Creating a src/main/java folder structure in Eclipse without Maven
As you've discovered, you can't have a source folder inside another source folder, so before you can create src/main/java you have to tell Eclipse not to treat src as a source folder. To do this, right click on the src folder and select build path -> remove from build path.
Once you have done this src will appear in the folder tree in its normal place, so you can create the main and java folders under it, and move the existing src/com to src/main/java/com. Finally right-click the newly-created java folder and select build path -> use as source folder.
I have solved this issue by below steps:
Right click the Maven Project -> Build Path -> Configure Build PathIn Order and Export tab, you can see the message like '2 build path entries are missing'Now select 'JRE System Library' and 'Maven Dependencies' checkboxClick OK
Open Properties > Java Build Path for the project. Select the Source tab and use Add Folders to add source folders.