This error occurs for a long time, and today all of a sudden want to see how, see https://blog.csdn.net/hongchangfirst/article/details/7663287 , thumbs up!
1、原因：因为java compiler level does not match the version of the installed java，
Solution: So we open the properties of the project, there is an option for the project facets, click Open, select the latest version of Java on the right, and then apply, should be ok.
2 reasons: because the referenced jar package caused invalid
Solution: This situation usually occurs on the project is a red exclamation mark, the general practice is to clean up all references to the jar, add it again.
3, see above Dynamic Web Module, sometimes After importing the project, wants to put the project in tomcat, but found not find, a few times because they did not set this caused.
