来源：tomcat如何共享多个web应用会话
作者：seaboat 汪洋之舟

问题

今天有位朋友问了个问题，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。

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);
        }

    }

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"); 

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;
        }
    }

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