Why does the Eclipse project name display a red exclamation mark?

The project name is highlighted in red because the jar package is lost. Open buildpath and check the libearies. A red packet is displayed because the referenced package cannot be found. When you reference a jar package in a path on the hard disk, it can be developed normally. However, when this package is moved or deleted, eclipse cannot find it, so packet loss occurs.

Solution:

1. Remove the red package in buildpath and re-reference it in the project. (This may happen again)

2. create a new folder in the project and paste the jar package you need into this folder. In libearies in buildpath, click Add jars and find the newly created folder, select the jar package and click OK. In this way, the jar package you need is put into the project. References the project path. It's okay with the outside.