ExceptionTranslationFilter过滤器对应的类路径为
org.springframework.security.web.access.ExceptionTranslationFilter
从类名就看出这个过滤器用于异常翻译的。但是从这个过滤器在filterchain中的位置来看，它仅仅处于倒数第三的位置（这个filter后面分为是FilterSecurityInterceptor、SwitchUserFilter），所以ExceptionTranslationFilter只能捕获到后面两个过滤器所抛出的异常。
这里需要强调一下，spring security中的异常类基本上都继承RuntimeException。
接着看ExceptionTranslationFilter执行过程
//doFilter拦截到请求时，不做处理。仅仅处理后面filter所抛出的异常
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
try {
chain.doFilter(request, response);
}
catch (IOException ex) {
throw ex;
}
catch (Exception ex) {
//这里主要是从异常堆栈中提取SpringSecurityException
Throwable[] causeChain = throwableAnalyzer.determineCauseChain(ex);
RuntimeException ase = (AuthenticationException)
throwableAnalyzer.getFirstThrowableOfType(AuthenticationException.class, causeChain);
if (ase == null) {
ase = (AccessDeniedException)throwableAnalyzer.getFirstThrowableOfType(AccessDeniedException.class, causeChain);
}
//如果提取到安全异常，则进行处理
if (ase != null) {
handleException(request, response, chain, ase);
} else {
//没有安全异常，继续抛出
// Rethrow ServletExceptions and RuntimeExceptions as-is
if (ex instanceof ServletException) {
throw (ServletException) ex;
}
else if (ex instanceof RuntimeException) {
throw (RuntimeException) ex;
}
throw new RuntimeException(ex);
}
}
}
//处理安全异常
private void handleException(HttpServletRequest request, HttpServletResponse response, FilterChain chain,
RuntimeException exception) throws IOException, ServletException {
//如果是认证异常，由sendStartAuthentication处理
if (exception instanceof AuthenticationException) {
sendStartAuthentication(request, response, chain, (AuthenticationException) exception);
}
//如果是访问拒绝异常，由访问拒绝处理类的handle处理
else if (exception instanceof AccessDeniedException) {
if (authenticationTrustResolver.isAnonymous(SecurityContextHolder.getContext().getAuthentication())) {
sendStartAuthentication(request, response, chain, new InsufficientAuthenticationException(
"Full authentication is required to access this resource"));
}
else {
accessDeniedHandler.handle(request, response, (AccessDeniedException) exception);
}
}
}
先分析如何处理认证异常
//处理认证异常
protected void sendStartAuthentication(HttpServletRequest request, HttpServletResponse response, FilterChain chain,
AuthenticationException reason) throws ServletException, IOException {
// SEC-112: Clear the SecurityContextHolder's Authentication, as the
// existing Authentication is no longer considered valid
//首先把SecurityContext中的认证实体置空
SecurityContextHolder.getContext().setAuthentication(null);
//通过cache保存当前的请求信息（分析RequestCacheAwareFilter时再深入）
requestCache.saveRequest(request, response);
logger.debug("Calling Authentication entry point.");
//由认证入口点开始处理
authenticationEntryPoint.commence(request, response, reason);
}
这里补充一下
[color=red]authenticationEntryPoint是由配置http标签时，通过什么认证入口来决定注入相应的入口点bean的。请看下面的对应关系列表
form-login认证:LoginUrlAuthenticationEntryPoint
http-basic认证:BasicAuthenticationEntryPoint
openid-login认证:LoginUrlAuthenticationEntryPoint
x509认证:Http403ForbiddenEntryPoint[/color]
就不一一分析每个EntryPoint了，着重看一下LoginUrlAuthenticationEntryPoint
//主要目的是完成跳转任务
//创建该bean时，只注入了loginFormUrl属性，其他类变量均为默认值
public void commence(HttpServletRequest request, HttpServletResponse response, AuthenticationException authException)
throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest) request;
HttpServletResponse httpResponse = (HttpServletResponse) response;
String redirectUrl = null;
//默认为false
if (useForward) {
if (forceHttps && "http".equals(request.getScheme())) {
redirectUrl = buildHttpsRedirectUrlForRequest(httpRequest);
}
if (redirectUrl == null) {
String loginForm = determineUrlToUseForThisRequest(httpRequest, httpResponse, authException);
RequestDispatcher dispatcher = httpRequest.getRequestDispatcher(loginForm);
dispatcher.forward(request, response);
return;
}
} else {
//返回的url为loginFormUrl配置的值，如果未配置，跳转到默认登录页面/spring_security_login
redirectUrl = buildRedirectUrlToLoginPage(httpRequest, httpResponse, authException);
}
redirectStrategy.sendRedirect(httpRequest, httpResponse, redirectUrl);
}
接着分析访问拒绝类异常的处理过程，看AccessDeniedHandlerImpl的handle方法
public void handle(HttpServletRequest request, HttpServletResponse response, AccessDeniedException accessDeniedException)
throws IOException, ServletException {
if (!response.isCommitted()) {
//如果配置了access-denied-page属性，跳转到指定的url
if (errorPage != null) {
// Put exception into request scope (perhaps of use to a view)
request.setAttribute(SPRING_SECURITY_ACCESS_DENIED_EXCEPTION_KEY, accessDeniedException);
// Set the 403 status code.
response.setStatus(HttpServletResponse.SC_FORBIDDEN);
// forward to error page.
RequestDispatcher dispatcher = request.getRequestDispatcher(errorPage);
dispatcher.forward(request, response);
//如果没有配置，则直接响应403禁止访问的错误信息到浏览器端
} else {
response.sendError(HttpServletResponse.SC_FORBIDDEN, accessDeniedException.getMessage());
}
}
}
通过以上分析，可以大体上认识到ExceptionTranslationFilter主要拦截两类安全异常：认证异常、访问拒绝异常。而且仅仅是捕获FilterSecurityInterceptor、SwitchUserFilter以及自定义拦截器的异常。所以在自定义拦截器时，需要注意在链中的顺序。
在上面分析过程中，有requestCache.saveRequest(request, response);的语句，具体requestCache的用途下篇分析。
