springmvc处理请求的流程：
问题一、SpringMVC的八大组件何时初始化？
利用Spring事件监听机制。Tomcat启动，触发了DispatchServlet的初始化，初始化结束后，容器会发送事件，调用方法将从容器中获取的bean赋值到DispatchServlet的八大组件中。
问题二、SpringMVC的八大组件初始化的时候都能获取到值吗？
文件处理器不能，其他都能。所以要使用文件处理功能 需要引入额外的包。
java
//初始化八大组件
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context); //容器中有就赋值 没有就没有
initLocaleResolver(context); //容器中有就赋值，容器中没有就创建默认的
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context); //这个没啥用
initViewResolvers(context);
initFlashMapManager(context);
}
java
//文件处理器 ： 如果容器里有就用默认的。 没有就没有
private void initMultipartResolver(ApplicationContext context) {
try {
this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
if (logger.isTraceEnabled()) {
logger.trace("Detected " + this.multipartResolver);
}
else if (logger.isDebugEnabled()) {
logger.debug("Detected " + this.multipartResolver.getClass().getSimpleName());
}
}
catch (NoSuchBeanDefinitionException ex) {
// Default is no multipart resolver.
this.multipartResolver = null;
if (logger.isTraceEnabled()) {
logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
}
}
}
java
//举例一个主题解析器。从容器中获取。有就赋值。没有就获取默认的
private void initThemeResolver(ApplicationContext context) {
try {
this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class);
if (logger.isTraceEnabled()) {
logger.trace("Detected " + this.themeResolver);
}
else if (logger.isDebugEnabled()) {
logger.debug("Detected " + this.themeResolver.getClass().getSimpleName());
}
}
catch (NoSuchBeanDefinitionException ex) {
// We need to use the default.
this.themeResolver = getDefaultStrategy(context, ThemeResolver.class);
if (logger.isTraceEnabled()) {
logger.trace("No ThemeResolver '" + THEME_RESOLVER_BEAN_NAME +
"': using default [" + this.themeResolver.getClass().getSimpleName() + "]");
}
}
}问题三、前端发送一个请求，怎么找到对应的处理类的？
1.容器启动完成，发送事件，调用DispatcherServlet的初始化八大组件方法。
2.初始化HandlerMappings的时候，在创建完对应的Bean后，会执行InitialzingBean的afterPropertiesSet方法：会将请求的路径以及对应的处理方法存储在一个map里面 。
3.前端发送请求后，会从这个map里找对应的目标方法，并且找到对应的拦截器链。
java
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//找到请求对应的目标方法
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String handlerName) {
handler = obtainApplicationContext().getBean(handlerName);
}
// Ensure presence of cached lookupPath for interceptors and others
if (!ServletRequestPathUtils.hasCachedPath(request)) {
initLookupPath(request);
}
//找到请求的目标方法后，还要构造一个处理器链（把拦截器加进去）
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (logger.isTraceEnabled()) {
logger.trace("Mapped to " + handler);
}
else if (logger.isDebugEnabled() && !DispatcherType.ASYNC.equals(request.getDispatcherType())) {
logger.debug("Mapped to " + executionChain.getHandler());
}
if (hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {
CorsConfiguration config = getCorsConfiguration(handler, request);
if (getCorsConfigurationSource() != null) {
CorsConfiguration globalConfig = getCorsConfigurationSource().getCorsConfiguration(request);
config = (globalConfig != null ? globalConfig.combine(config) : config);
}
if (config != null) {
config.validateAllowCredentials();
}
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
java
//寻找目标方法
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<Match> matches = new ArrayList<>();
List<T> directPathMatches = this.mappingRegistry.getMappingsByDirectPath(lookupPath);
if (directPathMatches != null) {
addMatchingMappings(directPathMatches, matches, request);
}
if (matches.isEmpty()) {
addMatchingMappings(this.mappingRegistry.getRegistrations().keySet(), matches, request);
}
if (!matches.isEmpty()) {
Match bestMatch = matches.get(0);
if (matches.size() > 1) {
Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
matches.sort(comparator);
bestMatch = matches.get(0);
if (logger.isTraceEnabled()) {
logger.trace(matches.size() + " matching mappings: " + matches);
}
if (CorsUtils.isPreFlightRequest(request)) {
for (Match match : matches) {
if (match.hasCorsConfig()) {
return PREFLIGHT_AMBIGUOUS_MATCH;
}
}
}
else {
Match secondBestMatch = matches.get(1);
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.getHandlerMethod().getMethod();
Method m2 = secondBestMatch.getHandlerMethod().getMethod();
String uri = request.getRequestURI();
throw new IllegalStateException(
"Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
}
}
}
request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.getHandlerMethod());
handleMatch(bestMatch.mapping, lookupPath, request);
return bestMatch.getHandlerMethod();
}
else {
return handleNoMatch(this.mappingRegistry.getRegistrations().keySet(), lookupPath, request);
}
}
java
//构造拦截器链
protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain handlerExecutionChain ?
handlerExecutionChain : new HandlerExecutionChain(handler));
for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
if (interceptor instanceof MappedInterceptor mappedInterceptor) {
if (mappedInterceptor.matches(request)) {
chain.addInterceptor(mappedInterceptor.getInterceptor());
}
}
else {
chain.addInterceptor(interceptor);
}
}
return chain;
}问题四、HandlerAdapters是干啥的？
1.根据参数类型，选择合适的参数处理器，处理参数。
2.使用method.invoke()反射执行目标方法
3.根据返回值类型，选择合适的返回值处理器处理返回值。
问题五、SpringMVC视图解析器的原理？
SpringMVC用视图解析器将我们返回的字符串解析成真正的view对象进行渲染。
问题六、SpringMVC的统一异常一般是怎么处理的？
一般使用@ControllerAdvice + @ExceptionHandle进行处理的。