Servlet容器初始化IOC容器

红太狼 2022-01-28 11:53 396阅读 0赞

# [Servlet容器初始化IOC容器][Servlet_IOC] #

<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>

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

[Servlet_IOC]: https://www.cnblogs.com/toov5/p/10614482.html