web应用会话之多个tomcat如何实现共享

面试问题

今天面试被问到了一个关于Tomcat之间如何实现信息共享的问题，大致是：tomcat下两个Java web，一个是商城，一个是直播，从商城登录后，再跳转到直播，发现处于非登录状态。

想到的解决思路

1. 将session抽出来成一个session服务，统一通过该服务操作session。
2. tomcat内部用会话管理器获取会话时遍历所有context内的会话。

方案1

重写获取session方法即可。

方案2

找了源码发现已经支持类似遍历所有context内的会话的形式，首先获取session时，如果cressContext属性为true，则会在获取不到时尝试遍历所有context是否存在该sessionid，如果存在则在本context根据sessionid创建自己的session对象。

 public HttpSession getSession(boolean create) {        if (crossContext) {            // There cannot be a session if no context has been assigned yet            if (context == null)                return (null);            // Return the current session if it exists and is valid            if (session != null && session.isValid()) {                return (session.getSession());            }            HttpSession other = super.getSession(false);            if (create && (other == null)) {                // First create a session in the first context: the problem is                // that the top level request is the only one which can                 // create the cookie safely                other = super.getSession(true);            }            if (other != null) {                Session localSession = null;                try {                    localSession =                        context.getManager().findSession(other.getId());                    if (localSession != null && !localSession.isValid()) {                        localSession = null;                    }                } catch (IOException e) {                    // Ignore                }                if (localSession == null && create) {                    localSession =                         context.getManager().createSession(other.getId());                }                if (localSession != null) {                    localSession.access();                    session = localSession;                    return session.getSession();                }            }            return null;        } else {            return super.getSession(create);        }    }
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context（web应用）获取跨应用session时通过类似下面操作获取：

request.getSession().getServletContext().getContext("/app2").getAttribute("att2"); 
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这是因为request会根据cookies的sessionid获取到session对象，这时不会报找不到，因为前面已经根据其他sessionid创建了一个session对象，然后getContext操作会获取对应url的context，接着进行会话操作。

public ServletContext getContext(String uri) {        // Validate the format of the specified argument        if (uri == null || !uri.startsWith("/")) {            return null;        }        Context child = null;        try {            // Look for an exact match            Container host = context.getParent();            child = (Context) host.findChild(uri);            // Non-running contexts should be ignored.            if (child != null && !child.getState().isAvailable()) {                child = null;            }            // Remove any version information and use the mapper            if (child == null) {                int i = uri.indexOf("##");                if (i > -1) {                    uri = uri.substring(0, i);                }                // Note: This could be more efficient with a dedicated Mapper                //       method but such an implementation would require some                //       refactoring of the Mapper to avoid copy/paste of                //       existing code.                MessageBytes hostMB = MessageBytes.newInstance();                hostMB.setString(host.getName());                MessageBytes pathMB = MessageBytes.newInstance();                pathMB.setString(uri);                MappingData mappingData = new MappingData();                ((Engine) host.getParent()).getService().findConnectors()[0].getMapper().map(                        hostMB, pathMB, null, mappingData);                child = (Context) mappingData.context;            }        } catch (Throwable t) {            ExceptionUtils.handleThrowable(t);            return null;        }        if (child == null) {            return null;        }        if (context.getCrossContext()) {            // If crossContext is enabled, can always return the context            return child.getServletContext();        } else if (child == context) {            // Can still return the current context            return context.getServletContext();        } else {            // Nothing to return            return null;        }    }