How to get AuditorAware to store CreatedBy as UUID from Spring Security?

I am trying to get the UUID from my logged in user to be stored as the "CreatedBy" and "LastUpdatedBy" instead of just the string. However, I am getting a cast error:

class java.lang.String cannot be cast to class com.example.lims.container.Container

All code below...thanks!

BIG EDIT:

I changed ApplicationUser to implement the UserDetails from SpringSecurity and added legit Mapped relationships to my BaseEntity. Now I get a new exception. I'm pretty sure it has to do with what I'm returning from SecurityAuditAware. I don't know how to get it to return the correct ApplicationUser instance.

java.lang.ClassCastException: class java.lang.String cannot be cast to class com.example.lims.user.ApplicationUser (java.lang.String is in module java.base of loader 'bootstrap'; me.tmpjr.lims.user.ApplicationUser is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @3d3e8a83) at com.example.lims.security.SecurityAuditorAware.getCurrentAuditor(SecurityAuditorAware.java:20) ~[classes/:na]

Schema:

CREATE TABLE IF NOT EXISTS "containers" (
    id uuid NOT NULL,
    container_name VARCHAR(100) NOT NULL,
    created_by UUID NOT NULL,
    created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
    updated_by UUID,
    updated_at TIMESTAMP WITHOUT TIME ZONE,
    PRIMARY KEY (id)
);

CREATE TABLE IF NOT EXISTS "users" (
    id uuid NOT NULL,
    username VARCHAR(100) NOT NULL,
    password VARCHAR(100) NOT NULL,
    created_by UUID,
    created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
    updated_by UUID,
    updated_at TIMESTAMP WITHOUT TIME ZONE,
    PRIMARY KEY (id)
);

BaseEntity AuditorAware:

@Data
@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)
public abstract class BaseEntity implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    protected UUID id;

    @CreatedDate
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "created_at")
    protected Date createdAt;

    @LastModifiedDate
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "updated_at")
    protected Date updatedAt;

    @CreatedBy
    @JoinColumn(name = "created_by", nullable = true, insertable = true, updatable = false)
    @ManyToOne
    protected ApplicationUser createdBy;

    @LastModifiedBy
    @JoinColumn(name = "updated_by", nullable = true, insertable = false, updatable = false)
    @ManyToOne
    protected ApplicationUser updatedBy;
}

ApplicationUser:

@Data
@EqualsAndHashCode(callSuper = true)
@Entity
@Valid
@Table(name = "users")
@NoArgsConstructor
public class ApplicationUser extends BaseEntity implements UserDetails 
{
    @Column(nullable = false)
    private String username;

    @Column(nullable = false)
    private String password;

    @Override
    public Collection<? extends GrantedAuthority> getAuthorities() {
        return null;
    }

    @Override
    public boolean isAccountNonExpired() {
        return false;
    }

    @Override
    public boolean isAccountNonLocked() {
        return false;
    }

    @Override
    public boolean isCredentialsNonExpired() {
        return false;
    }

    @Override
    public boolean isEnabled() {
        return false;
    }
}

Security AuditorAware Implementation that returns ApplicationUser:

@Component
public class SecurityAuditorAware implements AuditorAware<ApplicationUser> {
    public Optional<ApplicationUser> getCurrentAuditor() {
        Authentication auth = SecurityContextHolder.getContext().getAuthentication();

        if (auth == null || !auth.isAuthenticated()) {
            return Optional.empty();
        }
        // THIS is where I think it's going wrong.. what to return here? How to return ApplicationUser?
        return Optional.ofNullable((ApplicationUser) auth.getPrincipal());
    }
}

UserDetailsServiceImpl:

@RequiredArgsConstructor
@Service
public class UserDetailsServiceImpl implements UserDetailsService {
    private final ApplicationUserRepository userRepository;

    @Override
    public ApplicationUser loadUserByUsername(String username) throws UsernameNotFoundException {
        ApplicationUser applicationUser = userRepository.findByUsername(username);
        if (applicationUser == null) {
            throw new UsernameNotFoundException(username);
        }

        return applicationUser;
    }
}

Bean Config:

@Configuration
@EnableJpaAuditing(auditorAwareRef = "auditorAware")
public class PersistenceConfig {
    @Bean
    public AuditorAware<ApplicationUser> auditorAware() {
        return new SecurityAuditorAware();
    }
}

Full Exception:

java.lang.ClassCastException: class java.lang.String cannot be cast to class com.example.lims.user.ApplicationUser (java.lang.String is in module java.base of loader 'bootstrap'; com.example.lims.user.ApplicationUser is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @3d3e8a83)
    at com.example.lims.security.SecurityAuditorAware.getCurrentAuditor(SecurityAuditorAware.java:21) ~[classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na]

VALID WORKAROUND?

I believe I have found the issue. Since I'm using UsernamePasswordAuthenticationToken getPrinciple does not return a UserDetails object. I simply cannot get it to cast to my custom Entity class. However, if I manually user a repository search inside my SecurityAuditorAware and return it directly it works!

@Component
@RequiredArgsConstructor
public class SecurityAuditorAware implements AuditorAware<ApplicationUser> {
    private final ApplicationUserRepository userRepository;

    public Optional<ApplicationUser> getCurrentAuditor() {
        Authentication auth = SecurityContextHolder.getContext().getAuthentication();

        if (auth == null || !auth.isAuthenticated()) {
            return Optional.empty();
        }

        return Optional.ofNullable((ApplicationUser) userRepository.findByUsername((auth.getName())));
    }
}