Message Title

@Entity
public class File {
	
	@Id
	@GeneratedValue(strategy=GenerationType.IDENTITY)
	private Long id;
	
	@ManyToOne(fetch=FetchType.LAZY)
	private Folder folder;
	
	public Long getId() {
		return id;
	}

	public Folder getFolder() {
		return folder;
	}
	
	public void setFolder(Folder route) {
		this.folder = route;
	}
}

@Entity
public class Folder {

	@Id
	@GeneratedValue(strategy=GenerationType.IDENTITY)
	private Long id;
	
	@OneToMany(fetch = FetchType.LAZY, mappedBy = "folder")
	private List<File> folders = new ArrayList<>();
	
	public Long getId() {
		return id;
	}
	
	public List<File> getATMs() {
		return Collections.unmodifiableList(folders);
	}
}

@Entity
public class FolderEx extends Folder {

	private String description;
	
	public void setDescription(String description) {
		this.description = description;
	}
	
	public String getDescription() {
		return description;
	}
}

...
FolderEx folder = new FolderEx();
folder.setDescription("Description");
em.persist(folder);
		
File file = new File();
file.setFolder(folder);
em.persist(file);
...

After that, if we get File entity by em.find method and get Folder entity by file.getFolder() method, than we can't access to FolderEx.getDescription() method, even with cast:

File file = em.find(File.class, fileId);
Folder folder = file.getFolder();
FolderEx folderEx = (FolderEx) folder; // ClassCastException!