ContextLoaderListener初始化

ContextLoaderListener初始化了webapp的root context（應用根上下文），過程以下

ContextLoaderListener繼承了ContextLoader，主要方法以下

private WebApplicationContext  context;
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    //判斷是否已經有根上下文，若是已經有了，報錯，不然，建立根上下文
    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!");
    } else {
        //開始建立根上下文
        try {
            if(this.context == null) {
                //建立根上下文
                this.context = this.createWebApplicationContext(servletContext);
            }

            if(this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext err = (ConfigurableWebApplicationContext)this.context;
                if(!err.isActive()) {
                    if(err.getParent() == null) {
                        ApplicationContext elapsedTime = this.loadParentContext(servletContext);
                        err.setParent(elapsedTime);
                    }
                    // 給上下文設置各類屬性
                    // 給上下文設置id
                    // 給上下文設置配置文件的路徑（applicationcontext.xml相似）
                    // 經過refresh方法設置beanfactory？這裏不是太清楚
                    this.configureAndRefreshWebApplicationContext(err, servletContext);
                }
            }
            //建立好的根上下文放到servletContext裏，整個webapp只有一個servletContext
            //servletContext是ServletConfig的一個成員變量
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
            ClassLoader err1 = Thread.currentThread().getContextClassLoader();
            if(err1 == ContextLoader.class.getClassLoader()) {
                currentContext = this.context;
            } else if(err1 != null) {
                currentContextPerThread.put(err1, this.context);
            }
            return this.context;
        } catch (RuntimeException var8) {
  
        }
    }
}

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    //獲取上下文類型，只能爲ConfigurableWebApplicationContext類型的
    Class contextClass = this.determineContextClass(sc);
    if(!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    } else {
        //若是類型正確，實例化並返回
        return (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);
    }
}

protected Class<?> determineContextClass(ServletContext servletContext) {
    //首先，看在web.xml中有沒有配置咱們本身的上下文類型，很明顯，咱們沒有，此處爲null
    String contextClassName = servletContext.getInitParameter("contextClass");
    if(contextClassName != null) {
        try {
            return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
        } catch (ClassNotFoundException var4) {
            throw new ApplicationContextException("Failed to load custom context class [" + contextClassName + "]", var4);
        }
    } else {
        //因爲咱們本身沒有配置，spring加載本身的默認類型（XmlWebApplicaionContext.java）
        contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());

        try {
            return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
        } catch (ClassNotFoundException var5) {
            throw new ApplicationContextException("Failed to load default context class [" + contextClassName + "]", var5);
        }
    }
}

//這個是默認類型
static {
    try {
        ClassPathResource ex = new ClassPathResource("ContextLoader.properties", ContextLoader.class);
        defaultStrategies = PropertiesLoaderUtils.loadProperties(ex);
    } catch (IOException var1) {
        throw new IllegalStateException("Could not load \'ContextLoader.properties\': " + var1.getMessage());
    }

    currentContextPerThread = new ConcurrentHashMap(1);
}

ContextLoader.properties文件內容以下

org.springframework.web.context.WebApplicationContext=
org.springframework.web.context.support.XmlWebApplicationContext

XmlWebApplicationContext是web應用中定製的getBean的方法

ServletContext servletContext = request.getSession().getServletContext();
WebApplicationContext was = WebApplicationContextUtils.getWebApplicationContext(servletContext);
User user = (User)was.getBean("user");

其他兩種方法爲：

ApplicationContext ctx =

new FileSystemXmlApplicationContext( "G:/Test/applicationcontext.xml ");

或

ApplicationContext ctx =

new ClassPathXmlApplicationContext( "/applicationcontext.xml ");

 

最後一句話：一個請求一個request，一個用戶一個session，一個應用一個ServletContext

ServletContext中能夠存儲共享的數據，由於它的生命週期很長，從應用啓動直到消亡。因此裏面不要存大的數據，會很佔內存

ServletContext對象是由容器建立的，也就是tomcat

和request同樣，ServletContext也有setAttribute以及getAttribute和removeAttribute方法

經過ServletContext，咱們也能夠在servlet之間傳遞信息，也能夠讀取web.xml中的初始化參數context-param，如web.xml中context-param以下

<context-param>
    <param-name>log4jConfigLocation</param-name>
    <param-value>classpath:${env}-conf/log4j.properties</param-value>
  </context-param>
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:spring.xml</param-value>
<!--     <param-value>classpath:spring-mybatis.xml</param-value> -->
  </context-param>
  <context-param>
    <param-name>webAppRootKey</param-name>
    <param-value>ott-vms-admin.root</param-value>
  </context-param>

ServletContext servletContext = request.getSession().getServletContext();
String value = (String)servletContext.getInitParameter("contextConfigLocation");