Web是一个多用户环境，所以在一个静态对象中（如singleton）保持应用状态是有可能引起问题的。

The web is a multiunser environment, so holding application state in static objects such as singletons is likely to cause issues

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该关系分别为这四个业务对象的非主键 state 属性定义一个静态关系。

The relationship defines a static relationship for the non-primary key state property for each of the four business objects.

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这种模式的典型实现方式是创建一个抽象类，其中具有静态方法create，这个方法返回的是实现了这个抽象类的一个具体对象。

Typically this is implemented by having an abstract class that has a static create method that returns a concrete object implementing the abstract class.

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