The default implementation of AuthenticationManager is ProviderManager:

public class ProviderManager implements AuthenticationManager

A ProviderManager typically depends on a number of list of authentication providers to check for authentication. It is sufficient if one of the authentication providers returns a successful authentication.

The following code shows an extract from ProviderManager:

for (AuthenticationProvider provider: getProviders()) {
    result = provider.authenticate(authentication);
    if (result != null) {
        copyDetails(authentication, result);
        break;
    }
}