ContextLoaderListener容器初始化

http://blog.csdn.net/qq924862077/article/details/52769754

1<context-param>  
2    <param-name>contextConfigLocation</param-name>  
3    <param-value>  
4        classpath:applicationContext.xml  
5    </param-value>  
6</context-param>  
7<listener>  
8    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>  
9</listener>

在tomcat启动时，spring通过上述配置会初始化spring容器，注入applicationContext.xml中配置的bean以及其他一些配置。

1public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
2
3	public ContextLoaderListener() {
4	}
5	public ContextLoaderListener(WebApplicationContext context) {
6		super(context);
7	}
8	@Override
9	public void contextInitialized(ServletContextEvent event) {
10		initWebApplicationContext(event.getServletContext());
11	}
12	@Override
13	public void contextDestroyed(ServletContextEvent event) {
14		closeWebApplicationContext(event.getServletContext());
15		ContextCleanupListener.cleanupAttributes(event.getServletContext());
16	}
17}

ContextLoderListener实现ServletContextListener接口，而该接口继承java的监听标记接口EventListener，表示当servlet容器启动时，就会接受通知，进而进行一系列的初始化操作。

1public interface ServletContextListener extends EventListener {
2
3    /**
4     * Receives notification that the web application initialization
5     * process is starting.
6     *
7     * <p>All ServletContextListeners are notified of context
8     * initialization before any filters or servlets in the web
9     * application are initialized.
10     *
11     * @param sce the ServletContextEvent containing the ServletContext
12     * that is being initialized
13     */
14    public void contextInitialized(ServletContextEvent sce);
15
16    /**
17     * Receives notification that the ServletContext is about to be
18     * shut down.
19     *
20     * <p>All servlets and filters will have been destroyed before any
21     * ServletContextListeners are notified of context
22     * destruction.
23     *
24     * @param sce the ServletContextEvent containing the ServletContext
25     * that is being destroyed
26     */
27    public void contextDestroyed(ServletContextEvent sce);
28}

spring的具体初始化工作均在父类ContextLoder中进行，具体初始化步骤如下：

判断是否已经存在WebApplicationContext(spring容器)，如果存在，则抛出异常。
创建WebApplicationContext容器，容web.xml中获取context-param名称为contextClass的value，该类必须实现WebApplicationContext接口，如果没有指定，则默认使用xmlWebApplicationContext。
判断创建的WebApplicationContext容器是否为ConfigurableApplicationContext的实例或子类，如果是，则判断该容器是否至少刷新一次并且还没有关闭（是否有效），如果无效，则重新设置容器的id，并加载对应的contextConfigLocation配置（spring配置)，加载父容器，spring提供一个模板方法，如果需要配置父容器，可以继承ContextLoder，并覆盖loadParentContext方法。

线程绑定容器

/** * Initialize Spring's web application context for the given servlet context, * using the application context provided at construction time, or creating a new one * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params. * @param servletContext current servlet context * @return the new WebApplicationContext * @see #ContextLoader(WebApplicationContext) * @see #CONTEXT_CLASS_PARAM * @see #CONFIG_LOCATION_PARAM / 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!"); }

1	Log logger = LogFactory.getLog(ContextLoader.class);
2	servletContext.log("Initializing Spring root WebApplicationContext");
3	if (logger.isInfoEnabled()) {
4		logger.info("Root WebApplicationContext: initialization started");
5	}
6	long startTime = System.currentTimeMillis();
7
8	try {
9		// Store context in local instance variable, to guarantee that
10		// it is available on ServletContext shutdown.
11		if (this.context == null) {
12		    //创建容器
13			this.context = createWebApplicationContext(servletContext);
14		}
15		if (this.context instanceof ConfigurableWebApplicationContext) {
16			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
17			if (!cwac.isActive()) {
18				// The context has not yet been refreshed -> provide services such as
19				// setting the parent context, setting the application context id, etc
20				if (cwac.getParent() == null) {
21					// The context instance was injected without an explicit parent ->
22					// determine parent for root web application context, if any.
23					ApplicationContext parent = loadParentContext(servletContext);
24					cwac.setParent(parent);
25				}
26				//如果容器无效，则重置contextId，且重新加载contextConfigLocation配置
27				configureAndRefreshWebApplicationContext(cwac, servletContext);
28			}
29		}
30		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
31        //线程绑定容器context
32		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
33		if (ccl == ContextLoader.class.getClassLoader()) {
34			currentContext = this.context;
35		}
36		else if (ccl != null) {
37			currentContextPerThread.put(ccl, this.context);
38		}
39
40		if (logger.isDebugEnabled()) {
41			logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
42					WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
43		}
44		if (logger.isInfoEnabled()) {
45			long elapsedTime = System.currentTimeMillis() - startTime;
46			logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
47		}
48
49		return this.context;
50	}
51	catch (RuntimeException ex) {
52		logger.error("Context initialization failed", ex);
53		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
54		throw ex;
55	}
56	catch (Error err) {
57		logger.error("Context initialization failed", err);
58		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
59		throw err;
60	}
61}

ContextLoaderListener容器初始化

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