createSession: Too many active sessions？？？

createSession: Too many active sessions 
java.lang.IllegalStateException: createSession: Too many active sessions at org.apache.catalina.session.StandardManager.createSession(StandardManager.java:286) at org.apache.catalina.connector.Request.doGetSession(Request.java:2225) at org.apache.catalina.connector.Request.getSession(Request.java:2024) at org.apache.catalina.connector.RequestFacade.getSession(RequestFacade.java:831) at org.apache.catalina.connector.RequestFacade.getSession(RequestFacade.java:842) at qicaise.Test.doGet(Test.java:30) at javax.servlet.http.HttpServlet.service(HttpServlet.java:689) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) at filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:122) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148) at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:833) at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:639) at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1285) at java.lang.Thread.run(Unknown Source) 
网站打开时显示就是这个异常，好像是sessions太多了。
这种情况一般采用什么方式解决？有没有办法可以将当前我的网站上的所有session清空一下。 
在程序里控制。如果在程序里能不能得到当前我的网站上的所有session列表？