IDEA opens the project, there is a J solution in the lower left corner of the java file in the project

When the idea opens the project, there is a red j in the lower left corner of the java file, and the file cannot be compiled at this time.
Solution: Open the project structure in File and find the Modules, among which there are Sources, click the first X on the right, remove the root path, and add it again
as shown below:

Last thanks: https://blog.csdn.net/mys_mys/article/details/82465540