When there is no error reported in the project and there is a red cross on the project name, the modification at this time:

    Reason: When using Eclipse to create a web project with a Maven structure, the Artifact Id was selected as maven-artchetype-webapp. Because this catalog is relatively old, the servlet used is still 2.3, and generally it is at least 2.5 now. Modify
    Dynamic in Project Facets When the web module is 2.5, Cannot change version of project facet Dynamic web module to 2.5 will appear

    . 1. Right-click the project, ----"properties----"project facets-----> put the Dynamic Web Module The version was changed to 2.5.
    2. Open the web.xml of the project, change the Servlet to 2.5
    3. Modify the settings of the project, open the org.eclipse.jdt.core.prefs in the project.settings directory under show view --->Navigator, and check the version number .
    4. Open org.eclipse.wst.common.component and check the version number
    5. Open org.eclipse.wst.common.project.facet.core.xml and modify the java version to <installed facet="java" version=" 1.8" /> is 1.8
    6. Okay, the preparation work is completed, open it again to see if the Dynamic web module has been changed to 2.5