这是文件系统不同步的问题，是因为在eclipse之外对工程中的resource进行修改引起的；但是，有时没有在eclipse之外进行修改，也会报类似的错误。
解决办法：需要手动刷新一下资源管理器。
（1）在eclipse中，工程目录右键，选择F5（refresh）
（2）设置eclipse自动刷新。
通过Window->Preferences->General->Workspace，选中Refresh automatically
eclipse中的项目无法查看源文件，报错：resource is out of sync with the file system。唯一的问题，在eclipse工程之外的地方修改了源文件但是没有刷新。MyEclipse自带帮助文档：
Don't forget to refresh!
Whenever you use external tools to modify workspace files, you must perform a Refresh from within Eclipse to make the workspace aware of the changes.If you get a resource out of sync error in Eclipse it is a sign that there are resources in Eclipse that have been modified outside of Eclipse.
One solution is to perform a refresh (available from a resource's popup menu) on any resources or projects that where modified outside of Eclipse.
There is also a preference to refresh automatically. 但是设置为自动刷新在工程比较大时代价有点大！ Window->Preferences->General->Workspace，选中Refresh automatically。