spring容器启动的加载过程(一)

使用spring，我们在web.xml都会配置ContextLoaderListener

  <listener>
    <listener-class>
            org.springframework.web.context.ContextLoaderListener
        </listener-class>
  </listener>

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

//由于他继承自ContextLoader，加载的时候当然会先加载他啦，那我们来看看这个类

}

ContextLoader这个类有一段静态代码块

private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";//这个文件是在org.springframework.web.context.ContextLoader.properties他里面是

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext //这个后面再研究 以后就会把spring所有重要的东西都能看到了。

    private static final Properties defaultStrategies;

    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);//根据上面的properties文件，就能得到一个默认的properties了。
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
    }

第一步：我们在看看ContextLoaderListener的这个方法，在容器启动的时候，他会扫描web.xml的listener配置，然后自动调用Listener的contextInitialized初始化方法。

public void contextInitialized(ServletContextEvent event) {
        this.contextLoader = createContextLoader();
        if (this.contextLoader == null) {
            this.contextLoader = this;
        }
        this.contextLoader.initWebApplicationContext(event.getServletContext());
    }

 第二步：initWebApplicationContext初始化web应用上下文

    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);//判断是否有WebApplicationContext，没有则创建，创建得到的是ConfigurableWebApplicationContext
            }
            if (this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
                if (!cwac.isActive()) {  //上下文尚未活动
                    // 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);//决定根Web应用程序环境是否存在父应用程序环境，一般是返回null
                        cwac.setParent(parent);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);//在这里是初始化web应用上下文的入口 ,spring的依赖注入也从这里开始了。
                }
            }
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);//把创建的根Web应用程序环境保存到Servlet环境中，每个派遣器Servlet加载的子环境会应用这个环境作为父环境    

            ClassLoader ccl = Thread.currentThread().getContextClassLoader();//取得线程的类加载器
            if (ccl == ContextLoader.class.getClassLoader()) {
                currentContext = this.context;//如果线程和本类拥有相同的类加载器，则使用静态变量保存即可，因为同一类加载器加载同一份静态变量    
            }
            else if (ccl != null) {
                currentContextPerThread.put(ccl, this.context);//如果线程和本类拥有不同的类加载器，则使用线程的类加载器作为键值保存在一个映射对象里，保证析构时能拿到Web应用程序环境进行关闭操作
            }

            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);//如果产生任何异常，则保存异常对象到Servlet环境里 
            throw ex;
        }
        catch (Error err) {
            logger.error("Context initialization failed", err);
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);//如果产生任何错误，则保存错误对象到Servlet环境里
            throw err;
        }
    }

第三步：得到具体的应用上下文

public static final String CONTEXT_CLASS_PARAM = "contextClass";


private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";


private static final Properties defaultStrategies;

//这个静态代码块，会去加载相同目录下的ContextLoader.properties，这个目录是org.springframework.web.context，这个配置文件的配置信息就只有org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
}

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        Class<?> contextClass = determineContextClass(sc);//决定要得到哪个ContextClass
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);   //实例化Web应用程序环境类 
}
    
protected Class<?> determineContextClass(ServletContext servletContext) {
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);//获取web.xml配置的contextClass
        if (contextClassName != null) {//配置了contextClass,返回具体的Class
            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());//在上面的静态代码块中已经加载了XmlWebApplicationContext，返回的是XmlWebApplicationContext的全路径名：org.springframework.web.context.support.XmlWebApplicationContext
            try {
                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());//没有配置contextClass,则通过配置文件，返回了XmlWebApplicationContext的class
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load default context class [" + contextClassName + "]", ex);
            }
        }
}

第四步：用于封装ApplicationContext数据并且初始化所有相关Bean对象。它会从web.xml中读取名为contextConfigLocation的配置，这就是spring xml数据源设置，然后放到ApplicationContext中，最后调用传说中的refresh方法执行所有Java对象的创建。

    public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";  在wen.xml中配置的位置
    
    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);
            //设置Web应用程序环境的ID  
            if (idParam != null) {
                wac.setId(idParam);
            }
            else {
                // Generate default id...
                 //如果 Servlet规范 <= 2.4，则使用web.xml里定义的应用程序名字定义Web应用程序名
                if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
                    // Servlet <= 2.4: resort to name specified in web.xml, if any.
                    wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                            ObjectUtils.getDisplayString(sc.getServletContextName()));
                }
                else {
                // 如果Servlet规范是 2.5， 则使用配置的ContextPath定义Web应用程序名 
                    wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                            ObjectUtils.getDisplayString(sc.getContextPath()));
                }
            }
        }
        //保存Servlet环境    
        wac.setServletContext(sc);
        String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);//这里得到"contextConfigLocation"配置的值:classpath*:spring-config.xml,
        if (initParameter != null) {
            wac.setConfigLocation(initParameter);//设置保存ConfigLocation，待后面加载的时候，扫描里面的配置
        }
        customizeContext(sc, wac);//提供子类可互换Web应用程序环境的机会 占位符方法  
        wac.refresh();//刷新Web应用程序环境以加载Bean定义,这里才是把我们XML里定义的bean放入容器的时候  
    }

"contextConfigLocation"在web.xml的配置

<context-param>
     <param-name>contextConfigLocation</param-name>
    <param-value>
                classpath*:spring-config.xml,
    </param-value>
</context-param>

第五步：bean的注入

    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();//告诉子类刷新内部bean工厂。

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);//准备bean工厂给上下文使用

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);//允许在上下文后处理bean工厂的子类

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);//调用处理器工厂注册的bean上下文

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);注册bean拦截器拦截bean的创建

                // Initialize message source for this context.
                initMessageSource();//给上下文初始化消息来源

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();//给上下文初始化事件多播

                // Initialize other special beans in specific context subclasses.//在特定上下文初始化其他特殊bean子类
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();//检查监听器bean并注册他们

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);//实例化所有剩余(non-lazy-init)单例对象

                // Last step: publish corresponding event.
                finishRefresh();//发布相应的事件
            }

            catch (BeansException ex) {
                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();//销毁已经创建的单例，避免悬挂的资源

                // Reset 'active' flag.
                cancelRefresh(ex);//重置“活跃”标志

                // Propagate exception to caller.
                throw ex;
            }
        }
    }