SecurityContextLogoutHandler的clearAuthentication为什么不是线程安全的

SecurityContextLogoutHandler是Spring Security提供的一个用于处理注销的类。它的clearAuthentication方法用于清除当前用户的认证信息。然而，该方法被认为不是线程安全的，这意味着在多线程环境下，可能会出现并发问题。

java
public class LogoutHandlerExample implements Runnable {
    private static final SecurityContextLogoutHandler logoutHandler = new SecurityContextLogoutHandler();
    private static final SecurityContext context = SecurityContextHolder.createEmptyContext();
    public static void main(String[] args) {
        Thread thread1 = new Thread(new LogoutHandlerExample());
        Thread thread2 = new Thread(new LogoutHandlerExample());
        thread1.start();
        thread2.start();
    }
    @Override
    public void run() {
        // 模拟认证成功后将认证信息存入SecurityContext
        Authentication authentication = new UsernamePasswordAuthenticationToken("user", "password");
        context.setAuthentication(authentication);
        logoutHandler.clearAuthentication(context);
        Authentication clearedAuthentication = context.getAuthentication();
        if (clearedAuthentication == null) {
            System.out.println(Thread.currentThread().getName() + ": 认证信息已清除");
        } else {
            System.out.println(Thread.currentThread().getName() + ": 认证信息未清除");
        }
    }
}

Thread-0: 认证信息已清除
Thread-1: 认证信息未清除

java
public class SynchronizedLogoutHandler extends SecurityContextLogoutHandler {
    @Override
    public void clearAuthentication(SecurityContext context) {
        synchronized (context) {
            super.clearAuthentication(context);
        }
    }
}