Java cannot find symbol solution

1. The project keeps reporting that the symbol cannot be found, resulting in failure to start.

2. Solution 1
: Clear the idea cache,


recompile, package and run.

3. Solution 2:
Delete the following files in the project folder

and reopen the maven clean package.

4. Solution 3
   It may be that the jdk version used when maven is compiled is incorrect, and it needs to be added to the pom file:
   the location is build -> plugins

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <source>11</source>
        <target>11</target>
    </configuration>
</plugin>