerMethodArgumentResolvers}
* and invoke the one that supports it.
* @throws IllegalArgumentException if no suitable argument resolver is found
*/
@Override
@Nullable
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
// 获取合适的参数解析器
HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
if (resolver == null) {
throw new IllegalArgumentException("Unsupported parameter type [" +
parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
}
// 执行参数绑定
return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}
复制代码getArgumentResolver
getArgumentResolver该方法用于执行参数的绑定，定义如下
/**
* Find a registered {@link HandlerMethodArgumentResolver} that supports
* the given method parameter.
*/
@Nullable
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
if (result == null) {
for (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {
if (resolver.supportsParameter(parameter)) {
result = resolver;
this.argumentResolverCache.put(parameter, result);
break;
}
}
}
return result;
}
复制代码该方法的逻辑就是先从argumentResolver缓存中找到能够执行参数绑定的HandlerMethodArgumentResolver，如果找不到就从HandlerMethodArgumentResolver找，SpringMVC支持的HandlerMethodArgumentResolver一共有26种，用来解析各种类型的参数
根据博主的调试可以知道
- RequestParamMethodArgumentResolver：处理普通参数（基本类型、包装类型、String），不管加不加@RequestParam注解
- ServletModelAttributeMethodProcessor：处理POJO类型的参数，比如自定义的Student对象
- RequestResponseBodyMethodProcessor：处理@RequestBody注解类型的参数
resolveArgument
由于不同类型的参数有不同的HandlerMethodArgumentResolver来处理，此处选取POJO类型参数的注入实现，对应的参数解析类是ModelAttributeMethodProcessor，其中resolveArgument方法用来解析（绑定）参数方法定义如下
/**
* Resolve the argument from the model or if not found instantiate it with
* its default if it is available. The model attribute is then populated
* with request values via data binding and optionally validated
* if {@code @java.validation.Valid} is present on the argument.
* @throws BindException if data binding and validation result in an error
* and the next method parameter is not of type {@link Errors}
* @throws Exception if WebDataBinder initialization fails
*/
@Override
@Nullable
public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
Assert.state(mavContainer != null, "ModelAttributeMethodProcessor requires ModelAndViewContainer");
Assert.state(binderFactory != null, "ModelAttributeMethodProcessor requires WebDataBinderFactory");
// 获取参数名
String name = ModelFactory.getNameForParameter(parameter);
// 获取参数上的ModelAttribute注解
ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
if (ann != null) {
mavContainer.setBinding(name, ann.binding());
}
Object attribute = null;
BindingResult bindingResult = null;
if (mavContainer.containsAttribute(name)) {
attribute = mavContainer.getModel().get(name);
}
else {
// Create attribute instance
try {
// 创建参数类型的实例（未注入值），底层就是通过反射调用构造方法
attribute = createAttribute(name, parameter, binderFactory, webRequest);
}
catch (BindException ex) {
if (isBindExceptionRequired(parameter)) {
// No BindingResult parameter -> fail with BindException
throw ex;
}
// Otherwise, expose null/empty value and associated BindingResult
if (parameter.getParameterType() == Optional.class) {
attribute = Optional.empty();
}
bindingResult = ex.getBindingResult();
}
}
if (bindingResult == null) {
// Bean property binding and validation;
// skipped in case of binding failure on construction.
WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name);
if (binder.getTarget() != null) {
if (!mavContainer.isBindingDisabled(name)) {
// 真正执行绑定（值注入）的方法
bindRequestParameters(binder, webRequest);
}
validateIfApplicable(binder, parameter);
if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
throw new BindException(binder.getBindingResult());
}
}
// Value type adaptation, also covering java.util.Optional
if (!parameter.getParameterType().isInstance(attribute)) {
attribute = binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter);
}
bindingResult = binder.getBindingResult();
}
// Add resolved attribute and BindingResult at the end of the model
Map bindingResultModel = bindingResult.getModel();
mavContainer.removeAttributes(bindingResultModel);
mavContainer.addAllAttributes(bindingResultModel);
return attribute;
}
复制代码 
根据调试信息也可以看到bindRequestParameters(binder, webRequest)执行完成之后，POJO类型的参数已经完成了绑定。
bindRequestParameters
/**
* This implementation downcasts {@link WebDataBinder} to
* {@link ServletRequestDataBinder} before binding.
* @see ServletRequestDataBinderFactory
*/
@Override
protected void bindRequestParameters(WebDataBinder binder, NativeWebRequest request) {
ServletRequest servletRequest = request.getNativeRequest(ServletRequest.class);
Assert.state(servletRequest != null, "No ServletRequest");
ServletRequestDataBinder servletBinder = (ServletRequestDataBinder) binder;
// 执行绑定的方法
servletBinder.bind(servletRequest);
}
复制代码bind
继续深入bind方法
public void bind(ServletRequest request) {
// 获取所有参数的键值对
MutablePropertyValues mpvs = new ServletRequestParameterPropertyValues(request);
// 处理文件上传请求
MultipartRequest multipartRequest = WebUtils.getNativeRequest(request, MultipartRequest.class);
if (multipartRequest != null) {
bindMultipart(multipartRequest.getMultiFileMap(), mpvs);
}
// 把url中携带的参数也加入到MutablePropertyValues
addBindValues(mpvs, request);
// 执行绑定（注入值）
doBind(mpvs);
}
复制代码由于调用层次过深，所以无法一步步列出下面的步骤，doBind方法的原理还是通过调用POJO对象里的setter方法设置值，可以查看最终的调试信息
根据调试信息可以看到，最终执行的还是POJO对象的setter方法，具体执行的类是BeanWrapperImpl。
了解了参数的绑定，再来看返回值的处理。
返回值处理
invokeAndHandle
回到源码invokeAndHandle方法处（ServletInvocableHandlerMethod类中），该方法定义如下
/**
* Invoke the method and handle the return value through one of the
* configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
* @param webRequest the current request
* @param mavContainer the ModelAndViewContainer for this request
* @param providedArgs "given" arguments matched by type (not resolved)
*/
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
disableContentCachingIfNecessary(webRequest);
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
try {
// 真正处理不同类型返回值的方法
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(formatErrorForReturnValue(returnValue), ex);
}
throw ex;
}
}
复制代码真正处理不同类型的返回值的方法是handleReturnValue方法
handleReturnValue
/**
* Iterate over registered {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers} and invoke the one that supports it.
* @throws IllegalStateException if no suitable {@link HandlerMethodReturnValueHandler} is found.
*/
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
// 根据返回值个返回值类型选取合适的HandlerMethodReturnValueHandler
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
// 真正的处理返回值
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
复制代码selectHandler
@Nullable
private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {
boolean isAsyncValue = isAsyncReturnValue(value, returnType);
for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
continue;
}
if (handler.supportsReturnType(returnType)) {
return handler;
}
}
return null;
}
复制代码
根据调试信息可以看到，SpringMVC为返回值提供了15个HandlerMethodReturnValueHandler的实现了来处理不同类型的返回值。
事实上，用来处理@ResponseBody类型的是RequestResponseBodyMethodProcessor。
如果对前文参数绑定还有印象的话，会发现@RequestBody类型参数绑定也是用的这个类。
继续跟进RequestResponseBodyMethodProcessor类的handleReturnValue方法
handleReturnValue
RequestResponseBodyMethodProcessor类的handleReturnValue方法定义如下
这里设置了一个非常重要的属性requestHandled，这个属性关系到是否需要返回ModelAndView对象
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
// 设置该请求是否已在处理程序中完全处理，例如@ResponseBody方法不需要视图解析器，此处就可以设置为true。
// 当控制器方法声明类型为ServletResponse或OutputStream的参数时，也可以设置此标志为true。
// 这个属性设置成true之后，上层getModelAndView获取ModelAndView时会返回Null，因为不需要视图。
// 默认值为false
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
// Try even with null return value. ResponseBodyAdvice could get involved.
// 底层就是利用java.io.OutputStreamWriter类把返回值写到网络IO
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}
复制代码继续深入writeWithMessageConverters方法，一步步调试到最后，底层就是利用java.io.OutputStreamWriter类把返回值写到网络IO
由于handleReturnValue把requestHandled设置成了true，上层在调用getModelAndView方法时会返回null，表示该请求不需要视图。感兴趣的同学自己调试一下便知。
总结
本文主要从源码的阅读和调试的角度，整体的讲解了SpringMVC处理请求的整个流程，并且讲解了参数的绑定以及返回值的处理。相信大家看完后，结合自己的调试信息，会对SpringMVC的请求处理过程有一个更深入的理解。