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

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()

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.

You can simply use HttpServletRequest also to get user principle,

using HttpServletRequest request,

String user=request.getUserPrincipal().getName();

One way is to add java.security.Principal as a parameter as follows:

@RequestMapping("/resource")
public Map<String, Object> home(Principal principal) {
    Map<String, Object> model = new HashMap<String, Object>();
    model.put("id", UUID.randomUUID().toString());
    model.put("content", "Hello " + principal.getName());
    return model;
}