为了实现多域名共享同一个Session实例，我们可以通过配置web.xml文件中的context-param和listener标签来完成。以下是一个具体的例子：

1. 在所有需要共享Session的域名的web.xml文件中，添加以下context-param和listener配置：

```xml

sessionCookieName

mySessionCookie

org.springframework.web.context.request.RequestContextListener

```

2. 在任意一个域名下的web.xml中，添加以下filter配置，用于设置跨域共享Session：

```xml

SessionSharingFilter

com.example.SessionSharingFilter

SessionSharingFilter

/*

```

3. 编写SessionSharingFilter类，实现跨域共享Session的逻辑：

```java

package com.example;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;

import java.io.IOException;

public class SessionSharingFilter implements Filter {

@Override

public void init(FilterConfig filterConfig) throws ServletException {

}

@Override

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

HttpServletRequest httpRequest = (HttpServletRequest) request;

HttpServletResponse httpResponse = (HttpServletResponse) response;

Cookie[] cookies = httpRequest.getCookies();

if (cookies != null) {

for (Cookie cookie : cookies) {

if ("