Oauth2 get Username from token

You are using Spring Security for authentication.

You can get User Detail from SecurityContext

Authentication authentication = SecurityContextHolder.getContext()
    .getAuthentication();

UserDetails userDetail = authentication.getPrincipal();
userDetail.getUsername();

or in Rest Controller

@RequestMapping(value = "/username", method = RequestMethod.GET)
public String currentUserName(Principal principal) {
    return principal.getName();
}

or

@RequestMapping(value = "/username", method = RequestMethod.GET)
public String currentUserName(HttpServletRequest request) {
    Principal principal = request.getUserPrincipal();

    return principal.getName();
}