Servlet容器初始化IOC容器

<!-- ServletContext参数，配置Ioc容器的xml文件名 -->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:applicationContext.xml</param-value>
    </context-param>
    
<!-- 初始化Ioc容器的监听器（重点就是这个监听器）-->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

看下这个监听器：

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
 
    public ContextLoaderListener() {
    }
    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }
 
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
 
}

这个监听器首先实现了ServletContextListener，所以会在Servlet容器初始化和销毁的时候分别调用contextInitialized( )与contextDestroyed( )方法。

注意这里的带参构造方法 ContextLoaderListener（WebApplicationContext context）是调用了父类ContextLoader的构造方法，并传参进去。

而ContextLoader的主要作用就是初始化Ioc容器，下面我们来看看ContextLoader这个类

public class ContextLoader {
public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";//ServletContext上下文参数
private WebApplicationContext context;

public ContextLoader(WebApplicationContext context) {
        this.context = context;
    }

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!");
        }//不能初始化Ioc容器，因为已经有一个根Application上下文，检查在web.xml中是否有多个ContextLoader
 
        Log logger = LogFactory.getLog(ContextLoader.class);
        servletContext.log("Initializing Spring root WebApplicationContext");
        if (logger.isInfoEnabled()) {
            logger.info("Root WebApplicationContext: initialization started");
        }//开始初始化WebApplicationContext
        long startTime = System.currentTimeMillis();//当前毫秒数
 
        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()) {／／Ｉｏｃ容器并未初始化，所以暂时不能提供服务
                    // 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) {／／设置父applicationContext
                        // 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;
        }
    }

这里是总过程的描述：

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        Class<?> contextClass = determineContextClass(sc);//获取context是由那个类定义的
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {//判断是否为同一个类，或是子类
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

通过determineContextClass(　）知道了实例化IOC容器方式，然后用这个方式，再通过反射机制实例化容器，即实例化webApplicationContext。

protected Class<?> determineContextClass(ServletContext servletContext) {
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);//该属性为"ContextClass"
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        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);
            }
        }
    }

因为实例化Ioc容器的方式有几种，而这个方法是判定需要实例化那种类来实例化当前Web容器的Spring Ioc根容器。要判定需要实例化哪种类来实例化当前web容器的Spring根容器，如果我们设置了名称为“contextClass”的context-param，则取我们设置的类，该类应当实现ConfigurableWebApplicationContext接口或继承自实现了该接口的子类（如XmlWebApplicationContext、GroovyWebApplicationContext和AnnotationConfigWebApplicationContext），通常我们都不会设置，Spring会默认取与ContextLoader同目录下的ContextLoader.properties中记录的类名作为根容器的类型（默认是org.springframework.web.context.support.XmlWebApplicationContext）；

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();

}

虽然实例化了IOC容器，但是并未完成IOC容器的初始化，还不能提供服务。该方法的逻辑主要有一下几点：设置一个contextId（从contextId这个param获取，如果没有则默认是WebApplicationContext的类名 + “:” + servlet context的路径）；设置配置位置（从contextConfigLocation 这个param获取，如果未配置，则默认是/WEB-INF/applicationContext.xml，在XmlWebApplicationContext中可以看出）；自定义该congtext；调用该Context的refresh()方法。

上面描述了WebApplicationContext的构造，默认还是使用反射的方式构建XmlWebApplicationContext实例

public void closeWebApplicationContext(ServletContext servletContext) {
        servletContext.log("Closing Spring root WebApplicationContext");
        try {
            if (this.context instanceof ConfigurableWebApplicationContext) {
                ((ConfigurableWebApplicationContext) this.context).close();
            }
        }
        finally {
            ClassLoader ccl = Thread.currentThread().getContextClassLoader();
            if (ccl == ContextLoader.class.getClassLoader()) {
                currentContext = null;
            }
            else if (ccl != null) {
                currentContextPerThread.remove(ccl);
            }
            servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
            if (this.parentContextRef != null) {
                this.parentContextRef.release();
            }
        }
    }

该方法完成了对Ioc容器的销毁。

PS：webApplicationContext实现了ApplicationContext接口的子类。是专门为WEB应用准备的。

1. 它允许从相对于Web根目录的路径中加载配置文件完成初始化工作。从WebApplicationContext中可以获取ServletContext引用，整个Web应用上下文对象将作为属性放置在ServletContext中，以便Web应用环境可以访问Spring上下文。 
2.WebApplicationContext还为Bean提供了三个新的作用域，request、session和globalsession。 
其中两个参数HttpServletRequest：服务器从客户端拿去数据 
HttpServletResponse：服务器向前台传送数据

原文：https://blog.csdn.net/Wayne_y/article/details/79778207