Struts2 线程中的拦截器不安全吗?

(1) 为什么为每个线程创建一个动作实例不会影响性能?或者是吗?

使用现代 JVM，创建对象的成本可以忽略不计.如果您通过避免昂贵的初始化(例如通过不在操作内创建数据库连接，而是使用连接池.

(2) 你是否建议永远不要使用默认的拦截器堆栈，并始终使用自定义拦截器堆栈(其中所有使用实例变量的未使用的拦截器都被删除)以便线程安全?

我认为 Struts 2 附带和配置的任何默认拦截器都不是线程安全的.即使它们使用实例字段(因为它们要么仅用于配置，要么本身是线程安全的，如 Logger).

根据我的个人经验，只有在有充分理由的情况下才应该触摸/更改拦截器堆栈(内置拦截器的线程安全性不是其中之一).如果您更改堆栈，很多事情都会以意想不到的方式运行/中断 - 从长远来看，运行default"或paramPrepareParam"等内置堆栈之一可以节省很多挫败感.添加您自己的自定义拦截器通常比从现有堆栈中删除/重新排列拦截器的破坏性更小.

I understand that Struts2 Action classes are thread-safe, because the actions are put in the Value Stack. The Value Stack in turn is one part of the Action Context. Since the Action Context is thread-local, the values stored in the Action Context (including the value stack) are unique per thread. So, Actions are thread-safe.

But consider the interceptors: They are really useful, they do all those tedious little jobs for the programmer... like validations, fetching the param values, etc. But the thing to consider is that: Interceptors can be shared between multiple requests. So does that make interceptors thread unsafe?

With this question in mind, I tried to surf the net for some good articles related to this problem. And I found a very good article, where they have clearly mentioned with an example How interceptors are NOT thread safe.

The web page is: http://www.bullraider.com/java/struts2/tutorials/interceptors-and-thread-safety

What I got to know from this article is, the major reason behind Interceptors being thread un-safe is that the interceptors are created only once. i.e. each interceptor has only one object. So, the instance fields are not safe, when the same instance of an Interceptor is shared between threads.

At the end of the article it's mentioned that there are cases, where even the interceptors are thread safe. But they didn't mentioned any such cases. I surfed the net to find the answer... but in vain :(

Can anybody tell me or provide me with a link, where I can find out how to make interceptors thread-safe (or what are the scenarios when an interceptor is thread safe)?

Any Interceptor that does not use instance fields or other shared state is thread-safe:

For examples, look at all the built-in interceptors that parse HTTP request parameters and cookies, do logging, access checks, exception handling: They do not use instance fields for mutable state(*) but just operate on the ActionInvocation instance they get as parameters.

(*) some do have instance fields for configuration parameters which are set when Struts starts up (in a single thread), like ExceptionMappingInterceptor, or thread-safe instance fields like the Logger in LoggingInterceptor.

If you plan on writing your own Interceptor, work just with the ActionInvocation parameter you get passed in and with local variables in your intercept() method. Avoid the temptation to make your intercept method synchronized or put things into a synchronized{} block -- this will create a bottleneck with Struts' single-instance approach to Interceptors.

To answer the questions from the comments:

(1) How come creating an instance of action for every thread doesn't affect the performance?or does it?

With modern JVMs, the cost of creating an object is negligible. There should be no noticeable effect on performance if you keep your actions light-weight by avoiding expensive initializtion, e.g. by not creating database connections inside an action but using a connection pool.

(2) Do u recommend NEVER to use the default interceptor stack, and always use custom interceptor stack (where all the unused interceptors which use instance variables are removed) so that it will be thread safe?

I don't think any of the default interceptors that are shipped and configured with Struts 2 are not thread-safe; even if they use instance fields (because they're either used for configuration only or itself thread-safe like Logger).

From my personal experience, you should only ever touch/change the interceptor stack if you have a good reason (thread-safety of the built-in interceptors isn't one). A lot of things behave/break in unexpected ways if you change the stacks -- running one of the built-in stacks like "default" or "paramPrepareParam" saves a lot of frustration in the long run. Adding your own custom interceptors is usually less disruptive than removing/rearranging interceptors from an existing stack.

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