The project was not built since its build path is incomplete解决方法

错误提示:The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project 

The type java.lang.Object cannot be resolved. It is indirectly referenced from required .class files 

今天在eclipse3.2+myeclipse5.1+tomcat5.5重新部署时出了这问题.搞了很久才找到原因.解决办法写出来分享:
出现以上错误的原因是居然是装jdk5时了多装了个jre。本来Eclipse在建立项目时，会自动参照你的jre路径，但多个版本就没办法加载了。 
解决办法：
1. 进入window \ preferences \ java \ Installed JREs 
1)按Add 
2)输入JRE Name, 例JDK1.5.0.09
3)JRE home directory, 选择安装的路径 
4)按OK 
2. 进入Project \ properties \ Java Bulid Path 
1)Add library 
2)选JRE System Library后按Next 
3)选workplace default JRE后按 finish... 

这样就行了