Spring 初始化 ContextLoaderListener 与 DispatcherServlet

青旅半醒 2022-05-18 04:37 148阅读 0赞

## 0. 概述 ##

分析web项目中spring的配置，初始化，bean解析和注入的过程，以及spring bean 的生命周期和扩展点，深入学习spring框架的使用。  
示例项目: [springinside][]

## 1. spring web 配置文件 ##

项目常见的spring web 的web.xml， 这里去掉了一些无关的配置，尽量最小化配置

<?xml version="1.0" encoding="UTF-8"?>
    <web-app version="2.4"
             xmlns="http://java.sun.com/xml/ns/j2ee"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    
        <context-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath*:META-INF/spring/*.xml</param-value>
        </context-param>
    
        <listener>
            <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
        </listener>
    
        <servlet>
            <description>spring inside</description>
            <display-name>spring inside</display-name>
            <servlet-name>springinside</servlet-name>
            <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
            <init-param>
                <param-name>contextConfigLocation</param-name>
                <param-value>WEB-INF/applicationContext.xml</param-value>
            </init-param>
            <load-on-startup>1</load-on-startup>
        </servlet>
    
        <servlet-mapping>
            <servlet-name>springinside</servlet-name>
            <url-pattern>/*</url-pattern>
        </servlet-mapping>
    </web-app>

1.  配置context-param,contextConfigLocation 指定ContextLoaderListener使用的配置文件
2.  配置了ContextLoaderListener，监听web容器的启动，用于做些初始化工作
3.  配置了DispatcherServlet处理web请求，指定了DispatcherServlet要加载的配置文件

对于spring web 项目来说，其中1和2不是必须的，可以没有。但是，这里配置ContextLoaderListener，是为了和部门的项目一致。部门的sof开发框架在web容器启动的时候有一些初始化操作，就是在ContextLoaderListener这里触发的。接下来我们分析ContextLoaderListener和DispatcherServlet源码，来看看spring是怎么完成初始化的。

## 2. ContextLoaderListener分析 ##

### 2.1 继承关系 ###

首先，看一下ContextLoaderListener的继承关系

![Image 1][]

可以看到，ContextLoaderListener实现了ServletContextListener接口。ServletContextListener接口够监听 ServletContext 对象的生命周期，实际上就是监听 Web应用的生命周期。当Servlet 容器启动或终止Web 应用时，会触发ServletContextEvent 事件，该事件由ServletContextListener 来处理。

public interface ServletContextListener extends EventListener {
        /**
         ** Notification that the web application initialization
         ** process is starting.
         ** All ServletContextListeners are notified of context
         ** initialization before any filter or servlet in the web
         ** application is initialized.
         */
    
        public void contextInitialized ( ServletContextEvent sce );
    
        /**
         ** Notification that the servlet context is about to be shut down.
         ** All servlets and filters have been destroy()ed before any
         ** ServletContextListeners are notified of context
         ** destruction.
         */
        public void contextDestroyed ( ServletContextEvent sce );
    }

当Servlet启动时就会调用contextInitialized方法；当Servlet销毁时会调用contextDestroyed方法。

### 2.2 ContextLoaderListener ###

接下来我们看ContextLoaderListener源码。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    
        public ContextLoaderListener() {
        }
    
    
        public ContextLoaderListener(WebApplicationContext context) {
            super(context);
        }
    
    
        /**
         * Initialize the root web application context.
         */
        @Override
        public void contextInitialized(ServletContextEvent event) {
            initWebApplicationContext(event.getServletContext());
        }
    
    
        /**
         * Close the root web application context.
         */
        @Override
        public void contextDestroyed(ServletContextEvent event) {
            closeWebApplicationContext(event.getServletContext());
            ContextCleanupListener.cleanupAttributes(event.getServletContext());
        }
    
    }

ContextLoaderListener的代码很简单，在Servlet启动时，调用initWebApplicationContext方法初始化WebApplicationContext这个web应用上下文；在Servlet销毁时，关闭应用上下文。Spring是怎么初始化web应用上下文的，我们跟踪代码进入initWebApplicationContext方法一探究竟。

### 2.3 initWebApplicationContext 分析 ###

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
            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.
                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);
                }
    
                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方法一共做了三件事：

1. 如果this.context为null, 则创建WebApplicationContext

if (this.context == null) {
        this.context = createWebApplicationContext(servletContext);
    }

我们在web.xml里，只配置了一个Listener，所以在ContextLoaderListener之前没有创建过WebApplicationContext，固这里this.context为null，所以会创建WebApplicationContext。

2. 如果this.context是ConfigurableWebApplicationContext的实例，并且没有刷新过WebApplicationContext，则配置并刷新WebApplicationContext。

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);
        }
    }

什么意思呢？首先，我们看看this.context的起继承关系 。

![Image 1][]

this.contexnt的类型是WebApplicationContext， 可以看到WebApplicationContext是个接口，它继承了BeanFactory。this.context是Spring的IoC容器，所以下面的配置和刷新应用上下文就不足为奇了，因为它要解析，注入bean。所以configureAndRefreshWebApplicationContext方法，会完成bean解析，加载，注入的过程。现在我们知道了WebApplicationContext是一个接口，这里的this.context到底会是什么那个具体的实例呢？其实，这里的this.context是XmlWebApplicationContext的具体实例。我们看一下XmlWebApplicationContext的继承关系。

![Image 1][]

可以看到，XmlWebApplicationContext实现了ConfigurableWebApplicationContext 接口，ConfigurableWebApplicationContext 继承了WebApplicationContext。继承关系理清楚了，这里的代码也就很容易看懂了。

3. 将WebApplicationContext放入servletContext中，供后续流程使用

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

这里讲this.context放入servletContext中，后续的DispatcherServlet会从servletContext取出该容器，作为其创建的容器的父容器。

### 2.4 configureAndRefreshWebApplicationContext 分析 ###

现在我们进入configureAndRefreshWebApplicationContext方法，看看是如何配置和刷新上下文的。

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);
        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();
    }

方法进来首先检查是否设置过id，如果没有设置过id，那么就设置容器id。

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()));
        }
    }

容器id优先从配置文web.xml中获取，如果配置文件配置了容器id，那个就使用配置的容器id，否则使用默认生成的容器id。

然后是获取spring配置文件。配置文件也是我们在web.xml配置的context-param

String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
    if (configLocationParam != null) {
        wac.setConfigLocation(configLocationParam);
    }

<context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath*:META-INF/spring/*.xml</param-value>
    </context-param>

Spring的环境配置，暂时不深入了解，后续在学习Spring的bean装配时在介绍。

接下来是一个扩展点————定制上下文，在容器刷新之前，给用户一个机会，可以做一些事情。

customizeContext(sc, wac);

跟进去看看

protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
        List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
                determineContextInitializerClasses(sc);
    
        for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
            Class<?> initializerContextClass =
                    GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
            if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {
                throw new ApplicationContextException(String.format(
                        "Could not apply context initializer [%s] since its generic parameter [%s] " +
                        "is not assignable from the type of application context used by this " +
                        "context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),
                        wac.getClass().getName()));
            }
            this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
        }
    
        AnnotationAwareOrderComparator.sort(this.contextInitializers);
        for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
            initializer.initialize(wac);
        }
    }

怎么定制上下文呢？看代码可以知道，要实现ApplicationContextInitializer接口

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
    
        /**
         * Initialize the given application context.
         * @param applicationContext the application to configure
         */
        void initialize(C applicationContext);
    
    }

接口就一个初始化方法，传入上下文。实现该接口，自己操作上下文，完成定制。实现了ApplicationContextInitializer接口之后，怎么配置让其生效呢？我们先看看是怎么找实现该接口的类的，进入determineContextInitializerClasses看看。

protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
                determineContextInitializerClasses(ServletContext servletContext) {
    
        List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
                new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();
    
        String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
        if (globalClassNames != null) {
            for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
                classes.add(loadInitializerClass(className));
            }
        }
    
        String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
        if (localClassNames != null) {
            for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) {
                classes.add(loadInitializerClass(className));
            }
        }
    
        return classes;
    }

可以看到，这里是通过 servletContext.getInitParameter(GLOBAL\_INITIALIZER\_CLASSES\_PARAM)和servletContext.getInitParameter(CONTEXT\_INITIALIZER\_CLASSES\_PARAM) 获取类名，和Spring配置文件一样，也是在web.xml中配置的。

最后是刷新上下文。

这里的刷新上下文，其实就是Spring读取配置稳定，解析bean定义，注册BeanDefinition，实例化bean，完成注入的过程，在此暂不深入。

wac.refresh();

## 3. DispatcherServlet分析 ##

### 3.1继承关系 ###

![Image 1][]

DispatcherServlet间接实现了Servlet接口。在初始化阶段，会调用Servlet的init()方法，这是Servlet的入口，接下来我们找到入口，从入口分析。

### 3.2 追根溯源 Servlet的init() ###

本次分析的目的是研究DispatcherServlet的上下文和ContextLoaderListener的上下文是如何初始化和关联起来的，其它逻辑不是本文重点，将会被忽略。

在GenericServlet类中实现了Servlet接口的init方法

public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

GenericServlet的init(ServletConfig config)方法除了设置了config并没有干其它的逻辑，而是调用了自身的init()方法，将具体逻辑委托给自己的子类处理。我们接着找谁实现了init()方法。在HttpServletBean类中找到了init()方法，看一下代码

public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }
    
        // Set bean properties from init parameters.
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                initBeanWrapper(bw);
                bw.setPropertyValues(pvs, true);
            }
            catch (BeansException ex) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }
    
        // Let subclasses do whatever initialization they like.
        initServletBean();
    
        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }

HttpServletBean的init()方法也没有看到有处理上下文的逻辑，但是看了initServletBean()这个方法，我们继续跟代码。  
在FrameworkServlet类中重写了initServletBean()方法。

@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");
        }
    }

在这里终于看到了与上下文有关的逻辑————initWebApplicationContext()方法，跟之。

protected WebApplicationContext initWebApplicationContext() {
        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
                        cwac.setParent(rootContext);
                    }
                    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
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            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.
            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;
    }

我们看我们感兴趣的逻辑。

WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;
    .
    .
    .
    if (wac == null) {
        // No context instance is defined for this servlet -> create a local one
        wac = createWebApplicationContext(rootContext);
    }

这里获取根上下文rootContext，并将rootContext作为参数，创建WebApplicationContext。这里获取的rootContext就是在ContextLoaderListener设置的WebApplicationContext。Talk is cheap. Show me the code.

public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
        return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
    }

看到 WebApplicationContext.ROOT\_WEB\_APPLICATION\_CONTEXT\_ATTRIBUTE，我们回忆一下ContextLoaderListener是怎么设置上下文到ServletContext的。

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

证据确凿。  
再来看createWebApplicationContext(rootContext)

protected WebApplicationContext createWebApplicationContext(WebApplicationContext parent) {
        return createWebApplicationContext((ApplicationContext) parent);
    }

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;
    }

看到createWebApplicationContext方法的代码，是不是似曾相识的感觉？这和ContextLoaderListener的 initWebApplicationContext方法的逻辑是不是很相似？

wac.setParent(parent);

我们将目光放在上面一行代码上。这行代码将ContextLoaderListener的创建的WebApplicationContext和DispatcherServlet创建的WebApplicationContext关联了起来，将Web应用上下文的层次体现出来，我们也知道WebApplicationContext其实也是Spring的BeanFactory，所以这里也是设置了Spring Ioc容器的父子关系。最后就是configureAndRefreshWebApplicationContext了，这里就不分析了。

作者：RayJY  
链接：https://www.jianshu.com/p/3d3532f6647f  
來源：简书  
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。

[springinside]: https://link.jianshu.com?t=https://github.com/jylaxp/springinside
[Image 1]: