DispatcherServlet以及其线程安全性
DispatcherServlet
探究到DispatcherServlet这个MVC中最重要的一个前端请求调度器时，突然考虑到了它本身的线程安全性，由此引入对DispatcherServlet的了解
首先是一个大概的MVC执行框图
看代码是spring-webmvc-4.3.17版本，首先看一下DispatcherServlet的继承结构
DispatcherServlet直接继承于FrameworkServlet，然后是HttpServletBean，再往上是HttpServlet直到GenericServlet定义了最基本的servlet标准。
我们知道在spring的托管下，所有的servlet在IOC容器初始化的时候都已经加载完毕，包含这个最重要的DispatcherServlet，作为web服务入口的前端控制器，所需要的参数都在web.xml中进行配置，它会拦截匹配规则的请求。
DispatcherServlet初始化
在这个继承体系下，当ServletWrappingController.afterPropertiesSet()初始化所有的Servlet时，会调用HttpServlet的init()方法进行初始化，实际调用的是DispatcherServlet.initStrategies()方法
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
initLocaleResolver(context); //本地化解析
initThemeResolver(context); //主题解析
initHandlerMappings(context); //通过HandlerMapping，将请求映射到处理器
initHandlerAdapters(context); //通过HandlerAdapter支持多种类型的处理器
initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
initRequestToViewNameTranslator(context); //直接解析请求到视图名
initViewResolvers(context); //通过ViewResolver解析逻辑视图名到具体视图实现
initFlashMapManager(context); //flash映射管理器
}
在初始化的时候，DispatcherServlet已经持有了所有url对应的Controller的映射关系，方便后续将request中请求的url分发给适合的Controller。
DispatcherServlet线程安全？
其实这是主要想关注的点，DispatcherServlet请求调度器是否是线程安全的呢？
在web容器中对DispatcherServlet初始化的时候是放在Synchronized修饰的同步代码块中实现的，可以保证Servlet创建和初始化的时候是对线程可见的，因此可以保证DispatcherServlet调用initStrategies方法是可以把成员变量刷新到主存中的。但是值得注意的是在DispatcherServlet中有一些变量是未被static final修饰的，这不会引起线程不安全么
/** Detect all HandlerMappings or just expect "handlerMapping" bean? */
private boolean detectAllHandlerMappings = true;
/** Detect all HandlerAdapters or just expect "handlerAdapter" bean? */
private boolean detectAllHandlerAdapters = true;
/** Detect all HandlerExceptionResolvers or just expect "handlerExceptionResolver" bean? */
private boolean detectAllHandlerExceptionResolvers = true;
/** Detect all ViewResolvers or just expect "viewResolver" bean? */
private boolean detectAllViewResolvers = true;
/** Throw a NoHandlerFoundException if no Handler was found to process this request? **/
private boolean throwExceptionIfNoHandlerFound = false;
/** Perform cleanup of request attributes after include request? */
private boolean cleanupAfterInclude = true;
/** MultipartResolver used by this servlet */
private MultipartResolver multipartResolver;
/** LocaleResolver used by this servlet */
private LocaleResolver localeResolver;
/** ThemeResolver used by this servlet */
private ThemeResolver themeResolver;
/** List of HandlerMappings used by this servlet */
private List<HandlerMapping> handlerMappings;
/** List of HandlerAdapters used by this servlet */
private List<HandlerAdapter> handlerAdapters;
/** List of HandlerExceptionResolvers used by this servlet */
private List<HandlerExceptionResolver> handlerExceptionResolvers;
/** RequestToViewNameTranslator used by this servlet */
private RequestToViewNameTranslator viewNameTranslator;
/** FlashMapManager used by this servlet */
private FlashMapManager flashMapManager;
/** List of ViewResolvers used by this servlet */
private List<ViewResolver> viewResolvers;
但仔细查看这些变量的使用处，发现都是在initStrategies()初始化时调用的init方法中进行了赋值，这样就可以保证未被安全修饰符修饰的变量始终是可见的，尽管它们未被修饰，但是并不会影响到DispatcherServlet的可见性。为什么要这么写？特别是那几个未被修改的Boolean类型的参数，尚未得知。
DispatcherServlet处理（分发）请求
按照上一篇blog中所述的在filter过滤后请求到达DispatcherServlet，大概的流程是
ServletHolder.handle() ->
FrameworkServlet.service() -> FrameworkServlet.processRequest()
DispatcherServlet.doService() -> DispatcherServlet.doDispatch()
其中，FrameworkServlet定义了Servlet的service抽象处理流程
/** FrameworkServlet
*/
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
processRequest(request, response);
}
else {
super.service(request, response);
}
}
针对FrameworkServlet处理不同类型的请求（GET、POST、PUT、DELETE、HEAD、PATCH、OPTIONS、TRACE），以POST请求为例，接着到了FrameworkServlet.processRequest()方法。 FrameworkServlet 抽象定义了处理流程，留待子类来实现该方法，完成具体的请求处理。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
initContextHolders(request, localeContext, requestAttributes);
try {
// 子类DispatcherServlet去实现
doService(request, response);
}
catch (ServletException ex) {
failureCause = ex;
throw ex;
}
catch (IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
看看DispatcherServlet中具体的处理流程
/**
* 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 {
if (logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
" processing " + request.getMethod() + " request for [" + getRequestUri(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<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<String, Object>();
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());
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 {
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);
}
}
}
}
DispatcherServlet分发请求的细节
具体有三个功能：
- 根据映射关系获取注册到容器中的Servlet配置对应的handler
- 将请求分发到具体的handler，这里会获取HandlerMapping中最匹配的HandlerMethod，它对应了具体处理request的Controller以及对应的方法
- 反射执行方法处理请求并获取处理结果ModelAndView
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 {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (logger.isDebugEnabled()) {
logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
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);
}
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);
}
}
}
}