“rt.jar is not on its project's build path”

引用src.zip时、报“rt.jar is not on its project's build path”的解决方法
当通过引用src.zip想看JDK标准类的源码时、有时会报如下错误：
“an error occurred while opening the source attachment form ”
“jre\lib\rt.jar is not on its project's build path”
 
解决办法如下： 
1. Open project properties. 
2. Select Java Build Path node. 
3. Select Libraries tab. 
4. Remove JRE System Library 一次 
5. 再次 Add Library JRE System Library