@AuthenticationPrincipal null

CustomService 에서 반환하는 Class 인 User 클래스로 @AuthenticationPrincipal  어노테이션에 들어온다는 정보가 많은데, 그것은 스프링 기본인 AbstractUserDetailAuthenticationProvider 를 사용하면 UserDetailService 에서 반환한 객체를 principal 로 세팅하기 때문이고, 나는 Provider 를 직접 생성했고 principal 에 username (이메일) 정보만 꺼내서 넣었기 때문에 String 으로 값이 들어온다.

@AuthenticationPrincipal  필드 타입을 잘못 지정하면 null 이 들어온다.

CustomUserDetail.class

//AbstractUserDetailAuthenticationProvider.class
public Authentication authenticate(Authentication authentication) throws AuthenticationException {
        Assert.isInstanceOf(UsernamePasswordAuthenticationToken.class, authentication, () -> {
            return this.messages.getMessage("AbstractUserDetailsAuthenticationProvider.onlySupports", "Only UsernamePasswordAuthenticationToken is supported");
        });
        String username = this.determineUsername(authentication);
        boolean cacheWasUsed = true;
        UserDetails user = this.userCache.getUserFromCache(username);
        if (user == null) {
            cacheWasUsed = false;

            try {
                user = this.retrieveUser(username, (UsernamePasswordAuthenticationToken)authentication);
            } catch (UsernameNotFoundException var6) {
                this.logger.debug("Failed to find user '" + username + "'");
                if (!this.hideUserNotFoundExceptions) {
                    throw var6;
                }

                throw new BadCredentialsException(this.messages.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", "Bad credentials"));
            }

            Assert.notNull(user, "retrieveUser returned null - a violation of the interface contract");
        }

        try {
            this.preAuthenticationChecks.check(user);
            this.additionalAuthenticationChecks(user, (UsernamePasswordAuthenticationToken)authentication);
        } catch (AuthenticationException var7) {
            if (!cacheWasUsed) {
                throw var7;
            }

            cacheWasUsed = false;
            user = this.retrieveUser(username, (UsernamePasswordAuthenticationToken)authentication);
            this.preAuthenticationChecks.check(user);
            this.additionalAuthenticationChecks(user, (UsernamePasswordAuthenticationToken)authentication);
        }

        this.postAuthenticationChecks.check(user);
        if (!cacheWasUsed) {
            this.userCache.putUserInCache(user);
        }

        Object principalToReturn = user;
        if (this.forcePrincipalAsString) {
            principalToReturn = user.getUsername();
        }

        return this.createSuccessAuthentication(principalToReturn, authentication, user);
    }
    
protected Authentication createSuccessAuthentication(Object principal, Authentication authentication, UserDetails user) {
    UsernamePasswordAuthenticationToken result = UsernamePasswordAuthenticationToken.authenticated(principal, authentication.getCredentials(), this.authoritiesMapper.mapAuthorities(user.getAuthorities()));
    result.setDetails(authentication.getDetails());
    this.logger.debug("Authenticated user");
    return result;
}

//UsernamePasswordAuthenticationToken.class
public static UsernamePasswordAuthenticationToken authenticated(Object principal, Object credentials,
      Collection<? extends GrantedAuthority> authorities) {
   return new UsernamePasswordAuthenticationToken(principal, credentials, authorities);
}