前言

在使用Spring和SpringMVC的老版本進(jìn)行開(kāi)發(fā)時(shí)，我們需要配置很多的xml文件，非常的繁瑣，總是讓用戶自行選擇配置也是非常不好的?；诩s定大于配置的規(guī)定，Spring提供了很多注解幫助我們簡(jiǎn)化了大量的xml配置；但是在使用SpringMVC時(shí)，我們還會(huì)使用到WEB-INF/web.xml，但實(shí)際上我們是完全可以使用Java類來(lái)取代xml配置的，這也是后來(lái)SpringBoott的實(shí)現(xiàn)原理。本篇就來(lái)看看Spring是如何實(shí)現(xiàn)完全的零XML配置。

正文

先來(lái)看一下原始的web.xml配置：

<!DOCTYPE web-app PUBLIC '-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN' 'http://java.sun.com/dtd/web-app_2_3.dtd' ><web-app> <context-param> <param-name>contextConfigLocation</param-name> <param-value> <!--加載spring配置--> classpath:spring.xml </param-value> </context-param> <context-param> <param-name>webAppRootKey</param-name> <param-value>ServicePlatform.root</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> <!--<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>--> </listener> <servlet> <servlet-name>spring-dispatcher</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <!--springmvc的配置文件--> <param-name>contextConfigLocation</param-name> <param-value>classpath:spring-dispatcher.xml</param-value> </init-param> <load-on-startup>0</load-on-startup> </servlet> <servlet-mapping> <servlet-name>spring-dispatcher</servlet-name> <url-pattern>/</url-pattern> </servlet-mapping></web-app>

這里各個(gè)配置的作用簡(jiǎn)單說(shuō)下，context-param是加載我們主的sping.xml配置，比如一些bean的配置和開(kāi)啟注解掃描等；listener是配置監(jiān)聽(tīng)器，Tomcat啟動(dòng)會(huì)觸發(fā)監(jiān)聽(tīng)器調(diào)用；servlet則是配置我們自定義的Servlet實(shí)現(xiàn)，比如DispatcherServlet。還有其它很多配置就不一一說(shuō)明了，在這里主要看到記住context-param和servlet配置，這是SpringIOC父子容器的體現(xiàn)。

在之前的I文章中講過(guò)IOC容器是以父子關(guān)系組織的，但估計(jì)大部分人都不能理解，除了看到復(fù)雜的繼承體系，并沒(méi)有看到父容器作用的體現(xiàn)，稍后來(lái)分析。

了解了配置，我們就需要思考如何替換掉這些繁瑣的配置。實(shí)際上Tomcat提供了一個(gè)規(guī)范，有一個(gè)ServletContainerInitializer接口：

public interface ServletContainerInitializer { void onStartup(Set<Class<?>> var1, ServletContext var2) throws ServletException;}

Tomcat啟動(dòng)時(shí)會(huì)調(diào)用該接口實(shí)現(xiàn)類的onStartup方法，這個(gè)方法有兩個(gè)參數(shù)，第二個(gè)不用說(shuō)，主要是第一個(gè)參數(shù)什么？從哪里來(lái)？另外我們自定義的實(shí)現(xiàn)類又怎么讓Tomcat調(diào)用呢？

首先解答最后一個(gè)問(wèn)題，這里也是利用SPI來(lái)實(shí)現(xiàn)的，因此我們實(shí)現(xiàn)了該接口后，還需要在META-INF.services下配置。其次，這里傳入的第一個(gè)參數(shù)也是我們自定義的擴(kuò)展接口的實(shí)現(xiàn)類，我們可以通過(guò)我們自定義的接口實(shí)現(xiàn)很多需要在啟動(dòng)時(shí)做的事，比如加載Servlet，但是Tomcat又是怎么知道我們自定義的接口是哪個(gè)呢？

這就需要用到@HandlesTypes注解，該注解就是標(biāo)注在ServletContainerInitializer的實(shí)現(xiàn)類上，其值就是我們擴(kuò)展的接口，這樣Tomcat就知道需要傳入哪個(gè)接口實(shí)現(xiàn)類到這個(gè)onStartup方法了。

來(lái)看一個(gè)簡(jiǎn)單的實(shí)現(xiàn)：

@HandlesTypes(LoadServlet.class)public class MyServletContainerInitializer implements ServletContainerInitializer { @Override public void onStartup(Set<Class<?>> set, ServletContext servletContext) throws ServletException { Iterator var4; if (set != null) { var4 = set.iterator(); while (var4.hasNext()) {Class<?> clazz = (Class<?>) var4.next();if (!clazz.isInterface() && !Modifier.isAbstract(clazz.getModifiers()) && LoadServlet.class.isAssignableFrom(clazz)) { try { ((LoadServlet) clazz.newInstance()).loadOnstarp(servletContext); } catch (Exception e) { e.printStackTrace(); }} } } }}public interface LoadServlet { void loadOnstarp(ServletContext servletContext);}public class LoadServletImpl implements LoadServlet { @Override public void loadOnstarp(ServletContext servletContext) { ServletRegistration.Dynamic initServlet = servletContext.addServlet('initServlet', 'org.springframework.web.servlet.DispatcherServlet'); initServlet.setLoadOnStartup(1); initServlet.addMapping('/init'); }}

這就是Tomcat給我們提供的規(guī)范，通過(guò)這個(gè)規(guī)范我們就能實(shí)現(xiàn)Spring的零xml配置啟動(dòng)，直接來(lái)看Spring是如何做的。根據(jù)上面所說(shuō)我們可以在spring-web工程下找到META-INF/services/javax.servlet.ServletContainerInitializer配置：

@HandlesTypes(WebApplicationInitializer.class)public class SpringServletContainerInitializer implements ServletContainerInitializer { @Override public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException { List<WebApplicationInitializer> initializers = new LinkedList<>(); if (webAppInitializerClasses != null) { for (Class<?> waiClass : webAppInitializerClasses) { // Be defensive: Some servlet containers provide us with invalid classes, // no matter what @HandlesTypes says... if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) { try { initializers.add((WebApplicationInitializer) ReflectionUtils.accessibleConstructor(waiClass).newInstance()); } catch (Throwable ex) { throw new ServletException('Failed to instantiate WebApplicationInitializer class', ex); } } } } if (initializers.isEmpty()) { servletContext.log('No Spring WebApplicationInitializer types detected on classpath'); return; } servletContext.log(initializers.size() + ' Spring WebApplicationInitializers detected on classpath'); AnnotationAwareOrderComparator.sort(initializers); for (WebApplicationInitializer initializer : initializers) { initializer.onStartup(servletContext); } }}

核心的實(shí)現(xiàn)就是WebApplicationInitializer，先看看其繼承體系

AbstractReactiveWebInitializer不用管，主要看另外一邊，但是都是抽象類，也就是說(shuō)真的實(shí)例也是由我們自己實(shí)現(xiàn)，但需要我們實(shí)現(xiàn)什么呢？我們一般直接繼承AbstractAnnotationConfigDispatcherServletInitializer類，有四個(gè)抽象方法需要我們實(shí)現(xiàn)：

//父容器 @Override protected Class<?>[] getRootConfigClasses() { return new Class<?>[]{SpringContainer.class}; } //SpringMVC配置子容器 @Override protected Class<?>[] getServletConfigClasses() { return new Class<?>[]{MvcContainer.class}; } //獲取DispatcherServlet的映射信息 @Override protected String[] getServletMappings() { return new String[]{'/'}; } // filter配置 @Override protected Filter[] getServletFilters() { MyFilter myFilter = new MyFilter(); CorsFilter corsFilter = new CorsFilter(); return new Filter[]{myFilter,corsFilter}; }

這里主要注意getRootConfigClasses和getServletConfigClasses方法，分別加載父、子容器：

@ComponentScan(value = 'com.dark',excludeFilters = { @ComponentScan.Filter(type = FilterType.ANNOTATION,classes = {Controller.class})})public class SpringContainer {}@ComponentScan(value = 'com.dark',includeFilters = { @ComponentScan.Filter(type = FilterType.ANNOTATION,classes = {Controller.class})},useDefaultFilters = false)public class MvcContainer {}

看到這兩個(gè)類上的注解應(yīng)該不陌生了吧，父容器掃描裝載了所有不帶@Controller注解的類，子容器則相反，但需要對(duì)象時(shí)首先從當(dāng)前容器中找，如果沒(méi)有則從父容器中獲取，為什么要這么設(shè)計(jì)呢？

直接放到一個(gè)容器中不行么？先思考下， 稍后解答?；氐給nStartup方法中，直接回調(diào)用到AbstractDispatcherServletInitializer類：

public void onStartup(ServletContext servletContext) throws ServletException { super.onStartup(servletContext); //注冊(cè)DispatcherServlet registerDispatcherServlet(servletContext); }

先是調(diào)用父類：

public void onStartup(ServletContext servletContext) throws ServletException { registerContextLoaderListener(servletContext); } protected void registerContextLoaderListener(ServletContext servletContext) { //創(chuàng)建spring上下文，注冊(cè)了SpringContainer WebApplicationContext rootAppContext = createRootApplicationContext(); if (rootAppContext != null) { //創(chuàng)建監(jiān)聽(tīng)器 ContextLoaderListener listener = new ContextLoaderListener(rootAppContext); listener.setContextInitializers(getRootApplicationContextInitializers()); servletContext.addListener(listener); } }

然后調(diào)用createRootApplicationContext創(chuàng)建父容器：

protected WebApplicationContext createRootApplicationContext() { Class<?>[] configClasses = getRootConfigClasses(); if (!ObjectUtils.isEmpty(configClasses)) { AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext(); context.register(configClasses); return context; } else { return null; } }

可以看到就是創(chuàng)建了一個(gè)AnnotationConfigWebApplicationContext對(duì)象，并將我們的配置類SpringContainer注冊(cè)了進(jìn)去。接著創(chuàng)建Tomcat啟動(dòng)加載監(jiān)聽(tīng)器ContextLoaderListener，該監(jiān)聽(tīng)器有一個(gè)contextInitialized方法，會(huì)在Tomcat啟動(dòng)時(shí)調(diào)用。

public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } */ public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { long startTime = System.currentTimeMillis(); try { // Store context in local instance variable, to guarantee that // it is available on ServletContext shutdown. if (this.context == null) { this.context = createWebApplicationContext(servletContext); } if (this.context instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context; if (!cwac.isActive()) { // The context has not yet been refreshed -> provide services such as // setting the parent context, setting the application context id, etc if (cwac.getParent() == null) { // The context instance was injected without an explicit parent -> // determine parent for root web application context, if any. ApplicationContext parent = loadParentContext(servletContext); cwac.setParent(parent); } configureAndRefreshWebApplicationContext(cwac, servletContext); } } servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); ClassLoader ccl = Thread.currentThread().getContextClassLoader(); if (ccl == ContextLoader.class.getClassLoader()) { currentContext = this.context; } else if (ccl != null) { currentContextPerThread.put(ccl, this.context); } return this.context; } }

可以看到就是去初始化容器，這個(gè)和之前分析xml解析是一樣的，主要注意這里封裝了ServletContext對(duì)象，并將父容器設(shè)置到了該對(duì)象中。

父容器創(chuàng)建完成后自然就是子容器的創(chuàng)建，來(lái)到registerDispatcherServlet方法：

protected void registerDispatcherServlet(ServletContext servletContext) { String servletName = getServletName(); Assert.hasLength(servletName, 'getServletName() must not return null or empty'); //創(chuàng)建springmvc的上下文，注冊(cè)了MvcContainer類 WebApplicationContext servletAppContext = createServletApplicationContext(); Assert.notNull(servletAppContext, 'createServletApplicationContext() must not return null'); //創(chuàng)建DispatcherServlet FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext); Assert.notNull(dispatcherServlet, 'createDispatcherServlet(WebApplicationContext) must not return null'); dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers()); ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet); if (registration == null) { throw new IllegalStateException('Failed to register servlet with name ’' + servletName + '’. ' + 'Check if there is another servlet registered under the same name.'); } /* * 如果該元素的值為負(fù)數(shù)或者沒(méi)有設(shè)置，則容器會(huì)當(dāng)Servlet被請(qǐng)求時(shí)再加載。 如果值為正整數(shù)或者0時(shí)，表示容器在應(yīng)用啟動(dòng)時(shí)就加載并初始化這個(gè)servlet， 值越小，servlet的優(yōu)先級(jí)越高，就越先被加載 * */ registration.setLoadOnStartup(1); registration.addMapping(getServletMappings()); registration.setAsyncSupported(isAsyncSupported()); Filter[] filters = getServletFilters(); if (!ObjectUtils.isEmpty(filters)) { for (Filter filter : filters) { registerServletFilter(servletContext, filter); } } customizeRegistration(registration); } protected WebApplicationContext createServletApplicationContext() { AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext(); Class<?>[] configClasses = getServletConfigClasses(); if (!ObjectUtils.isEmpty(configClasses)) { context.register(configClasses); } return context; }

這里也是創(chuàng)建了一個(gè)AnnotationConfigWebApplicationContext對(duì)象，不同的只是這里注冊(cè)的配置類就是我們的Servlet配置了。然后創(chuàng)建了DispatcherServlet對(duì)象，并將上下文對(duì)象設(shè)置了進(jìn)去。

看到這你可能會(huì)疑惑，既然父子容器創(chuàng)建的都是相同類的對(duì)象，何來(lái)的父子容器之說(shuō)？

別急，這個(gè)在初始化該上文時(shí)就明白了。但是這里的初始化入口在哪呢？沒(méi)有看到任何監(jiān)聽(tīng)器的創(chuàng)建和調(diào)用。

實(shí)際上這里的上下文對(duì)象初始化是在Servlet初始化時(shí)實(shí)現(xiàn)的，即init方法，直接來(lái)到HttpServletBean的init方法（分析SpringMVC源碼時(shí)講過(guò)）：

public final void init() throws ServletException { ...省略 // Let subclasses do whatever initialization they like. initServletBean(); } protected final void initServletBean() throws ServletException { try { this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet(); } } protected WebApplicationContext initWebApplicationContext() { //這里會(huì)從servletContext中獲取到父容器，就是通過(guò)監(jiān)聽(tīng)器加載的容器 WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext()); WebApplicationContext wac = null; if (this.webApplicationContext != null) { // A context instance was injected at construction time -> use it wac = this.webApplicationContext; if (wac instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac; if (!cwac.isActive()) { if (cwac.getParent() == null) { cwac.setParent(rootContext); } //容器加載 configureAndRefreshWebApplicationContext(cwac); } } } if (wac == null) { wac = findWebApplicationContext(); } if (wac == null) { wac = createWebApplicationContext(rootContext); } if (!this.refreshEventReceived) { synchronized (this.onRefreshMonitor) { onRefresh(wac); } } if (this.publishContext) { // Publish the context as a servlet context attribute. String attrName = getServletContextAttributeName(); getServletContext().setAttribute(attrName, wac); } return wac; }

看到這里想你也應(yīng)該明白了，首先從ServletContext中拿到父容器，然后設(shè)置到當(dāng)前容器的parent中，實(shí)現(xiàn)了父子容器的組織，而這樣設(shè)計(jì)好處我想也是很清楚的，子容器目前裝載的都是MVC的配置和Bean，簡(jiǎn)單點(diǎn)說(shuō)就是Controller，父容器中都是Service，Controller是依賴于Service的，如果不構(gòu)建這樣的層級(jí)關(guān)系并優(yōu)先實(shí)例化父容器，你怎么實(shí)現(xiàn)Controller層的依賴注入成功呢？

總結(jié)

本篇結(jié)合之前的文章，分析了SpringMVC零XML配置的實(shí)現(xiàn)原理，也補(bǔ)充了之前未分析到父子容器關(guān)系，讓我們能從細(xì)節(jié)上更加全面的理解SpringIOC的實(shí)現(xiàn)原理，相信看完本篇對(duì)于SpringBoot的實(shí)現(xiàn)你也會(huì)有自己的想法。希望能給大家一個(gè)參考，也希望大家多多支持好吧啦網(wǎng)。