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概述
此过滤器过滤请求,检测SecurityContextHolder中是否存在Authentication对象，如果不存在，说明用户尚未登录，此时为其提供一个匿名Authentication对象:AnonymousAuthentication。
注意:在整个请求处理的开始,无论当前请求所对应的
session中用户是否已经登录,SecurityContextPersistenceFilter
都会确保SecurityContextHolder中保持一个SecurityContext对象。但如果用户尚未登录，这个的SecurityContext对
象会是一个空对象，也就是其属性Authentication为null。然后在该请求处理过程中，如果一直到当前Filter执
行,SecurityContextHolder中SecurityContext对象属性Authentication仍是null,该AnonymousAuthenticationFilter就将其
修改为一个AnonymousAuthentication对象，表明这是一个匿名访问。
源代码解析
package org.springframework.security.web.authentication;
import java.io.IOException;
import java.util.*;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.security.authentication.AnonymousAuthenticationToken;
import org.springframework.security.authentication.AuthenticationDetailsSource;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.GrantedAuthority;
import org.springframework.security.core.authority.AuthorityUtils;
import org.springframework.security.core.context.SecurityContextHolder;
import org.springframework.util.Assert;
import org.springframework.web.filter.GenericFilterBean;
/**
* Detects if there is no Authentication object in the
* SecurityContextHolder, and populates it with one if needed.
*
* 检测 SecurityContextHolder中是否存在Authentication对象，如果不存在，说明
* 用户尚未登录，此时为其提供一个匿名 Authentication: AnonymousAuthentication对象。
*
* @author Ben Alex
* @author Luke Taylor
*/
public class AnonymousAuthenticationFilter extends GenericFilterBean implements
InitializingBean {
// ~ Instance fields
// ================================================================================================
// 用于构造匿名Authentication中详情属性的详情来源对象，这里使用一个WebAuthenticationDetailsSource
private AuthenticationDetailsSource<HttpServletRequest, ?> authenticationDetailsSource =
new WebAuthenticationDetailsSource();
private String key;
private Object principal;
private List<GrantedAuthority> authorities;
/**
* Creates a filter with a principal named "anonymousUser" and the single authority
* "ROLE_ANONYMOUS".
* 使用外部指定的key构造一个AnonymousAuthenticationFilter:
* 1. 缺省情况下，Spring Security 配置机制为这里指定的key是一个随机的uuid;
* 2. 所对应的 princpial(含义指当前登录主体) 是一个字符串"anonymousUser";
* 3. 所拥护的角色是 "ROLE_ANONYMOUS";
* @param key the key to identify tokens created by this filter
*/
public AnonymousAuthenticationFilter(String key) {
this(key, "anonymousUser", AuthorityUtils.createAuthorityList("ROLE_ANONYMOUS"));
}
/**
* 使用外部指定的参数构造一个AnonymousAuthenticationFilter
* @param key key the key to identify tokens created by this filter
* @param principal the principal which will be used to represent anonymous users
* @param authorities the authority list for anonymous users
*/
public AnonymousAuthenticationFilter(String key, Object principal,
List<GrantedAuthority> authorities) {
Assert.hasLength(key, "key cannot be null or empty");
Assert.notNull(principal, "Anonymous authentication principal must be set");
Assert.notNull(authorities, "Anonymous authorities must be set");
this.key = key;
this.principal = principal;
this.authorities = authorities;
}
// ~ Methods
// ========================================================================================================
@Override
public void afterPropertiesSet() {
// 在当前Filter bean被创建时调用，主要目的是断言三个主要属性都必须已经有效设置
Assert.hasLength(key, "key must have length");
Assert.notNull(principal, "Anonymous authentication principal must be set");
Assert.notNull(authorities, "Anonymous authorities must be set");
}
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
if (SecurityContextHolder.getContext().getAuthentication() == null) {
// 如果SecurityContextHolder中SecurityContext对象的属性authentication是null,
// 将其替换成一个匿名 Authentication: AnonymousAuthentication
SecurityContextHolder.getContext().setAuthentication(
createAuthentication((HttpServletRequest) req));
if (logger.isDebugEnabled()) {
logger.debug("Populated SecurityContextHolder with anonymous token: '"
+ SecurityContextHolder.getContext().getAuthentication() + "'");
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("SecurityContextHolder not populated with anonymous token, as it already contained: '"
+ SecurityContextHolder.getContext().getAuthentication() + "'");
}
}
// 对SecurityContextHolder中SecurityContext对象的属性authentication做过以上处理之后，继续
// filter chain 的执行
chain.doFilter(req, res);
}
// 根据指定属性key,princpial,authorities和当前环境(servlet web环境)构造一个AnonymousAuthenticationToken
protected Authentication createAuthentication(HttpServletRequest request) {
AnonymousAuthenticationToken auth = new AnonymousAuthenticationToken(key,
principal, authorities);
auth.setDetails(authenticationDetailsSource.buildDetails(request));
return auth;
}
// 可以外部指定Authentication对象的详情来源, 缺省情况下使用的是WebAuthenticationDetailsSource,
// 已经在属性authenticationDetailsSource初始化指定
public void setAuthenticationDetailsSource(
AuthenticationDetailsSource<HttpServletRequest, ?> authenticationDetailsSource) {
Assert.notNull(authenticationDetailsSource,
"AuthenticationDetailsSource required");
this.authenticationDetailsSource = authenticationDetailsSource;
}
public Object getPrincipal() {
return principal;
}
public List<GrantedAuthority> getAuthorities() {
return authorities;
}
}