SpringMVC Root WebApplicationContext启动流程

传统的SpringMVC项目中，需要在web.xml中配置Contextlistener。ContextLoaderListener是负责引导启动和关闭Spring的Root上下文的监听器。主要将处理委托给ContextLoader和ContextCleanupListener。
类的继承关系。

ContextLoaderListener实现了ServletContextListener接口。该接口主要定义了两个行为：监听上下文创建（contextInitialized）和监听上下文销毁（contextDestroyed）。
ContextLoaderListener只是将方法的处理委托给ContextLoader。

package org.springframework.web.context;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    public ContextLoaderListener() {
    }

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }

    //初始化Root WebApplication
    @Override
    public void contextInitialized(ServletContextEvent event) {
    //委托给ContextLoader
        initWebApplicationContext(event.getServletContext());
    }

    //销毁Root WebApplication
    @Override
    public void contextDestroyed(ServletContextEvent event) {
    //委托给ContextLoader
        closeWebApplicationContext(event.getServletContext());
        //委托给ContextCleanupListener
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }

}

ContextLoader的initWebApplicationContext方法负责创建root web application context。

//初始化root web application context
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        //校验是否已经存在root application，
        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.
            //创建WebApplicationContext，并保存到变量中，等关闭时使用
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
            //如果是ConfigurableWebApplicationContext，则配置上下文并刷新
            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);
                }
            }
            //在ServletContext中设置属性，表明已经配置了root web application context
            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;
        }
    }

该方法主要分为四步：

1. 校验是否已经存在root WebApplicationContext
2. 创建root WebApplicationContext
3. 配置上下文并刷新
4. 绑定root WebApplicationContext到servlet context上

这四步具体分析如下：

1.校验

校验的过程比较简单，主要是判断Servlet Context是否已经绑定过WebApplicationContext

2.创建对象

createWebApplicationContext()方法负责创建WebApplicationContext对象，主要过程是确定WebApplicationContext类，并实例化：

    protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    //查找WebApplicationContext的类
        Class<?> contextClass = determineContextClass(sc);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        //实例化对象
        return
        (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

决定WebApplicationContext类的主要策略是先判断ServletContext是否配置了contextClass属性，如果是，则用该属性指定的类作为WebApplicationContext类，否则则是否默认策略。默认策略是根绝classpath下的ContextLoader.properties中配置的类作为WebApplicatioNCOntext类：

protected Class<?> determineContextClass(ServletContext servletContext) {
        //优先根据servletContext配置的contextClass属性
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        //在通过classpath下的`ContextLoader.properties`文件制定的类作为WebApplicationContext类
        //默认是XmlWebApplicationContext
        else {
            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
            try {
                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load default context class [" + contextClassName + "]", ex);
            }
        }
    }

配置与刷新

创建WebApplicationContext对象后，ContextLoader会判断是否是ConfigurableWebApplicationContext。
ConfigurableWebApplicationContext拓展了WebApplicationContext的能力。其不仅扩展了自身方法，还继承了ConfigurableApplicationContext接口。

配置与刷新的步骤也正是利用了这些接口提供的能力。

    //配置并刷新上下文
    protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
        //设置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()));
            }
        }

        //WebApplicationContext绑定Servlet Context
        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);
        }

        //自定义上下文初始过程，留给用户的拓展机制
        customizeContext(sc, wac);
        //刷新上下文，加载bean
        wac.refresh();
    }

创建流程图如下：

原文地址：https://www.cnblogs.com/insaneXs/p/11115862.html