How to find out the currently logged-in user in Spring Boot?

Since Spring Security 3.2 you can get currently logged in user (your implementation of UserDetails) by adding a parameter inside your controller method:

import org.springframework.security.web.bind.annotation.AuthenticationPrincipal;

@RequestMapping("/resource")
public Map<String, Object> home(@AuthenticationPrincipal User user) {
   ..
}

Replace User with the name of your class which implements UserDetails interface.

Edit:

Since Spring Security 4.0 annotation was moved to a different package:

import org.springframework.security.core.annotation.AuthenticationPrincipal;

Addendum:

This will work even in WebFlux reactive environment versus the SecurityContextHolder.getContext().getAuthentication() which won't work because of paradigm shift from thread per request model to multiple requests per thread.

As per request:

Spring Boot which uses Spring Security internally provides a SecurityContextHolder class which allows the lookup of the currently authenticated user via:

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

The authentication instance now provides the following methods:

• Get the username of the logged in user: getPrincipal()
• Get the password of the authenticated user: getCredentials()
• Get the assigned roles of the authenticated user: getAuthorities()
• Get further details of the authenticated user: getDetails()