java: jdk 8 
spring-boot: 2.1.6.RELEASE 

public class UsernamePasswordAuthenticationFilter extends
    AbstractAuthenticationProcessingFilter {
  ...
  public UsernamePasswordAuthenticationFilter() {
    super(new AntPathRequestMatcher("/login", "POST"));
  }
  ...
}

public interface Authentication extends Principal, Serializable {
  
  // 該principal具有的權限。AuthorityUtils工具類提供了一些方便的方法。
  Collection<? extends GrantedAuthority> getAuthorities();
  // 證明Principal的身份的證書，比如密碼。
  Object getCredentials();
  // authentication request的附加信息，比如ip。
  Object getDetails();
  // 當事人。在username+password模式中為username，在有userDetails之后可以為userDetails。
  Object getPrincipal();
  // 是否已經通過認證。
  boolean isAuthenticated();
  // 設置通過認證。
  void setAuthenticated(boolean isAuthenticated) throws IllegalArgumentException;
}

// 設置
SecurityContextHolder.getContext().setAuthentication(anAuthentication);
// 獲取
Authentication existingAuth = SecurityContextHolder.getContext()
        .getAuthentication();

public String getName() {
  if (this.getPrincipal() instanceof UserDetails) {
    return ((UserDetails) this.getPrincipal()).getUsername();
  }
  if (this.getPrincipal() instanceof AuthenticatedPrincipal) {
    return ((AuthenticatedPrincipal) this.getPrincipal()).getName();
  }
  if (this.getPrincipal() instanceof Principal) {
    return ((Principal) this.getPrincipal()).getName();
  }

  return (this.getPrincipal() == null) ? "" : this.getPrincipal().toString();
}

public interface AuthenticationManager {
  Authentication authenticate(Authentication authentication)
      throws AuthenticationException;
}

public class ProviderManager implements AuthenticationManager, MessageSourceAware,
    InitializingBean {
  ...

  public ProviderManager(List<AuthenticationProvider> providers) {
    this(providers, null);
  }

  public ProviderManager(List<AuthenticationProvider> providers,
      AuthenticationManager parent) {
    Assert.notNull(providers, "providers list cannot be null");
    this.providers = providers;
    this.parent = parent;
    checkState();
  }

  public Authentication authenticate(Authentication authentication)
      throws AuthenticationException {
    Class<? extends Authentication> toTest = authentication.getClass();
    AuthenticationException lastException = null;
    AuthenticationException parentException = null;
    Authentication result = null;
    Authentication parentResult = null;
    boolean debug = logger.isDebugEnabled();

    for (AuthenticationProvider provider : getProviders()) {
      // #1, 檢查是否由該認證部門進行認證`AuthenticationProvider`
      if (!provider.supports(toTest)) {
        continue;
      }

      if (debug) {
        logger.debug("Authentication attempt using "
            + provider.getClass().getName());
      }

      try {
        // #2, 認證部門進行認證
        result = provider.authenticate(authentication);

        if (result != null) {
          copyDetails(authentication, result);
          // #3，認證通過則不再進行下一個認證部門的認證，否則拋出的異常被捕獲，執(zhí)行下一個認證部門(AuthenticationProvider)
          break;
        }
      }
      catch (AccountStatusException e) {
        prepareException(e, authentication);
        // SEC-546: Avoid polling additional providers if auth failure is due to
        // invalid account status
        throw e;
      }
      catch (InternalAuthenticationServiceException e) {
        prepareException(e, authentication);
        throw e;
      }
      catch (AuthenticationException e) {
        lastException = e;
      }
    }

    if (result == null && parent != null) {
      // Allow the parent to try.
      try {
        result = parentResult = parent.authenticate(authentication);
      }
      catch (ProviderNotFoundException e) {
        // ignore as we will throw below if no other exception occurred prior to
        // calling parent and the parent
        // may throw ProviderNotFound even though a provider in the child already
        // handled the request
      }
      catch (AuthenticationException e) {
        lastException = parentException = e;
      }
    }
    // #4, 如果認證通過，執(zhí)行認證通過之后的操作
    if (result != null) {
      if (eraseCredentialsAfterAuthentication
          && (result instanceof CredentialsContainer)) {
        // Authentication is complete. Remove credentials and other secret data
        // from authentication
        ((CredentialsContainer) result).eraseCredentials();
      }

      // If the parent AuthenticationManager was attempted and successful than it will publish an AuthenticationSuccessEvent
      // This check prevents a duplicate AuthenticationSuccessEvent if the parent AuthenticationManager already published it
      if (parentResult == null) {
        eventPublisher.publishAuthenticationSuccess(result);
      }
      return result;
    }

    // Parent was null, or didn't authenticate (or throw an exception).
    // #5，如果認證不通過，必然有拋出異常，否則表示沒有配置相應的認證部門(AuthenticationProvider)
    if (lastException == null) {
      lastException = new ProviderNotFoundException(messages.getMessage(
          "ProviderManager.providerNotFound",
          new Object[] { toTest.getName() },
          "No AuthenticationProvider found for {0}"));
    }

    // If the parent AuthenticationManager was attempted and failed than it will publish an AbstractAuthenticationFailureEvent
    // This check prevents a duplicate AbstractAuthenticationFailureEvent if the parent AuthenticationManager already published it
    if (parentException == null) {
      prepareException(lastException, authentication);
    }

    throw lastException;
  }
  ...
}

public interface AuthenticationProvider {
  // 進行認證
  Authentication authenticate(Authentication authentication)
      throws AuthenticationException;
  // 是否由該AuthenticationProvider進行認證
  boolean supports(Class<?> authentication);
}

// 獲取 UserDetails
protected abstract UserDetails retrieveUser(String username,
    UsernamePasswordAuthenticationToken authentication)
    throws AuthenticationException;

protected abstract void additionalAuthenticationChecks(UserDetails userDetails,
    UsernamePasswordAuthenticationToken authentication)
    throws AuthenticationException;

public interface UserDetails extends Serializable {
  
  Collection<? extends GrantedAuthority> getAuthorities();

  String getPassword();
  
  String getUsername();
  // 賬號是否過期
  boolean isAccountNonExpired();
  // 賬號是否被鎖
  boolean isAccountNonLocked();
  // 證書(password)是否過期
  boolean isCredentialsNonExpired();
  // 賬號是否可用
  boolean isEnabled();
}

protected Authentication createSuccessAuthentication(Object principal,
    Authentication authentication, UserDetails user) {
  // Ensure we return the original credentials the user supplied,
  // so subsequent attempts are successful even with encoded passwords.
  // Also ensure we return the original getDetails(), so that future
  // authentication events after cache expiry contain the details
  UsernamePasswordAuthenticationToken result = new UsernamePasswordAuthenticationToken(
      principal, authentication.getCredentials(),
      authoritiesMapper.mapAuthorities(user.getAuthorities()));
  result.setDetails(authentication.getDetails());

  return result;
}

// 檢查密碼是否正確
protected void additionalAuthenticationChecks(UserDetails userDetails,
    UsernamePasswordAuthenticationToken authentication)
    throws AuthenticationException {
  if (authentication.getCredentials() == null) {
    logger.debug("Authentication failed: no credentials provided");

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

  String presentedPassword = authentication.getCredentials().toString();

  if (!passwordEncoder.matches(presentedPassword, userDetails.getPassword())) {
    logger.debug("Authentication failed: password does not match stored value");

    throw new BadCredentialsException(messages.getMessage(
        "AbstractUserDetailsAuthenticationProvider.badCredentials",
        "Bad credentials"));
  }
}
// 通過資料室(UserDetailsService)獲取UserDetails對象
protected final UserDetails retrieveUser(String username,
    UsernamePasswordAuthenticationToken authentication)
    throws AuthenticationException {
  prepareTimingAttackProtection();
  try {
    UserDetails loadedUser = this.getUserDetailsService().loadUserByUsername(username);
    if (loadedUser == null) {
      throw new InternalAuthenticationServiceException(
          "UserDetailsService returned null, which is an interface contract violation");
    }
    return loadedUser;
  }
  ...
}

public interface UserDetailsService {
  UserDetails loadUserByUsername(String username) throws UsernameNotFoundException;
}

public interface UserDetailsManager extends UserDetailsService {
  void createUser(UserDetails user);

  void updateUser(UserDetails user);

  void deleteUser(String username);

  void changePassword(String oldPassword, String newPassword);

  boolean userExists(String username);
}