使用SpringMVC集成SpringSession的问题

最近在使用SpringSession时遇到一个问题，错误日志如下：

Exception sending context initialized event to listener instance of class org.springframework.web.context.ContextLoaderListener
java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!

先说下项目的配置情况：
SpringSession完全按照官方文档配置如下，

@EnableRedisHttpSession 
public class Config {

        @Bean
        public JedisConnectionFactory connectionFactory(@RedisServerPort int port) {
                JedisConnectionFactory connection = new JedisConnectionFactory(); 
                connection.setPort(port);
                return connection;
        }
}

public class Initializer
                extends AbstractHttpSessionApplicationInitializer { 

        public Initializer() {
                super(Config.class); 
        }
}

web.xml也是标准的配置方法：

   <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath*:config/spring/rpc-service.xml</param-value>
    </context-param>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

   <servlet>
        <servlet-name>mvc</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath*:config/spring/spring-mvc-main.xml</param-value>
        </init-param>
        <load-on-startup>0</load-on-startup>
    </servlet>

要想知道这个错误产生的原因，先要弄清楚Spring的容器(ApplicationContext)的继承原理及SpringMVC如何使用这一机制的。

Spring容器的继承关系

如上图那样，容器之间可以像对象的继承关系一样，子容器通过setParent方法来设置自己的父容器。在调用容器的getBean查找实例时，依次从当前容器往父容器查找，直到找到满足的对象即返回，如果一直没有找到则返回Null.

SpringMVC中的容器即它们的关系

在使用SpringMVC时，必需要配置org.springframework.web.servlet.DispatcherServlet这样的一个servlet。在初始化此实例时，会生成一个WebApplicationContext容器，生成容器后会检查当前ServletContext环境下是否已经存在"rootContext"，如果存在，则通过setParent方法设置为父容器。源代码在这里(org.springframework.web.servlet.FrameworkServlet#initWebApplicationContext)：

protected WebApplicationContext initWebApplicationContext() {
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;

		if (this.webApplicationContext != null) {
			// A context instance was injected at construction time -> use it
			wac = this.webApplicationContext;
			if (wac instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
				if (!cwac.isActive()) {
					// 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 -> set
						// the root application context (if any; may be null) as the parent
						cwac.setParent(rootContext);
					}
					configureAndRefreshWebApplicationContext(cwac);
				}
			}
		}
		if (wac == null) {
			// No context instance was injected at construction time -> see if one
			// has been registered in the servlet context. If one exists, it is assumed
			// that the parent context (if any) has already been set and that the
			// user has performed any initialization such as setting the context id
			wac = findWebApplicationContext();
		}
		if (wac == null) {
			// No context instance is defined for this servlet -> create a local one
			wac = createWebApplicationContext(rootContext);
		}

		if (!this.refreshEventReceived) {
			// Either the context is not a ConfigurableApplicationContext with refresh
			// support or the context injected at construction time had already been
			// refreshed -> trigger initial onRefresh manually here.
			onRefresh(wac);
		}

		if (this.publishContext) {
			// Publish the context as a servlet context attribute.
			String attrName = getServletContextAttributeName();
			getServletContext().setAttribute(attrName, wac);
			if (this.logger.isDebugEnabled()) {
				this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
						"' as ServletContext attribute with name [" + attrName + "]");
			}
		}

		return wac;
	}

那这个rootContext从哪里来的呢？答案是org.springframework.web.context.ContextLoaderListener，它是一个标准的javax.servlet.ServletContextListener实现，在容器启动的时候创建一个全局唯一的rootContext，代码在:org.springframework.web.context.ContextLoader#initWebApplicationContext下:

   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);
			}
			if (this.context instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
				if (!cwac.isActive()) {
					// 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);
						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;
		}
	}

总结下结果：servlet容器通过ContextLoaderListener创建一个root容器，并设置为SpringMVC的父容器。 到止应该明白了文章最开始的报错信息的来源了，就是在这个方法里报错。出错的原因有且只有一个：就是给servlet容器注册了两个ContextLoaderListener。一个是在web.xml配置文件里配置的，那另一个在哪里注册的呢？接着分析。

SpringSession的加载机制

集成SpringSession是很简单的，只要实现一个"AbstractHttpSessionApplicationInitializer "的子类即可，然后在子类的构造器中传一个标注了EnableRedisHttpSession的注解类，此注解继承了Configuration，所以在类Initializer进行初始化时，会调用“org.springframework.session.web.context.AbstractHttpSessionApplicationInitializer#onStartup”方法，代码如下：

    public void onStartup(ServletContext servletContext)
            throws ServletException {
        beforeSessionRepositoryFilter(servletContext);
        if(configurationClasses != null) {
            AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
            rootAppContext.register(configurationClasses);
            servletContext.addListener(new ContextLoaderListener(rootAppContext));
        }
        insertSessionRepositoryFilter(servletContext);
        afterSessionRepositoryFilter(servletContext);
    }

现在我们找到了另外一个往servlet容器中注册ContextLoaderListener的地方了，也就是在这个地方抛错了。找到了问题的根源，解决问题就很简单了。

解决问题

其实只要保证ContextLoaderListener只注册一次就不会有这个问题了，所以有两个选择做法：要么别在web.xml里配置ContextLoaderListener，要么在Initializer类的构造方法中，不要调用父类的有参数构造器，而是调用空参构造器。为了遵守SpringMVC官方的开发规范，最好还是要配置下ContextLoaderListener，把非web层而的对象单独配置，比如service层对象。而web层的东西配置在dispatcher容器中。但是这样即使这样做了，会报别外一个错误，说"org.springframework.data.redis.connection.jedis.JedisConnectionFactory"找不到。所以要在spring的配置文件中加入如下配置：

   <bean class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory">
        <property name="hostName" value="xxxx"/>
        <property name="port" value="xxxx"/>
        <property name="password" value="xxx"/>
    </bean>

如果你加入这个配置，也还是报相同错误的话，那么就要检查下这个配置是放在哪个spring的配置文件下，如果放在ContextLoaderListener的配置文件下就不会报错，而放在DispatcherServlet的配置下就会报错。原因还是从代码（org.springframework.web.filter.DelegatingFilterProxy#findWebApplicationContext）里看：

   protected WebApplicationContext findWebApplicationContext() {
		if (this.webApplicationContext != null) {
			// the user has injected a context at construction time -> use it
			if (this.webApplicationContext instanceof ConfigurableApplicationContext) {
				if (!((ConfigurableApplicationContext)this.webApplicationContext).isActive()) {
					// the context has not yet been refreshed -> do so before returning it
					((ConfigurableApplicationContext)this.webApplicationContext).refresh();
				}
			}
			return this.webApplicationContext;
		}
		String attrName = getContextAttribute();
		if (attrName != null) {
			return WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
		}
		else {
			return WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		}
	}

这个方法的最后几行可以看出，SpringSession所需要的所有基础对象，比如Redis连接对象，Redis模板对象，都是从WebApplicationContext从获取。而WebApplicationContext根据getContextAttribute()的值不同先获取的方式也不同。如果getContextAttribute()返回为Null，则取的容器是rootContext，即ContextLoaderListener生成的容器。反之，获取的是DispatcherServlet容器。知道了原因，解决方式就清晰了。重写Initializer的getDispatcherWebApplicationContextSuffix方法。Initializer最终的代码如下：

@EnableRedisHttpSession
public class Initializer extends AbstractHttpSessionApplicationInitializer
{
    @Override
    protected String getDispatcherWebApplicationContextSuffix()
    {
        return "mvc"; # 这里返回的字符串就是你配置DispatcherServlet的名称
    

而本文前面提到的Config类可以删除不用了。