（Servlets，线程安全）保护会话和上下文状态((Servlets, Thread safety) Protecting the session and context state)

 参考：Head First Servlets＆JSP，第204页，第三个问题： 
 
 问：“实现SingleThreadModel的效果与同步服务方法几乎相同。两者都可以在不保护会话和上下文状态的情况下使Web应用程序瘫痪。” 
 
 这意味着什么：“在不保护会话和上下文状态的情况下，将Web应用程序放在膝盖上”？ 他们是否说实施SingleThreadModel是不够线程安全的？ 因为即使一次只有一个线程可以在一个servlet上运行，这也不会阻止其他servlet访问和改变上下文/会话作用域变量？ 如果Servlet实现SingleThreadingModel，那么为什么需要保护会话状态？ 它一次只允许一个线程。 即使您打开一个新的浏览器窗口，我认为servlet也不允许您发出两个请求。 也许他们的意思是，如果不同的Servlet在这个状态下不同步，那么一次只有一个线程的Servlet仍然可能破坏上下文/会话状态？ 
 
 为什么类（静态）变量不是线程安全的？ 因为所有线程都可以访问和修改（损坏）该值？ 
 
 它对我来说仍然有点模糊。 

Referring to: Head First Servlets & JSP, page 204, third question: 
 
Q. "The effect of implementing SingleThreadModel is virtually the same as synchronizing the service method. Both can bring a web app to its knees without protecting the session and context state." 
 
What does this mean: "bring a web app to its knees without protecting the session and context state"? Are they saying that it's not enough thread-safe to implement the SingleThreadModel? Because even though only one thread at a time can be running at one servlet, this doesn't stop other servlets from accessing and mutating the context/session scoped variables? If a Servlet implements the SingleThreadingModel, then why would you need to protect session state? It only allows one thread at a time. Even if you open up a new browser window, the servlet wouldn't allow you to make two requests, I think. Maybe they mean that different Servlets with one thread at a time could still corrupt the context/session state if they don't synchronize on that state? 
 
And why are class (static) variables not thread-safe? Because all threads can access and modify (corrupt) that value? 
 
It's still a little vague to me. 

原文：https://stackoverflow.com/questions/17806517