Red exclamation mark in front of Java project icon

Sometimes a red exclamation mark appears in front of the Java Project icon because the jar package introduced by the project does not exist (or is deleted, moved).

Right-click on the project, select Build Path->configure build Path, the popup window can be seen with the Red Fork jar package is a nonexistent jar package.

The solution is to select the file, then click on the right of the Edit button to re-select the correct path for the file, determine the back of the jar package before the Red Fork will not exist, the project before the exclamation mark has disappeared.