1. Check whether the jdk of the project is 8 or 11. Mine is 11, so you need to download jdk 11. Download and unzip it first.
2. Download Maven and modify the contents of the local warehouse in setting.xml in D:\Idea\maven\application\apache-maven-3.3.9\conf
3. If the project is not built as a Maven project at this time, you can modify the pom.xml. It was originally red, but after modifying the Add as Maven Project property selection in a position similar to this, the pom.xml becomes blue.
4. If there is no run tag in the startup class at this time, you can modify the properties of Sources in Modules in Project Structure and select it.
