Servlet是在多線程環境下的。即可能有多個請求發給一個servelt實例，每個請求是一個線程。 struts下的action也類似，同樣在多線程環境 下。可以參考struts user guide: http://struts.apache.org/struts-action/userGuide /building_controller.html 中的Action Class Design Guidelines一節: 4.4.1 Action Class Design Guidelines Remember the following design guidelines when coding Action classes: * Write code for a multi-threaded environment - Our controller servlet creates only one instance of your Action class, and uses this one instance to service all requests. Thus, you need to write thread-safe Action classes. Follow the same guidelines you would use to write thread-safe Servlets. Here are two general guidelines that will help you write scalable, thread-safe Action classes: o Only Use Local Variables - The most important principle that aids in thread-safe coding is to use only local variables, not instance variables , in your Action class. Local variables are created on a stack that is assigned (by your JVM) to each request thread, so there is no need to worry about sharing them. An Action can be factored into several local methods, so long as all variables needed are passed as method parameters. This assures thread safety, as the JVM handles such variables internally using the call stack which is associated with a single Thread. o Conserve Resources - As a general rule, allocating scarce resources and keeping them across requests from the same user (in the user's session) can cause scalability problems. For example, if your application uses JDBC and you allocate a separate JDBC connection for every user, you are probably going to run in some scalability issues when your site suddenly shows up on Slashdot. You should strive to use pools and release resources (such as database connections) prior to forwarding control to the appropriate View component -- even if a bean method you have called throws an exception. * Don't throw it, catch it! - Ever used a commercial website only to have a stack trace or exception thrown in your face after you've already typed in your credit card number and clicked the purchase button? Let's just say it doesn't inspire confidence. Now is your chance to deal with these application errors - in the Action class. If your application specific code throws expections you should catch these exceptions in your Action class, log them in your application's log (servlet.log("Error message", exception)) and return the appropriate ActionForward. It is wise to avoid creating lengthy and complex Action classes. If you start to embed too much logic in the Action class itself, you will begin to find the Action class hard to understand, maintain, and impossible to reuse. Rather than creating overly complex Action classes, it is generally a good practice to move most of the persistence, and "business logic" to a separate application layer. When an Action class becomes lengthy and procedural, it may be a good time to refactor your application architecture and move some of this logic to another conceptual layer; otherwise, you may be left with an inflexible application which can only be accessed in a web-application environment. The framework should be viewed as simply the foundation for implementing MVC in your applications. Struts provides a useful control layer, but it is not a fully featured platform for building MVC applications, soup to nuts. 譯:為多線程環境編寫代碼。我們的controller servlet指揮創建你的Action 類的一個實例，用此實例來服務所有的請求。因此，你必須編寫線程安全的Action類。遵循與寫線程安全的servlet同樣的方針。 1.什么是線程安全的代碼 在多線程環境下能正確執行的代碼就是線程安全的。 安全的意思是能正確執行，否則后果是程序執行錯誤，可能出現各種異常情況。 2.如何編寫線程安全的代碼 很多書籍里都詳細講解了如何這方面的問題，他們主要講解的是如何同步線程對共享資源的使用的問題。主要是對synchronized關鍵字的各種用法，以及鎖的概念。< BR> Java1.5中也提供了如讀寫鎖這類的工具類。這些都需要較高的技巧，而且相對難于調試。 但是，線程同步是不得以的方法,是比較復雜的,而且會帶來性能的損失。等效的代碼中，不需要同步在編寫容易度和性能上會更好些。 我這里強調的是什么代碼是始終為線程安全的、是不需要同步的。如下: 1)常量始終是線程安全的，因為只存在讀操作。 2)對構造器的訪問(new 操作)是線程安全的，因為每次都新建一個實例，不會訪問共享的資源。 3)最重要的是:局部變量是線程安全的。因為每執行一個方法，都會在獨立的空間創建局部變量，它不是共享的資源。局部變量包括方法的參數變量。 struts user guide里有： Only Use Local Variables - The most important principle that aids in thread-safe coding is to use only local variables, not instance variables , in your Action class. 譯:只使用用局部變量。--編寫線程安全的代碼最重要的原則就是，在Action類中只使用局部變量，不使用實例變量。 總結： 在Java的Web服務器環境下開發，要注意線程安全的問題。最簡單的實現方式就是在Servlet和Struts Action里不要使用類變量、實例變量，但可以使用類常量和實例常量。如果有這些變量，可以將它們轉換為方法的參數傳入，以消除它們。 注意一個容易混淆的地方：被Servlet或Action調用的類中(如值對象、領域模型類)中是否可以安全的使用實例變量？如果你在每次方法調用時 新建一個對 象，再調用它們的方法，則不存在同步問題---因為它們不是多個線程共享的資源，只有共享的資源才需要同步---而Servlet和Action的實例對于多個線程是共享 的。 換句話說，Servlet和Action的實例會被多個線程同時調用，而過了這一層,如果在你自己的代碼中沒有另外啟動線程，且每次調用后續業務對象時都是先 新建一個實例再調用，則都是線程安全的。

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