The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build

eclipse好久之前做的java项目，今天打开发现许多jsp页面有如下错误：
[color=red][b][size=medium]The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path[/size][/b][/color]
[img]http://dl2.iteye.com/upload/attachment/0117/8844/ca057449-c441-3d03-a248-962a15e96bd2.png[/img]
如果没有添加Tomcat Server ，需手动添加一下（不过我已经添加过了）
project->Java Build Path->Libraries-> Add Libray->Server Runtime ->Tomcat Server
继续查找原因
Windows->Preferences->Server->Runtime Environments->add->Apache Tomcat v7.0->next 找到相应tomcat7.0的安装路径（这个我也添加过了）
不过在点击我已配置过的Tomcat Server，发现了下面信息：
[color=red][b][size=medium]The specified Tomcat installation directory does not exist.[/size][/b][/color]
[img]http://dl2.iteye.com/upload/attachment/0117/8850/af84bb9f-dc41-33cd-85fe-f9533b0e9d71.png[/img]
指定Tomcat的安装目录不存在
原来是换了电脑后，重新放置了Tomcat的安装位置，重新Edit下
还是不行？
把项目从eclipse里面删掉
[img]http://dl2.iteye.com/upload/attachment/0117/8874/d3067338-ef89-390a-9b40-efd7d08b09df.png[/img]
在把刚从eclipse里刚删的项目import回来
[img]http://dl2.iteye.com/upload/attachment/0117/8876/bc7a6d48-40bb-3250-976a-202da7fb054b.png[/img]
完美解决！