关于struts线程问题 转

Servlet是在多线程环境下的。即可能有多个请求发给一个servelt实例，每个请求是一个线程。 struts下的action也类似，同样在多线程环境 下。可以参考struts user guide: http://struts.apache.org/struts-action/userGuide /building_controller.html 中的Action Class Design Guidelines一节: 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. 译:为多线程环境编写代码。我们的controller servlet指挥创建你的Action 类的一个实例，用此实例来服务所有的请求。因此，你必须编写线程安全的Action类。遵循与写线程安全的servlet同样的方针。 1.什么是线程安全的代码 在多线程环境下能正确执行的代码就是线程安全的。 安全的意思是能正确执行，否则后果是程序执行错误，可能出现各种异常情况。 2.如何编写线程安全的代码 很多书籍里都详细讲解了如何这方面的问题，他们主要讲解的是如何同步线程对共享资源的使用的问题。主要是对synchronized关键字的各种用法，以及锁的概念。< BR> Java1.5中也提供了如读写锁这类的工具类。这些都需要较高的技巧，而且相对难于调试。 但是，线程同步是不得以的方法,是比较复杂的,而且会带来性能的损失。等效的代码中，不需要同步在编写容易度和性能上会更好些。 我这里强调的是什么代码是始终为线程安全的、是不需要同步的。如下: 1)常量始终是线程安全的，因为只存在读操作。 2)对构造器的访问(new 操作)是线程安全的，因为每次都新建一个实例，不会访问共享的资源。 3)最重要的是:局部变量是线程安全的。因为每执行一个方法，都会在独立的空间创建局部变量，它不是共享的资源。局部变量包括方法的参数变量。 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. 译:只使用用局部变量。--编写线程安全的代码最重要的原则就是，在Action类中只使用局部变量，不使用实例变量。 总结： 在Java的Web服务器环境下开发，要注意线程安全的问题。最简单的实现方式就是在Servlet和Struts Action里不要使用类变量、实例变量，但可以使用类常量和实例常量。如果有这些变量，可以将它们转换为方法的参数传入，以消除它们。 注意一个容易混淆的地方：被Servlet或Action调用的类中(如值对象、领域模型类)中是否可以安全的使用实例变量？如果你在每次方法调用时 新建一个对 象，再调用它们的方法，则不存在同步问题---因为它们不是多个线程共享的资源，只有共享的资源才需要同步---而Servlet和Action的实例对于多个线程是共享 的。 换句话说，Servlet和Action的实例会被多个线程同时调用，而过了这一层,如果在你自己的代码中没有另外启动线程，且每次调用后续业务对象时都是先 新建一个实例再调用，则都是线程安全的。