What is the advantage of the ‘src/main/java” convention?

Main benefit is in having the test directory as subdirectory of src with the same directory structure as the one in main:

• Project-A
  • bin
  • lib
  • src
    • main
      • java
        • RootLevelPackageClass.java
      • resources
    • test
      • java
        • TestRootLevelPackageClass.java
      • resources

All package private methods of RootLevelPackageClass will be visible, i.e. testable from TestRootLevelPackageClass. Since the testing code is also source its place should be under src directory.