Object principal = SecurityContextHolder.getContext().getAuthentication().getPrincipal();

if (principal instanceof UserDetails) {
String username = ((UserDetails)principal).getUsername();
} else {
String username = principal.toString();
}

public interface Authentication extends Principal, Serializable {
 Collection<? extends GrantedAuthority> getAuthorities();
 /**
 * 通常是密碼
 */
 Object getCredentials();
 /**
 * Stores additional details about the authentication request. These might be an IP
 * address, certificate serial number etc.
 */
 Object getDetails();

 /**
 * 用來(lái)標(biāo)識(shí)是否已認(rèn)證，如果使用用戶名和密碼登錄,通常是用戶名 
 */
 Object getPrincipal();
 /**
 * 是否已認(rèn)證
 */
 boolean isAuthenticated();
 void setAuthenticated(boolean isAuthenticated) throws IllegalArgumentException;
}

public abstract class AbstractAuthenticationToken implements Authentication,
 CredentialsContainer {
 }
public class UsernamePasswordAuthenticationToken extends AbstractAuthenticationToken {
}

 UsernamePasswordAuthenticationToken authenticationToken =
 new UsernamePasswordAuthenticationToken(loginVM.getUsername(), loginVM.getPassword());

Authentication authentication = this.authenticationManager.authenticate(authenticationToken);
SecurityContextHolder.getContext().setAuthentication(authentication);

public interface UserDetails extends Serializable {
 /**
 * 用戶的授權(quán)信息，可以理解為角色
 */
 Collection<? extends GrantedAuthority> getAuthorities();
 /**
 * 用戶密碼
 *
 * @return the password
 */
 String getPassword();
 /**
 * 用戶名 
 * */
 String getUsername();
 boolean isAccountNonExpired();
 boolean isAccountNonLocked();
 boolean isCredentialsNonExpired();
 boolean isEnabled();
}

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

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

public interface AuthenticationProvider {
 Authentication authenticate(Authentication authentication)
 throws AuthenticationException;
 boolean supports(Class<?> authentication);
}

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

 for (AuthenticationProvider provider : getProviders()) {
 if (!provider.supports(toTest)) {
 continue;
 }

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

 try {
 result = provider.authenticate(authentication);

 if (result != null) {
  copyDetails(authentication, result);
  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 = 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 = e;
 }
 }

 if (result != null) {
 if (eraseCredentialsAfterAuthentication
  && (result instanceof CredentialsContainer)) {
 // Authentication is complete. Remove credentials and other secret data
 // from authentication
 ((CredentialsContainer) result).eraseCredentials();
 }
 eventPublisher.publishAuthenticationSuccess(result);
 return result;
 }

 // Parent was null, or didn't authenticate (or throw an exception).
 if (lastException == null) {
 lastException = new ProviderNotFoundException(messages.getMessage(
  "ProviderManager.providerNotFound",
  new Object[] { toTest.getName() },
  "No AuthenticationProvider found for {0}"));
 }
 prepareException(lastException, authentication);
 throw lastException;
 }

public Authentication authenticate(Authentication authentication)
 throws AuthenticationException {
 // 必須是UsernamePasswordAuthenticationToken
 Assert.isInstanceOf(UsernamePasswordAuthenticationToken.class, authentication,
 messages.getMessage(
  "AbstractUserDetailsAuthenticationProvider.onlySupports",
  "Only UsernamePasswordAuthenticationToken is supported"));

 // 獲取用戶名
 String username = (authentication.getPrincipal() == null) ? "NONE_PROVIDED"
 : authentication.getName();

 boolean cacheWasUsed = true;
 // 從緩存獲取
 UserDetails user = this.userCache.getUserFromCache(username);

 if (user == null) {
 cacheWasUsed = false;

 try {
 // retrieveUser 抽象方法，獲取用戶
 user = retrieveUser(username,
  (UsernamePasswordAuthenticationToken) authentication);
 }
 catch (UsernameNotFoundException notFound) {
 logger.debug("User '" + username + "' not found");

 if (hideUserNotFoundExceptions) {
  throw new BadCredentialsException(messages.getMessage(
  "AbstractUserDetailsAuthenticationProvider.badCredentials",
  "Bad credentials"));
 }
 else {
  throw notFound;
 }
 }
 
 Assert.notNull(user,
  "retrieveUser returned null - a violation of the interface contract");
 }

 try {
 // 預(yù)先檢查，DefaultPreAuthenticationChecks，檢查用戶是否被lock或者賬號(hào)是否可用
 preAuthenticationChecks.check(user);
 
 // 抽象方法，自定義檢驗(yàn)
 additionalAuthenticationChecks(user,
  (UsernamePasswordAuthenticationToken) authentication);
 }
 catch (AuthenticationException exception) {
 if (cacheWasUsed) {
 // There was a problem, so try again after checking
 // we're using latest data (i.e. not from the cache)
 cacheWasUsed = false;
 user = retrieveUser(username,
  (UsernamePasswordAuthenticationToken) authentication);
 preAuthenticationChecks.check(user);
 additionalAuthenticationChecks(user,
  (UsernamePasswordAuthenticationToken) authentication);
 }
 else {
 throw exception;
 }
 }
 
 // 后置檢查 DefaultPostAuthenticationChecks，檢查isCredentialsNonExpired
 postAuthenticationChecks.check(user);
 if (!cacheWasUsed) {
 this.userCache.putUserInCache(user);
 }

 Object principalToReturn = user;
 if (forcePrincipalAsString) {
 principalToReturn = user.getUsername();
 }
 
 return createSuccessAuthentication(principalToReturn, authentication, user);
 }

private UserDetailsService userDetailsService;
private PasswordEncoder passwordEncoder;

protected final UserDetails retrieveUser(String username,
 UsernamePasswordAuthenticationToken authentication)
 throws AuthenticationException {
 UserDetails loadedUser;

 try {
 loadedUser = this.getUserDetailsService().loadUserByUsername(username);
 }
 catch (UsernameNotFoundException notFound) {
 if (authentication.getCredentials() != null) {
 String presentedPassword = authentication.getCredentials().toString();
 passwordEncoder.isPasswordValid(userNotFoundEncodedPassword,
  presentedPassword, null);
 }
 throw notFound;
 }
 catch (Exception repositoryProblem) {
 throw new InternalAuthenticationServiceException(
  repositoryProblem.getMessage(), repositoryProblem);
 }

 if (loadedUser == null) {
 throw new InternalAuthenticationServiceException(
  "UserDetailsService returned null, which is an interface contract violation");
 }
 return loadedUser;
 }

protected void additionalAuthenticationChecks(UserDetails userDetails,
 UsernamePasswordAuthenticationToken authentication)
 throws AuthenticationException {
 Object salt = null;

 if (this.saltSource != null) {
 salt = this.saltSource.getSalt(userDetails);
 }

 if (authentication.getCredentials() == null) {
 logger.debug("Authentication failed: no credentials provided");

 throw new BadCredentialsException(messages.getMessage(
  "AbstractUserDetailsAuthenticationProvider.badCredentials",
  "Bad credentials"));
 }
 // 獲取用戶密碼
 String presentedPassword = authentication.getCredentials().toString();
 // 比較passwordEncoder后的密碼是否和userdetails的密碼一致
 if (!passwordEncoder.isPasswordValid(userDetails.getPassword(),
 presentedPassword, salt)) {
 logger.debug("Authentication failed: password does not match stored value");

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

@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(prePostEnabled = true, securedEnabled = true)
public class ApplicationSecurity extends WebSecurityConfigurerAdapter {
 public SecurityConfiguration(AuthenticationManagerBuilder authenticationManagerBuilder, UserDetailsService userDetailsService,TokenProvider tokenProvider,CorsFilter corsFilter, SecurityProblemSupport problemSupport) {
 this.authenticationManagerBuilder = authenticationManagerBuilder;
 this.userDetailsService = userDetailsService;
 this.tokenProvider = tokenProvider;
 this.corsFilter = corsFilter;
 this.problemSupport = problemSupport;
 }

 @PostConstruct
 public void init() {
 try {
 authenticationManagerBuilder
 .userDetailsService(userDetailsService)
 .passwordEncoder(passwordEncoder());
 } catch (Exception e) {
 throw new BeanInitializationException("Security configuration failed", e);
 }
 }

 @Override
 protected void configure(HttpSecurity http) throws Exception {
 http
 .addFilterBefore(corsFilter, UsernamePasswordAuthenticationFilter.class)
 .exceptionHandling()
 .authenticationEntryPoint(problemSupport)
 .accessDeniedHandler(problemSupport)
 .and()
 .csrf()
 .disable()
 .headers()
 .frameOptions()
 .disable()
 .and()
 .sessionManagement()
 .sessionCreationPolicy(SessionCreationPolicy.STATELESS)
 .and()
 .authorizeRequests()
 .antMatchers("/api/register").permitAll()
 .antMatchers("/api/activate").permitAll()
 .antMatchers("/api/authenticate").permitAll()
 .antMatchers("/api/account/reset-password/init").permitAll()
 .antMatchers("/api/account/reset-password/finish").permitAll()
 .antMatchers("/api/profile-info").permitAll()
 .antMatchers("/api/**").authenticated()
 .antMatchers("/management/health").permitAll()
 .antMatchers("/management/**").hasAuthority(AuthoritiesConstants.ADMIN)
 .antMatchers("/v2/api-docs/**").permitAll()
 .antMatchers("/swagger-resources/configuration/ui").permitAll()
 .antMatchers("/swagger-ui/index.html").hasAuthority(AuthoritiesConstants.ADMIN)
 .and()
 .apply(securityConfigurerAdapter());
 }
}

public void decide(Authentication authentication, Object object,
 Collection<ConfigAttribute> configAttributes) throws AccessDeniedException {
 int deny = 0;
 // 遍歷DecisionVoter 
 for (AccessDecisionVoter voter : getDecisionVoters()) {
 // 投票
 int result = voter.vote(authentication, object, configAttributes);

 if (logger.isDebugEnabled()) {
 logger.debug("Voter: " + voter + ", returned: " + result);
 }

 switch (result) {
 case AccessDecisionVoter.ACCESS_GRANTED:
 return;

 case AccessDecisionVoter.ACCESS_DENIED:
 deny++;

 break;

 default:
 break;
 }
 }
 
 // 一票否決
 if (deny > 0) {
 throw new AccessDeniedException(messages.getMessage(
  "AbstractAccessDecisionManager.accessDenied", "Access is denied"));
 }

 // To get this far, every AccessDecisionVoter abstained
 checkAllowIfAllAbstainDecisions();
 }

boolean supports(ConfigAttribute attribute);
boolean supports(Class<?> clazz);
int vote(Authentication authentication, S object,
 Collection<ConfigAttribute> attributes);

public boolean supports(ConfigAttribute attribute) {
 if ((attribute.getAttribute() != null)
 && attribute.getAttribute().startsWith(getRolePrefix())) {
 return true;
 }
 else {
 return false;
 }
 }
 
public boolean supports(Class<?> clazz) {
 return true;
 }

public int vote(Authentication authentication, Object object,
 Collection<ConfigAttribute> attributes) {
 if(authentication == null) {
 return ACCESS_DENIED;
 }
 int result = ACCESS_ABSTAIN;
 
 // 獲取GrantedAuthority信息
 Collection<? extends GrantedAuthority> authorities = extractAuthorities(authentication);

 for (ConfigAttribute attribute : attributes) {
 if (this.supports(attribute)) {
 // 默認(rèn)拒絕訪問(wèn)
 result = ACCESS_DENIED;

 // Attempt to find a matching granted authority
 for (GrantedAuthority authority : authorities) {
  // 判斷是否有匹配的 authority
  if (attribute.getAttribute().equals(authority.getAuthority())) {
  // 可訪問(wèn)
  return ACCESS_GRANTED;
  }
 }
 }
 }
 return result;
 }

public void doFilter(ServletRequest request, ServletResponse response,
  FilterChain chain) throws IOException, ServletException {
 boolean clearContext = request.getAttribute(FILTER_APPLIED) == null;
 if (clearContext) {
  try {
  request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
  doFilterInternal(request, response, chain);
  }
  finally {
  SecurityContextHolder.clearContext();
  request.removeAttribute(FILTER_APPLIED);
  }
 }
 else {
  doFilterInternal(request, response, chain);
 }
 }

 private void doFilterInternal(ServletRequest request, ServletResponse response,
  FilterChain chain) throws IOException, ServletException {

 FirewalledRequest fwRequest = firewall
  .getFirewalledRequest((HttpServletRequest) request);
 HttpServletResponse fwResponse = firewall
  .getFirewalledResponse((HttpServletResponse) response);

 List<Filter> filters = getFilters(fwRequest);

 if (filters == null || filters.size() == 0) {
  if (logger.isDebugEnabled()) {
  logger.debug(UrlUtils.buildRequestUrl(fwRequest)
   + (filters == null ? " has no matching filters"
    : " has an empty filter list"));
  }

  fwRequest.reset();

  chain.doFilter(fwRequest, fwResponse);

  return;
 }

 VirtualFilterChain vfc = new VirtualFilterChain(fwRequest, chain, filters);
 vfc.doFilter(fwRequest, fwResponse);
 }
 
 private static class VirtualFilterChain implements FilterChain {
 private final FilterChain originalChain;
 private final List<Filter> additionalFilters;
 private final FirewalledRequest firewalledRequest;
 private final int size;
 private int currentPosition = 0;

 private VirtualFilterChain(FirewalledRequest firewalledRequest,
  FilterChain chain, List<Filter> additionalFilters) {
  this.originalChain = chain;
  this.additionalFilters = additionalFilters;
  this.size = additionalFilters.size();
  this.firewalledRequest = firewalledRequest;
 }

 public void doFilter(ServletRequest request, ServletResponse response)
  throws IOException, ServletException {
  if (currentPosition == size) {
  if (logger.isDebugEnabled()) {
   logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
    + " reached end of additional filter chain; proceeding with original chain");
  }

  // Deactivate path stripping as we exit the security filter chain
  this.firewalledRequest.reset();

  originalChain.doFilter(request, response);
  }
  else {
  currentPosition++;

  Filter nextFilter = additionalFilters.get(currentPosition - 1);

  if (logger.isDebugEnabled()) {
   logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
    + " at position " + currentPosition + " of " + size
    + " in additional filter chain; firing Filter: '"
    + nextFilter.getClass().getSimpleName() + "'");
  }

  nextFilter.doFilter(request, response, this);
  }
 }
 }