SpringMvc之启动流程

矫情吗；* 2023-02-13 00:25 19阅读 0赞

当一个Web应用部署到容器内时(eg.tomcat)，在Web应用开始响应执行用户请求前，以下步骤会被依次执行:

* 部署描述文件中(eg.tomcat的web.xml)由`<listener>`元素标记的事件监听器会被创建和初始化
 * 对于所有事件监听器，如果实现了`ServletContextListener`接口，将会执行其实现的`contextInitialized()`方法
 * 部署描述文件中由`<filter>`元素标记的过滤器会被创建和初始化，并调用其`init()`方法
 * 部署描述文件中由`<servlet>`元素标记的servlet会根据`<load-on-startup>`的权值按顺序创建和初始化，并调用其`init()`方法

通过上述官方文档的描述，可绘制如下`Web应用部署初始化`流程执行图。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MzEzNjM1_size_16_color_FFFFFF_t_70][]

可以发现，在`tomcat`下`web应用`的初始化流程是，先初始化`listener`接着初始化`filter`最后初始化`servlet`，当我们清楚认识到`Web应用`部署到容器后的初始化过程后，就可以进一步深入探讨`SpringMVC`的启动过程。

### Spring MVC启动过程 ###

接下来以一个常见的简单`web.xml`配置进行`Spring MVC`启动过程的分析，`web.xml`配置内容如下:

<web-app>
 
 <display-name>Web Application</display-name>
 
 
 <context-param>
 <param-name>contextConfigLocation</param-name>
 <param-value>classpath:applicationContext-*.xml</param-value>
 </context-param>
 
 
 <listener>
 <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
 </listener>
 
 
 <filter>
 <filter-name>CharacterEncodingFilter</filter-name>
 <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
 <init-param>
 <param-name>encoding</param-name>
 <param-value>utf-8</param-value>
 </init-param>
 </filter>
 
 <filter-mapping>
 <filter-name>CharacterEncodingFilter</filter-name>
 <url-pattern>/*</url-pattern>
 </filter-mapping>
 
 
 <servlet>
 <servlet-name>springMVC_rest</servlet-name>
 <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
 <init-param>
 <param-name>contextConfigLocation</param-name>
 <param-value>classpath:spring-mvc.xml</param-value>
 </init-param>
 </servlet>
 
 <servlet-mapping>
 <servlet-name>springMVC_rest</servlet-name>
 <url-pattern>/</url-pattern>
 </servlet-mapping>
 
 </web-app>

### Listener的初始化过程 ###

首先定义了`<context-param>`标签，用于配置一个全局变量，`<context-param>`标签的内容读取后会被放进`application`中，做为Web应用的全局变量使用，接下来创建`listener`时会使用到这个全局变量，因此，Web应用在容器中部署后，进行初始化时会先读取这个全局变量，之后再进行上述讲解的初始化启动过程。

接着定义了一个`ContextLoaderListener类`的`listener`。查看`ContextLoaderListener`的类声明源码如下图:

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    
    }

`ContextLoaderListener`类继承了`ContextLoader`类并实现了`ServletContextListener`接口，首先看一下前面讲述的`ServletContextListener`接口源码:

/*
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 * may not use this file except in compliance with the License. You can
 * obtain a copy of the License at
 * https://glassfish.dev.java.net/public/CDDL+GPL_1_1.html
 * or packager/legal/LICENSE.txt. See the License for the specific
 * language governing permissions and limitations under the License.
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 * file and include the License file at packager/legal/LICENSE.txt.
 *
 * GPL Classpath Exception:
 * Oracle designates this particular file as subject to the "Classpath"
 * exception as provided by Oracle in the GPL Version 2 section of the License
 * file that accompanied this code.
 *
 * Modifications:
 * If applicable, add the following below the License Header, with the fields
 * enclosed by brackets [] replaced by your own identifying information:
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 * Contributor(s):
 * If you wish your version of this file to be governed by only the CDDL or
 * only the GPL Version 2, indicate your decision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
 * Version 2] license." If you don't indicate a single choice of license, a
 * recipient has the option to distribute your version of this file under
 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above. However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
 * holder.
 *
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 * This file incorporates work covered by the following copyright and
 * permission notice:
 *
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
 package javax.servlet;
 
 import java.util.EventListener;
 
 /** 
 * Interface for receiving notification events about ServletContext
 * lifecycle changes.
 *
 * In order to receive these notification events, the implementation
 * class must be either declared in the deployment descriptor of the web
 * application, annotated with {@link javax.servlet.annotation.WebListener},
 * or registered via one of the addListener methods defined on
 * {@link ServletContext}.
 *
 * Implementations of this interface are invoked at their
 * {@link #contextInitialized} method in the order in which they have been
 * declared, and at their {@link #contextDestroyed} method in reverse
 * order.
 *
 * @see ServletContextEvent
 *
 * @since Servlet 2.3
 */
 public interface ServletContextListener extends EventListener {
 
 /**
 * Receives notification that the web application initialization
 * process is starting.
 *
 * All ServletContextListeners are notified of context
 * initialization before any filters or servlets in the web
 * application are initialized.
 *
 * @param sce the ServletContextEvent containing the ServletContext
 * that is being initialized
 */
 public void contextInitialized(ServletContextEvent sce);
 
 /**
 * Receives notification that the ServletContext is about to be
 * shut down.
 *
 * All servlets and filters will have been destroyed before any
 * ServletContextListeners are notified of context
 * destruction.
 *
 * @param sce the ServletContextEvent containing the ServletContext
 * that is being destroyed
 */
 public void contextDestroyed(ServletContextEvent sce);
 }

该接口只有两个方法`contextInitialized`和`contextDestroyed`，这里采用的是观察者模式，也称为订阅-发布模式，实现了该接口的`listener`会向发布者进行订阅，当`Web应用`初始化或销毁时会分别调用上述两个方法。

继续看`ContextLoaderListener`，该`listener`实现了`ServletContextListener`接口，因此在`Web应用`初始化时会调用该方法，该方法的具体实现如下：

/**
     * Initialize the root web application context.
     */
    @Override
    public void contextInitialized(ServletContextEvent event) {
    	initWebApplicationContext(event.getServletContext());
    }

`ContextLoaderListener`的`contextInitialized()`方法直接调用了`initWebApplicationContext()`方法，这个方法是继承自`ContextLoader类`，通过函数名可以知道，该方法是用于初始化Web应用上下文，即`IoC容器`，这里使用的是代理模式，继续查看`ContextLoader类`的`initWebApplicationContext()`方法的源码如下:

/**
         * Initialize Spring's web application context for the given servlet context,
         * using the application context provided at construction time, or creating a new one
         * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
         * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
         * @param servletContext current servlet context
         * @return the new WebApplicationContext
         * @see #ContextLoader(WebApplicationContext)
         * @see #CONTEXT_CLASS_PARAM
         * @see #CONFIG_LOCATION_PARAM
         */
        //servletContext，servlet上下文，即application对象
        public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        /*
        首先通过WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
        这个String类型的静态变量获取一个根IoC容器，根IoC容器作为全局变量
        存储在application对象中，如果存在则有且只能有一个
        如果在初始化根WebApplicationContext即根IoC容器时发现已经存在
        则直接抛出异常，因此web.xml中只允许存在一个ContextLoader类或其子类的对象
        */
            if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
                throw new IllegalStateException(
                        "Cannot initialize context because there is already a root application context present - " +
                        "check whether you have multiple ContextLoader* definitions in your web.xml!");
            }
    
            Log logger = LogFactory.getLog(ContextLoader.class);
            servletContext.log("Initializing Spring root WebApplicationContext");
            if (logger.isInfoEnabled()) {
                logger.info("Root WebApplicationContext: initialization started");
            }
            long startTime = System.currentTimeMillis();
    
            try {
                // Store context in local instance variable, to guarantee that
                // it is available on ServletContext shutdown.
                // 如果当前成员变量中不存在WebApplicationContext则创建一个根WebApplicationContext
                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.
                            //为根WebApplicationContext设置一个父容器
                            ApplicationContext parent = loadParentContext(servletContext);
                            cwac.setParent(parent);
                        }
                        //配置并刷新整个根IoC容器，在这里会进行Bean的创建和初始化
                        configureAndRefreshWebApplicationContext(cwac, servletContext);
                    }
                }
                /*
                将创建好的IoC容器放入到application对象中，并设置key为WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
                因此，在SpringMVC开发中可以在jsp中通过该key在application对象中获取到根IoC容器，进而获取到相应的Ben
                */
                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);
                }
    
                if (logger.isDebugEnabled()) {
                    logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
                            WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
                }
                if (logger.isInfoEnabled()) {
                    long elapsedTime = System.currentTimeMillis() - startTime;
                    logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
                }
    
                return this.context;
            }
            catch (RuntimeException ex) {
                logger.error("Context initialization failed", ex);
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
                throw ex;
            }
            catch (Error err) {
                logger.error("Context initialization failed", err);
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
                throw err;
            }
        }

`initWebApplicationContext()`方法如上注解讲述，主要目的就是创建`root WebApplicationContext对象`即`根IoC容器`，其中比较重要的就是，整个`Web应用`如果存在`根IoC容器`则有且只能有一个，`根IoC容器`作为全局变量存储在`ServletContext`即`application对象`中。将`根IoC容器`放入到`application对象`之前进行了`IoC容器`的配置和刷新操作，调用了`configureAndRefreshWebApplicationContext()`方法，该方法源码如下:

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
 if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
 // The application context id is still set to its original default value
 // -> assign a more useful id based on available information
 String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
 if (idParam != null) {
 wac.setId(idParam);
 }
 else {
 // Generate default id...
 wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
 ObjectUtils.getDisplayString(sc.getContextPath()));
 }
 }
 
 wac.setServletContext(sc);
 /*
 CONFIG_LOCATION_PARAM = "contextConfigLocation"
 获取web.xml中<context-param>标签配置的全局变量，其中key为CONFIG_LOCATION_PARAM
 也就是我们配置的相应Bean的xml文件名，并将其放入到WebApplicationContext中
 */
 String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
 if (configLocationParam != null) {
 wac.setConfigLocation(configLocationParam);
 }
 
 // The wac environment's #initPropertySources will be called in any case when the context
 // is refreshed; do it eagerly here to ensure servlet property sources are in place for
 // use in any post-processing or initialization that occurs below prior to #refresh
 ConfigurableEnvironment env = wac.getEnvironment();
 if (env instanceof ConfigurableWebEnvironment) {
 ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
 }
 
 customizeContext(sc, wac);
 wac.refresh();
 }

比较重要的就是获取到了`web.xml`中的`<context-param>标签`配置的全局变量`contextConfigLocation`，并最后一行调用了`refresh()`方法，`ConfigurableWebApplicationContext`是一个接口，通过对常用实现类`ClassPathXmlApplicationContext`逐层查找后可以找到一个抽象类`AbstractApplicationContext`实现了`refresh()`方法，其源码如下:

@Override
        public void refresh() throws BeansException, IllegalStateException {
            synchronized (this.startupShutdownMonitor) {
                // Prepare this context for refreshing.
                prepareRefresh();
    
                // Tell the subclass to refresh the internal bean factory.
                ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    
                // Prepare the bean factory for use in this context.
                prepareBeanFactory(beanFactory);
    
                try {
                    // Allows post-processing of the bean factory in context subclasses.
                    postProcessBeanFactory(beanFactory);
    
                    // Invoke factory processors registered as beans in the context.
                    invokeBeanFactoryPostProcessors(beanFactory);
    
                    // Register bean processors that intercept bean creation.
                    registerBeanPostProcessors(beanFactory);
    
                    // Initialize message source for this context.
                    initMessageSource();
    
                    // Initialize event multicaster for this context.
                    initApplicationEventMulticaster();
    
                    // Initialize other special beans in specific context subclasses.
                    onRefresh();
    
                    // Check for listener beans and register them.
                    registerListeners();
    
                    // Instantiate all remaining (non-lazy-init) singletons.
                    finishBeanFactoryInitialization(beanFactory);
    
                    // Last step: publish corresponding event.
                    finishRefresh();
                }
    
                catch (BeansException ex) {
                    if (logger.isWarnEnabled()) {
                        logger.warn("Exception encountered during context initialization - " +
                                "cancelling refresh attempt: " + ex);
                    }
    
                    // Destroy already created singletons to avoid dangling resources.
                    destroyBeans();
    
                    // Reset 'active' flag.
                    cancelRefresh(ex);
    
                    // Propagate exception to caller.
                    throw ex;
                }
    
                finally {
                    // Reset common introspection caches in Spring's core, since we
                    // might not ever need metadata for singleton beans anymore...
                    resetCommonCaches();
                }
            }
        }

该方法主要用于创建并初始化`contextConfigLocation类`配置的`xml文件`中的`Bean`，因此，如果我们在配置`Bean`时出错，在`Web应用`启动时就会抛出异常，而不是等到运行时才抛出异常。

整个`ContextLoaderListener类`的启动过程到此就结束了，可以发现，创建`ContextLoaderListener`是比较核心的一个步骤，主要工作就是为了创建`根IoC容器`并使用特定的`key`将其放入到`application`对象中，供整个`Web应用`使用，由于在`ContextLoaderListener类`中构造的`根IoC容器`配置的`Bean`是全局共享的，因此，在`<context-param>`标识的`contextConfigLocation`的`xml配置文件`一般包括:`数据库DataSource`、`DAO层`、`Service层`、`事务`等相关`Bean`。

在`JSP`中可以通过以下两种方法获取到`根IoC容器`从而获取相应`Bean`:

WebApplicationContext applicationContext = (WebApplicationContext) application.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    
    WebApplicationContext applicationContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext());

### Filter的初始化 ###

在监听器`listener`初始化完成后，按照文章开始的讲解，接下来会进行`filter`的初始化操作，`filter`的创建和初始化中没有涉及`IoC容器`的相关操作，因此不是本文讲解的重点，本文举例的`filter`是一个用于编码用户请求和响应的过滤器，采用`utf-8`编码用于适配中文。

### Servlet的初始化 ###

`Web应用`启动的最后一个步骤就是创建和初始化相关`Servlet`，在开发中常用的`Servlet`就是`DispatcherServlet类`前端控制器，前端控制器作为中央控制器是整个`Web应用`的核心，用于获取分发用户请求并返回响应，借用网上一张关于`DispatcherServlet类`的类图，其类图如下所示:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MzEzNjM1_size_16_color_FFFFFF_t_70 1][]

通过类图可以看出`DispatcherServlet类`的间接父类实现了`Servlet接口`，因此其本质上依旧是一个`Servlet`。`DispatcherServlet类`的设计很巧妙，上层父类不同程度的实现了相关接口的部分方法，并留出了相关方法用于子类覆盖，将不变的部分统一实现，将变化的部分预留方法用于子类实现。

通过对上述类图中相关类的源码分析可以绘制如下相关初始化方法调用逻辑:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MzEzNjM1_size_16_color_FFFFFF_t_70 2][]

通过类图和相关初始化函数调用的逻辑来看，`DispatcherServlet类`的初始化过程将模板方法使用的淋漓尽致，其父类完成不同的统一的工作，并预留出相关方法用于子类覆盖去完成不同的可变工作。

`DispatcherServelt`类的本质是`Servlet`，通过文章开始的讲解可知，在`Web应用`部署到容器后进行`Servlet`初始化时会调用相关的`init(ServletConfig)`方法，因此，`DispatchServlet类`的初始化过程也由该方法开始。上述调用逻辑中比较重要的就是`FrameworkServlet抽象类`中的`initServletBean()`方法、`initWebApplicationContext()`方法以及`DispatcherServlet类`中的`onRefresh()`方法，接下来会逐一进行讲解。

首先查看一下`initServletBean()`的相关源码如下图所示:

@Override
    protected final void initServletBean() throws ServletException {
    		getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
    		if (this.logger.isInfoEnabled()) {
    			this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
    		}
    		long startTime = System.currentTimeMillis();
    
    		try {
    			this.webApplicationContext = initWebApplicationContext();
    			initFrameworkServlet();
    		}
    		catch (ServletException ex) {
    			this.logger.error("Context initialization failed", ex);
    			throw ex;
    		}
    		catch (RuntimeException ex) {
    			this.logger.error("Context initialization failed", ex);
    			throw ex;
    		}
    
    		if (this.logger.isInfoEnabled()) {
    			long elapsedTime = System.currentTimeMillis() - startTime;
    			this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
    					elapsedTime + " ms");
    		}
    }

该方法是重写了`FrameworkServlet抽象类`父类`HttpServletBean抽象类`的`initServletBean()`方法，`HttpServletBean抽象类`在执行`init()`方法时会调用`initServletBean()`方法，由于多态的特性，最终会调用其子类`FrameworkServlet抽象类`的`initServletBean()`方法。该方法由`final`标识，子类就不可再次重写了。该方法中比较重要的就是`initWebApplicationContext()`方法的调用，该方法仍由`FrameworkServlet抽象类`实现，继续查看其源码如下所示:

/**
 * Initialize and publish the WebApplicationContext for this servlet.
 * Delegates to {@link #createWebApplicationContext} for actual creation
 * of the context. Can be overridden in subclasses.
 * @return the WebApplicationContext instance
 * @see #FrameworkServlet(WebApplicationContext)
 * @see #setContextClass
 * @see #setContextConfigLocation
 */
 protected WebApplicationContext initWebApplicationContext() {
 /*
 获取由ContextLoaderListener创建的根IoC容器
 获取根IoC容器有两种方法，还可通过key直接获取
 */
 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()) {
 // 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 -> set
 // the root application context (if any; may be null) as the parent
 /*
 如果当前Servelt存在一个WebApplicationContext即子IoC容器
 并且上文获取的根IoC容器存在，则将根IoC容器作为子IoC容器的父容器
 */
 cwac.setParent(rootContext);
 }
 //配置并刷新当前的子IoC容器，功能与前文讲解根IoC容器时的配置刷新一致，用于构建相关Bean
 configureAndRefreshWebApplicationContext(cwac);
 }
 }
 }
 if (wac == null) {
 // No context instance was injected at construction time -> see if one
 // has been registered in the servlet context. If one exists, it is assumed
 // that the parent context (if any) has already been set and that the
 // user has performed any initialization such as setting the context id
 //如果当前Servlet不存在一个子IoC容器则去查找一下
 wac = findWebApplicationContext();
 }
 if (wac == null) {
 // No context instance is defined for this servlet -> create a local one
 //如果仍旧没有查找到子IoC容器则创建一个子IoC容器
 wac = createWebApplicationContext(rootContext);
 }
 
 if (!this.refreshEventReceived) {
 // Either the context is not a ConfigurableApplicationContext with refresh
 // support or the context injected at construction time had already been
 // refreshed -> trigger initial onRefresh manually here.
 //调用子类覆盖的onRefresh方法完成“可变”的初始化过程
 onRefresh(wac);
 }
 
 if (this.publishContext) {
 // Publish the context as a servlet context attribute.
 String attrName = getServletContextAttributeName();
 getServletContext().setAttribute(attrName, wac);
 if (this.logger.isDebugEnabled()) {
 this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
 "' as ServletContext attribute with name [" + attrName + "]");
 }
 }
 
 return wac;
 }

通过函数名不难发现，该方法的主要作用同样是创建一个`WebApplicationContext`对象，即`Ioc容器`，不过前文讲过每个`Web应用`最多只能存在一个`根IoC容器`，这里创建的则是特定`Servlet`拥有的`子IoC容器`，可能有些读者会有疑问，为什么需要多个`Ioc容器`，首先介绍一个`父子IoC容器`的访问特性，有兴趣的读者可以自行实验。

### 父子IoC容器的访问特性 ###

在学习`Spring`时，我们都是从读取`xml配置文件`来构造`IoC容器`，常用的类有`ClassPathXmlApplicationContext类`，该类存在一个初始化方法用于传入`xml文件`路径以及一个`父容器`，我们可以创建两个不同的`xml配置文件`并实现如下代码:

//applicationContext1.xml文件中配置一个id为baseBean的Bean
    ApplicationContext baseContext = new ClassPathXmlApplicationContext("applicationContext1.xml");
    
    Object obj1 = baseContext.getBean("baseBean");
    
    System.out.println("baseContext Get Bean " + obj1);
    
    //applicationContext2.xml文件中配置一个id未subBean的Bean
    ApplicationContext subContext = new ClassPathXmlApplicationContext(new String[]{"applicationContext2.xml"}, baseContext);
    
    Object obj2 = subContext.getBean("baseBean");
    
    System.out.println("subContext get baseContext Bean " + obj2);
    
    Object obj3 = subContext.getBean("subBean");
    
    System.out.println("subContext get subContext Bean " + obj3);
    
    //抛出NoSuchBeanDefinitionException异常
    Object obj4 = baseContext.getBean("subBean");
    
    System.out.println("baseContext get subContext Bean " + obj4);

首先创建`baseContext`没有为其设置`父容器`，接着可以成功获取`id`为`baseBean`的`Bean`，接着创建`subContext`并将`baseContext`设置为其`父容器`，`subContext`可以成功获取`baseBean`以及`subBean`，最后试图使用`baseContext`去获取`subContext`中定义的`subBean`，此时会抛出异常`NoSuchBeanDefinitionException`，由此可见，`父子容器`类似于类的继承关系，子类可以访问父类中的成员变量，而父类不可访问子类的成员变量，同样的，`子容器`可以访问`父容器`中定义的`Bean`，但`父容器`无法访问`子容器`定义的`Bean`。

通过上述实验我们可以理解为何需要创建多个`Ioc容器`，`根IoC容器`做为全局共享的`IoC容器`放入`Web应用`需要共享的`Bean`，而`子IoC容器`根据需求的不同，放入不同的`Bean`，这样能够做到隔离，保证系统的安全性。

--------------------

接下来继续讲解`DispatcherServlet类`的`子IoC容器`创建过程，如果当前`Servlet`存在一个`IoC容器`则为其设置`根IoC容器`作为其父类，并配置刷新该容器，用于构造其定义的`Bean`，这里的方法与前文讲述的`根IoC容器类似`，同样会读取用户在`web.xml`中配置的`<servlet>`中的`<init-param>`值，用于查找相关的`xml配置文件`用于构造定义的`Bean`，这里不再赘述了。如果当前`Servlet`不存在一个`子IoC容器`就去查找一个，如果仍然没有查找到则调用 
`createWebApplicationContext()`方法去创建一个，查看该方法的源码如下图所示:

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
 		Class<?> contextClass = getContextClass();
 		if (this.logger.isDebugEnabled()) {
 			this.logger.debug("Servlet with name '" + getServletName() +
 					"' will try to create custom WebApplicationContext context of class '" +
 					contextClass.getName() + "'" + ", using parent context [" + parent + "]");
 		}
 		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
 			throw new ApplicationContextException(
 					"Fatal initialization error in servlet with name '" + getServletName() +
 					"': custom WebApplicationContext class [" + contextClass.getName() +
 					"] is not of type ConfigurableWebApplicationContext");
 		}
 		ConfigurableWebApplicationContext wac =
 				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
 
 		wac.setEnvironment(getEnvironment());
 		wac.setParent(parent);
 		wac.setConfigLocation(getContextConfigLocation());
 
 		configureAndRefreshWebApplicationContext(wac);
 
 		return wac;
 	}

该方法用于创建一个`子IoC容器`并将`根IoC容器`做为其父容器，接着进行配置和刷新操作用于构造相关的`Bean`。至此，`根IoC容器`以及相关`Servlet`的`子IoC容器`已经配置完成，`子容器`中管理的`Bean`一般只被该`Servlet`使用，因此，其中管理的`Bean`一般是“局部”的，如`SpringMVC`中需要的各种重要组件，包括`Controller`、`Interceptor`、`Converter`、`ExceptionResolver`等。相关关系如下图所示:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI0MzEzNjM1_size_16_color_FFFFFF_t_70 3][]

**从图中可以看到DispatcherServlet里面有一个 Servlet WebApplicationContext，继承自 Root WebApplicationContext。我们知道WebApplicationContext其实就是一个IOC容器，root WebApplicationContext是Spring容器。**

![format_png][]

当`IoC子容器`构造完成后调用了`onRefresh()`方法，该方法的调用与`initServletBean()`方法的调用相同，由父类调用但具体实现由子类覆盖，调用`onRefresh()`方法时将前文创建的`IoC子容器`作为参数传入，查看`DispatcherServletBean类`的`onRefresh()`方法源码如下:

/**
 * This implementation calls {@link #initStrategies}.
 */
 //context为DispatcherServlet创建的一个IoC子容器
 @Override
 protected void onRefresh(ApplicationContext context) {
 initStrategies(context);
 }
 
 /**
 * Initialize the strategy objects that this servlet uses.
 * May be overridden in subclasses in order to initialize further strategy objects.
 */
 protected void initStrategies(ApplicationContext context) {
 initMultipartResolver(context);
 initLocaleResolver(context);
 initThemeResolver(context);
 initHandlerMappings(context);
 initHandlerAdapters(context);
 initHandlerExceptionResolvers(context);
 initRequestToViewNameTranslator(context);
 initViewResolvers(context);
 initFlashMapManager(context);
 }

`onRefresh()`方法直接调用了`initStrategies()`方法，源码如上，通过函数名可以判断，该方法用于初始化创建`multipartResovle`来支持图片等文件的上传、本地化解析器、主题解析器、`HandlerMapping`处理器映射器、`HandlerAdapter`处理器适配器、异常解析器、视图解析器、flashMap管理器等，这些组件都是`SpringMVC`开发中的重要组件，相关组件的初始化创建过程均在此完成。

至此，`DispatcherServlet类`的创建和初始化过程也就结束了，整个`Web应用`部署到容器后的初始化启动过程的重要部分全部分析清楚了，通过前文的分析我们可以认识到层次化设计的优点，以及`IoC容器`的继承关系所表现的隔离性。分析源码能让我们更清楚的理解和认识到相关初始化逻辑以及配置文件的配置原理。

## 总结 ##

这里给出一个简洁的文字描述版`SpringMVC启动过程`:

tomcat web容器启动时会去读取`web.xml`这样的`部署描述文件`，相关组件启动顺序为: `解析<context-param>` => `解析<listener>` => `解析<filter>` => `解析<servlet>`，具体初始化过程如下:

* 1、解析`<context-param>`里的键值对。
 * 2、创建一个`application`内置对象即`ServletContext`，servlet上下文，用于全局共享。
 * 3、将`<context-param>`的键值对放入`ServletContext`即`application`中，`Web应用`内全局共享。
 * 4、读取`<listener>`标签创建监听器，一般会使用`ContextLoaderListener类`，如果使用了`ContextLoaderListener类`，`Spring`就会创建一个`WebApplicationContext类`的对象，`WebApplicationContext类`就是`IoC容器`，`ContextLoaderListener类`创建的`IoC容器`是`根IoC容器`为全局性的，并将其放置在`appication`中，作为应用内全局共享，键名为`WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE`，可以通过以下两种方法获取

WebApplicationContext applicationContext = (WebApplicationContext) application.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    
    WebApplicationContext applicationContext1 = WebApplicationContextUtils.getWebApplicationContext(application);

这个全局的`根IoC容器`只能获取到在该容器中创建的`Bean`不能访问到其他容器创建的`Bean`，也就是读取`web.xml`配置的`contextConfigLocation`参数的`xml文件`来创建对应的`Bean`。

* 5、`listener`创建完成后如果有`<filter>`则会去创建`filter`。
 * 6、初始化创建`<servlet>`，一般使用`DispatchServlet类`。
 * 7、`DispatchServlet`的父类`FrameworkServlet`会重写其父类的`initServletBean`方法，并调用`initWebApplicationContext()`以及`onRefresh()`方法。
 * 8、`initWebApplicationContext()`方法会创建一个当前`servlet`的一个`IoC子容器`，如果存在上述的全局`WebApplicationContext`则将其设置为`父容器`，如果不存在上述全局的则`父容器`为null。
 * 9、读取`<servlet>`标签的`<init-param>`配置的`xml文件`并加载相关`Bean`。
 * 10、`onRefresh()`方法创建`Web应用`相关组件。

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[format_png]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly91c2VyLWdvbGQtY2RuLnhpdHUuaW8vMjAxOS80LzIxLzE2YTNlZGU2MjA2ZDA1MjE?x-oss-process=image/format,png