图解Spring Security 中用户是如何实现登录的

public class UsernamePasswordAuthenticationFilter extends
  AbstractAuthenticationProcessingFilter {
 // 默认取账户名、密码的key
	public static final String SPRING_SECURITY_FORM_USERNAME_KEY = "username";
	public static final String SPRING_SECURITY_FORM_PASSWORD_KEY = "password";
 // 可以通过对应的set方法修改
	private String usernameParameter = SPRING_SECURITY_FORM_USERNAME_KEY;
	private String passwordParameter = SPRING_SECURITY_FORM_PASSWORD_KEY;
 // 默认只支持 POST 请求
	private boolean postOnly = true;
 
 // 初始化一个用户密码 认证过滤器 默认的登录uri 是 /login 请求方式是POST
 public UsernamePasswordAuthenticationFilter() {
  super(new AntPathRequestMatcher("/login", "POST"));
 }

 // 实现其父类 AbstractAuthenticationProcessingFilter 提供的钩子方法 用去尝试认证
 public Authentication attemptAuthentication(HttpServletRequest request,
   HttpServletResponse response) throws AuthenticationException {
  // 判断请求方式是否是POST
  if (postOnly && !request.getMethod().equals("POST")) {
   throw new AuthenticationServiceException(
    "Authentication method not supported: " + request.getMethod());
  }
  
  // 先去 HttpServletRequest 对象中获取账号名、密码
  String username = obtainUsername(request);
  String password = obtainPassword(request);

  if (username == null) {
   username = "";
  }

  if (password == null) {
   password = "";
  }

  username = username.trim();

  // 然后把账号名、密码封装到 一个认证Token对象中，这是就是一个通行证，但是这时的状态时不可信的，一旦通过认证就变为可信的
  UsernamePasswordAuthenticationToken authRequest = new UsernamePasswordAuthenticationToken(
   username, password);

  // 会将 HttpServletRequest 中的一些细节 request.getRemoteAddr() request.getSession 存入的到Token中
  setDetails(request, authRequest);

  // 然后 使用 父类中的 AuthenticationManager 对Token 进行认证 
  return this.getAuthenticationManager().authenticate(authRequest);
 }
 // 获取密码 很重要 如果你想改变获取密码的方式要么在此处重写，要么通过自定义一个前置的过滤器保证能此处能get到
 @Nullable
 protected String obtainPassword(HttpServletRequest request) {
  return request.getParameter(passwordParameter);
 }

  // 获取账户很重要 如果你想改变获取密码的方式要么在此处重写，要么通过自定义一个前置的过滤器保证能此处能get到
 @Nullable
 protected String obtainUsername(HttpServletRequest request) {
  return request.getParameter(usernameParameter);
 }

 // 参见上面对应的说明为凭据设置一些请求细节
 protected void setDetails(HttpServletRequest request,
   UsernamePasswordAuthenticationToken authRequest) {
  authRequest.setDetails(authenticationDetailsSource.buildDetails(request));
 }

 // 设置账户参数的key
 public void setUsernameParameter(String usernameParameter) {
  Assert.hasText(usernameParameter, "Username parameter must not be empty or null");
  this.usernameParameter = usernameParameter;
 }

 // 设置密码参数的key
 public void setPasswordParameter(String passwordParameter) {
  Assert.hasText(passwordParameter, "Password parameter must not be empty or null");
  this.passwordParameter = passwordParameter;
 }

 // 认证的请求方式是只支持POST请求
 public void setPostOnly(boolean postOnly) {
  this.postOnly = postOnly;
 }

 public final String getUsernameParameter() {
  return usernameParameter;
 }

 public final String getPasswordParameter() {
  return passwordParameter;
 }
}