java - session bean 中的静态变量限制

Question

不可能在 session bean 代码上使用静态变量。这种限制是武断的还是有根据的？为什么？

最好的问候

Answer 1

如 FAQ on EJB restrictions 中所述，使用 EJB 的限制之一是:

enterprise beans should not read or write nonfinal static fields

在discussion on static fields中进一步扩展:

Nonfinal static class fields are disallowed in EJBs because such fields make an enterprise bean difficult or impossible to distribute. Static class fields are shared among all instances of a particular class, but only within a single Java Virtual Machine (JVM). Updating a static class field implies an intent to share the field's value among all instances of the class. But if a class is running in several JVMs simultaneously, only those instances running in the same JVM as the updating instance will have access to the new value. In other words, a nonfinal static class field will behave differently if running in a single JVM, than it will running in multiple JVMs. The EJB container reserves the option of distributing enterprise beans across multiple JVMs (running on the same server, or on any of a cluster of servers). Nonfinal static class fields are disallowed because enterprise bean instances will behave differently depending on whether or not they are distributed.

It is acceptable practice to use static class fields if those fields are marked as final. Since final fields cannot be updated, instances of the enterprise bean can be distributed by the container without concern for those fields' values becoming unsynchronized.