Eclipse cannot automatically compile the project.

Eclipse does not compile the project. How to refresh the classes directory is empty.
All operations are performed:
Open project> build automatically.
Open the project --> build automatically. In addition, if you clean the project, the compilation may fail.
By default, automatic compilation is selected for eclipse. Even if you cancel Automatic compilation, you cannot manually build the project.

 

There are three reasons for this:
1. if a jar package is introduced in the project and you delete the package, no error will be reported but the class file cannot be compiled, you can delete all packages and then introduce them one by one (required). Do not introduce all packages at once. Useless Packages may cause adverse consequences.
2. You want to delete a class file and regenerate it. After deleting the class file, the folder under the classes directory is opened by other programs, such as total commander. At this time, the compilation will not pass. In problems, the system may prompt "Con't delete classes ......". Turn off other programs and re-compile them.

3. In other cases, remove the JRE system library and re-import it to compile. However, it is unclear why.