spring security中的默认登录页源码跟踪

​2021年的最后2个月，立个flag，要把Spring Security和Spring Security OAuth2的应用及主流程源码研究透彻！

​项目中使用过Spring Security的童鞋都知道，当我们没有单独自定义登录页时，Spring Security自己在初始化的时候会帮我们配置一个默认的登录页，之前一直疑问默认登录页是怎么配置的，今晚特地找了源码跟一下。

springboot项目依赖

<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-security</artifactId>
</dependency>

在项目中随意编写一个接口，然后进行访问

@GetMapping("/")
public String hello() {
   return "hello, spring security";
}

在tomcat默认端口8080，localhost:8080 下访问该接口，spring security会帮我们将路径重定向到默认的登录页

​那么这个默认页是怎么来的呢？
原来Spring Security有一个默认的WebSecurityConfigurerAdapter，发现其中有一个init方法，于是在这个方法打了断点，在应用启动的时候进行跟踪。

​跟踪getHttp()方法，this.disableDefaults变量默认为false，意味着将会执行applyDefaultConfiguration(this.http);方法。查看applyDefaultConfiguration方法

public void init(WebSecurity web) throws Exception {
    // 首先配置security要拦截的哪些http请求
   HttpSecurity http = getHttp();
   web.addSecurityFilterChainBuilder(http).postBuildAction(() -> {
      FilterSecurityInterceptor securityInterceptor = http.getSharedObject(FilterSecurityInterceptor.class);
      web.securityInterceptor(securityInterceptor);
   });
}

protected final HttpSecurity getHttp() throws Exception {
		if (this.http != null) {
			return this.http;
		}
		AuthenticationEventPublisher eventPublisher = getAuthenticationEventPublisher();
		this.localConfigureAuthenticationBldr.authenticationEventPublisher(eventPublisher);
		AuthenticationManager authenticationManager = authenticationManager();
		this.authenticationBuilder.parentAuthenticationManager(authenticationManager);
		Map<Class<?>, Object> sharedObjects = createSharedObjects();
		this.http = new HttpSecurity(this.objectPostProcessor, this.authenticationBuilder, sharedObjects);
		if (!this.disableDefaults) {
            // 默认的配置将会走这个分支
			applyDefaultConfiguration(this.http);
			ClassLoader classLoader = this.context.getClassLoader();
			List<AbstractHttpConfigurer> defaultHttpConfigurers = SpringFactoriesLoader
					.loadFactories(AbstractHttpConfigurer.class, classLoader);
			for (AbstractHttpConfigurer configurer : defaultHttpConfigurers) {
				this.http.apply(configurer);
			}
		}
		configure(this.http);
		return this.http;
	}

查看applyDefaultConfiguration(this.http)方法，发现http对象new了一个DefaultLoginPageConfigurer对象属性，

private void applyDefaultConfiguration(HttpSecurity http) throws Exception {
   http.csrf();
   http.addFilter(new WebAsyncManagerIntegrationFilter());
   http.exceptionHandling();
   http.headers();
   http.sessionManagement();
   http.securityContext();
   http.requestCache();
   http.anonymous();
   http.servletApi();
   http.apply(new DefaultLoginPageConfigurer<>());
   http.logout();
}

​查看DefaultLoginPageConfigurer类定义，发现它在初始化的同时，它也初始化了自己的2个私有成员变量，分别是DefaultLoginPageGeneratingFilter默认登录页面生成Filter,DefaultLogoutPageGeneratingFilter默认登录页面Filter, 名字起得很好，见名知意，我们马山知道这2个类的含义。

​查看DefaultLoginPageGeneratingFilter的类成员变量，发现定义了一系列跟登录有关的成员变量，包括登录、登录等路径，默认的登录页面路径是"/login"

public class DefaultLoginPageGeneratingFilter extends GenericFilterBean {

   public static final String DEFAULT_LOGIN_PAGE_URL = "/login";

   public static final String ERROR_PARAMETER_NAME = "error";

   private String loginPageUrl;

   private String logoutSuccessUrl;

   private String failureUrl;

   private boolean formLoginEnabled;
    .....

​再结合类名思考，发现是个Filter类，那么它们应该都会重新Filter的doFilter(ServletRequest request, ServletResponse response, FilterChain chain)方法，我们查看一下DefaultLoginPageConfigurer类的``doFilter方法，果然，在doFilter`方法中发现了生成默认登录页面的方法。

private void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
      throws IOException, ServletException {
    // 判断当前的请求是否被认证通过
   boolean loginError = isErrorPage(request);
   boolean logoutSuccess = isLogoutSuccess(request);
   if (isLoginUrlRequest(request) || loginError || logoutSuccess) {
       // 当前请求认证失败的话，将会执行这个分支
      String loginPageHtml = generateLoginPageHtml(request, loginError, logoutSuccess);
      response.setContentType("text/html;charset=UTF-8");
      response.setContentLength(loginPageHtml.getBytes(StandardCharsets.UTF_8).length);
      response.getWriter().write(loginPageHtml);
      return;
   }
   chain.doFilter(request, response);
}

private String generateLoginPageHtml(HttpServletRequest request, boolean loginError, boolean logoutSuccess) {
   String errorMsg = "Invalid credentials";
   if (loginError) {
      HttpSession session = request.getSession(false);
      if (session != null) {
         AuthenticationException ex = (AuthenticationException) session
               .getAttribute(WebAttributes.AUTHENTICATION_EXCEPTION);
         errorMsg = (ex != null) ? ex.getMessage() : "Invalid credentials";
      }
   }
   String contextPath = request.getContextPath();
   StringBuilder sb = new StringBuilder();
   sb.append("<!DOCTYPE html>\n");
   sb.append("<html lang=\"en\">\n");
   sb.append("  <head>\n");
   sb.append("    <meta charset=\"utf-8\">\n");
   sb.append("    <meta name=\"viewport\" content=\"width=device-width, initial-scale=1, shrink-to-fit=no\">\n");
   sb.append("    <meta name=\"description\" content=\"\">\n");
   sb.append("    <meta name=\"author\" content=\"\">\n");
   sb.append("    <title>Please sign in</title>\n");
   sb.append("    <link href=\"https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css\" "
         + "rel=\"stylesheet\" integrity=\"sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M\" crossorigin=\"anonymous\">\n");
   sb.append("    <link href=\"https://getbootstrap.com/docs/4.0/examples/signin/signin.css\" "
         + "rel=\"stylesheet\" crossorigin=\"anonymous\"/>\n");
   sb.append("  </head>\n");
   sb.append("  <body>\n");
   sb.append("     <div class=\"container\">\n");
   if (this.formLoginEnabled) {
      sb.append("      <form class=\"form-signin\" method=\"post\" action=\"" + contextPath
            + this.authenticationUrl + "\">\n");
      sb.append("        <h2 class=\"form-signin-heading\">Please sign in</h2>\n");
      sb.append(createError(loginError, errorMsg) + createLogoutSuccess(logoutSuccess) + "        <p>\n");
      sb.append("          <label for=\"username\" class=\"sr-only\">Username</label>\n");
      sb.append("          <input type=\"text\" id=\"username\" name=\"" + this.usernameParameter
            + "\" class=\"form-control\" placeholder=\"Username\" required autofocus>\n");
      sb.append("        </p>\n");
      sb.append("        <p>\n");
      sb.append("          <label for=\"password\" class=\"sr-only\">Password</label>\n");
      sb.append("          <input type=\"password\" id=\"password\" name=\"" + this.passwordParameter
            + "\" class=\"form-control\" placeholder=\"Password\" required>\n");
      sb.append("        </p>\n");
      sb.append(createRememberMe(this.rememberMeParameter) + renderHiddenInputs(request));
      sb.append("        <button class=\"btn btn-lg btn-primary btn-block\" type=\"submit\">Sign in</button>\n");
      sb.append("      </form>\n");
   }
   if (this.openIdEnabled) {
      sb.append("      <form name=\"oidf\" class=\"form-signin\" method=\"post\" action=\"" + contextPath
            + this.openIDauthenticationUrl + "\">\n");
      sb.append("        <h2 class=\"form-signin-heading\">Login with OpenID Identity</h2>\n");
  ......
   return sb.toString();
}

​我们发现generateLoginPageHtml(HttpServletRequest request, boolean loginError, boolean logoutSuccess)这个方法中使用了最原始的Servlet写html页面的方法，将登录页的html代码写到字符串中写出到前端展示。到这里，我们就大体知道默认登录页面及登出页面是怎么生成的了。

​默认登录页到这里就结束了，有兴趣的可以关注下，接下来会继续写springsecurity的自定义表单认证、授权、会话等内容剖析。距离2022年只剩54天!

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