Spring 启动过程详解

一、简述

1、我们在Java web 项目开发中，都会有一个web.xml文件，这个文件中一定会有如下一段代码：

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

这个ContextLoaderListener类中有一个方法如下：

/**
	 * Initialize the root web application context.
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

因此，我们可以知道web项目启动时调用contextInitialized进行初始化的，这个方法又会在
tomcat 或者Jetty等插件中被调用。当然了不管是Tomcat还是Jetty，操作的对象都是ServletContextListener
，这里不再详细描述，之后有时间学习一Jetty源码。

2、在web.xml中定义 contextConfigLocation参数.spring会使用这个参数加载所有逗号分割的xml，如果没有这个参数,Spring 自动加载applicationContext.xml 配置文件。

<init-param>
     <param-name>contextConfigLocation</param-name>
     <param-value>
         classpath*:conf/applicationContext.xml，
         classpath*:conf/spring-mvc.xml
     </param-value>
 </init-param>

这个ContextLoaderListener类中设置配置文件路径方法：

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);
		//public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";
		String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (initParameter != null) {
			wac.setConfigLocation(initParameter);
		}
		customizeContext(sc, wac);
		wac.refresh();
	}

二、ContextLoaderListener

1、在 Servlet API中有一个ServletContextListener接口，它能够监听ServletContext对象的生命周期，实际上就是监听Web应用的生命周期。
当Servlet容器启动或终止Web应用时，会触发ServletContextEvent事件，该事件由ServletContextListener来处理。

2、ContextLoaderListener监听器的作用就是启动Web容器时，自动装配ApplicationContext的配置信息。
因为它实现了ServletContextListener这个接口，在web.xml配置了这个监听器，启动容器时，就会默认执行它实现的方法。
由于在ContextLoaderListener中关联了ContextLoader这个类，所以整个加载配置过程由ContextLoader来完成。

/**
 * 所有事件监听器接口必须扩展
 */
public interface EventListener {
}


/**
  *接受ServletContext的生命周期的事件通知接口
  *
  * 为了能接收通知事件，实现类要么在web应用程序的部署描述符中声明 ，用WebListener注解 或者通过ServletContext中的addListener 方法注册
  *
  * 接口的实现在contextInitialized 方法中按顺序什么，在contextDestroyed 方法中则是相反的顺序。
   */
public interface ServletContextListener extends EventListener {

    /**
     * web应用程序的初始化时收到通知，在所有的servlet和filter都已初始化前收到通知前
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * ServletContext销毁时会收到通知，收到通知前所有的servlet和filter都已被销毁
     */
    public void contextDestroyed(ServletContextEvent sce);
}


/**
 * 实际执行 root Application Context初始化的是ContextLoaderListener
 *
 * 在web.xml文件中寻找 <context-param></context-param>
 * 这段标签指定上下文类型，如果找不到的话使用默认的XmlWebApplicationContext。
 * 使用默认ContextLoader实现,任何上下文类都需要实现ConfigurableWebApplicationContext指定的接口
 *
 *
 *<context-param><param-name>contextConfigLocation</param-name><param-value>classpath*:conf/applicationContext.xml</param-value></context-param>
 *
 *
 */
public class ContextLoader {


	public static final String CONTEXT_CLASS_PARAM = "contextClass";

	public static final String CONTEXT_ID_PARAM = "contextId";

	public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";

	public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";

	public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";

	public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";

	/**
	 * 类路径资源名称(相对于ContextLoader类)定义ContextLoader名称的默认策略。
   * 内容：org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
	 */
	private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";


	private static final Properties defaultStrategies;

	static {
		//加载默认策略，实现属性文件中内容。
		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());
		}
	}


	/**
	 *ClassLoader  WebApplicationContext映射
	 */
	private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread =
			new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);

	/**
	 * The 'current' WebApplicationContext, if the ContextLoader class is
	 * 如果ContextLoader类是部署在web应用程序类加载器本身，当前的WebApplicationContext对象
	 */
	private static volatile WebApplicationContext currentContext;

	/**
   * 这个加载器管理的WebApplicationContext实例
	 */
	private WebApplicationContext context;


	private BeanFactoryReference parentContextRef;


	public ContextLoader() {
	}


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

	/**
	 * 根据指定的servlet context初始化spring 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 {
			// 上下文存储在本地实例变量,保证它可以在ServletContext关闭时可用
			if (this.context == null) {
				this.context = createWebApplicationContext(servletContext);
			}
			if (this.context instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
				if (!cwac.isActive()) {
					// 上下文尚未刷新--> 提供 设置父母上下文、设置上下文Id的服务
					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);
				}
			}
			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;
		}
	}

	/**
	 *为当前的加载器实例化WebApplicationContext,默认上下文类或一个自定义上下文类（如果指定）。
	 *
	 */
	protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
		Class<?> contextClass = determineContextClass(sc);
		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);
	}


	@Deprecated
	protected WebApplicationContext createWebApplicationContext(ServletContext sc, ApplicationContext parent) {
		return createWebApplicationContext(sc);
	}

	protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
		if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
			// 这个应用上下文Id 设置原来默认的值 ->分配一个更有价值的Id
			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 initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
		if (initParameter != null) {
			wac.setConfigLocation(initParameter);
		}
		customizeContext(sc, wac);
		wac.refresh();
	}

	/**
	 * 返回WebApplicationContext实现类使用,要么是默认XmlWebApplicationContext要么是自定义上下文类（在指定的情况下）。
	 */
	protected Class<?> determineContextClass(ServletContext servletContext) {
		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);
			}
		}
		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);
			}
		}
	}

	/**
	 * Return the {@link ApplicationContextInitializer} implementation classes to use
	 * if any have been specified by {@link #CONTEXT_INITIALIZER_CLASSES_PARAM}.
	 * @param servletContext current servlet context
	 * @see #CONTEXT_INITIALIZER_CLASSES_PARAM
	 */
	@SuppressWarnings("unchecked")
	protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
			determineContextInitializerClasses(ServletContext servletContext) {
		String classNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
			new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();
		if (classNames != null) {
			for (String className : StringUtils.tokenizeToStringArray(classNames, ",")) {
				try {
					Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
					Assert.isAssignable(ApplicationContextInitializer.class, clazz,
							"class [" + className + "] must implement ApplicationContextInitializer");
					classes.add((Class<ApplicationContextInitializer<ConfigurableApplicationContext>>)clazz);
				}
				catch (ClassNotFoundException ex) {
					throw new ApplicationContextException(
							"Failed to load context initializer class [" + className + "]", ex);
				}
			}
		}
		return classes;
	}

	/**
	 * 在上下文刷新之前、在配置文位置配置好之后 加载器创建ConfigurableWebApplicationContext
	 */
	protected void customizeContext(ServletContext servletContext, ConfigurableWebApplicationContext applicationContext) {
		List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
				determineContextInitializerClasses(servletContext);
		if (initializerClasses.isEmpty()) {
			// no ApplicationContextInitializers have been declared -> nothing to do
			return;
		}

		Class<?> contextClass = applicationContext.getClass();
		ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerInstances =
				new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>();

		for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
			Class<?> initializerContextClass =
					GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
			if (initializerContextClass != null) {
				Assert.isAssignable(initializerContextClass, contextClass, String.format(
						"Could not add context initializer [%s] as its generic parameter [%s] " +
						"is not assignable from the type of application context used by this " +
						"context loader [%s]: ", initializerClass.getName(), initializerContextClass.getName(),
						contextClass.getName()));
			}
			initializerInstances.add(BeanUtils.instantiateClass(initializerClass));
		}

		ConfigurableEnvironment env = applicationContext.getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(servletContext, null);
		}

		AnnotationAwareOrderComparator.sort(initializerInstances);
		for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : initializerInstances) {
			initializer.initialize(applicationContext);
		}
	}

	/**
	 * 
	 */
	protected ApplicationContext loadParentContext(ServletContext servletContext) {
		ApplicationContext parentContext = null;
		String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
		String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);

		if (parentContextKey != null) {
			// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
			BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
			Log logger = LogFactory.getLog(ContextLoader.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Getting parent context definition: using parent context key of '" +
						parentContextKey + "' with BeanFactoryLocator");
			}
			this.parentContextRef = locator.useBeanFactory(parentContextKey);
			parentContext = (ApplicationContext) this.parentContextRef.getFactory();
		}

		return parentContext;
	}

	/**
	 * Close Spring's web application context for the given servlet context. If
	 * the default {@link #loadParentContext(ServletContext)} implementation,
	 * which uses ContextSingletonBeanFactoryLocator, has loaded any shared
	 * parent context, release one reference to that shared parent context.
	 * <p>If overriding {@link #loadParentContext(ServletContext)}, you may have
	 * to override this method as well.
	 * @param servletContext the ServletContext that the WebApplicationContext runs in
	 */
	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();
			}
		}
	}


	/**
	 * Obtain the Spring root web application context for the current thread
	 * (i.e. for the current thread's context ClassLoader, which needs to be
	 * the web application's ClassLoader).
	 * @return the current root web application context, or {@code null}
	 * if none found
	 * @see org.springframework.web.context.support.SpringBeanAutowiringSupport
	 */
	public static WebApplicationContext getCurrentWebApplicationContext() {
		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
		if (ccl != null) {
			WebApplicationContext ccpt = currentContextPerThread.get(ccl);
			if (ccpt != null) {
				return ccpt;
			}
		}
		return currentContext;
	}

}


/**
 *
 *
 * 如果使用Log4jConfigListener的话，在web.xml配置中，ContextLoaderListener要放在Log4jConfigListener后面
 *
 * 通过ContextLoaderListener(WebApplicationContext)构造函数引入web 应用上下文
 */
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

	/**
   * 在web.xml文件中声明<listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener>
   * 时调用这个无参构造器
   * 创建的应用上下文将被注册到ServletContext的属性名称WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE中，
	 * 当生命周期方法contextDestroyed调用时Spring应用程序上下文将被关闭时
	 */
	public ContextLoaderListener() {
	}

	/**
   * 根据给定的应用程序上下文创建一个新的ContextLoaderListener
	 */
	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}

	/**
	 * Initialize the root web application context.
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}


	/**
	 * Close the root web application context.
	 */
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}

}