java 在 JSF 中有一个“常量”类
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Having a "constants"-class in JSF
提问by javaMS
I'm building a web-application using JSF/Primefaces. I need to have a "constants"-class, i.e. a class that will consist of constants. These constants are mainly navigational commands that will be used throughout the application. The reason I am doing this is to avoid having instantiated Strings on an ad-hoc basis.
我正在使用 JSF/Primefaces 构建一个 Web 应用程序。我需要一个“常量”类，即一个包含常量的类。这些常量主要是将在整个应用程序中使用的导航命令。我这样做的原因是为了避免临时实例化字符串。
How do I achieve this, making the constants accessible from both backing beans and XHTML files?
我如何实现这一点，使常量可以从支持 bean 和 XHTML 文件中访问？
I have tried using @ApplicationScopedand using the Singleton-pattern (Singleton class) but I am unable to get it working due to scope issues.
我曾尝试使用@ApplicationScoped单例模式（单例类），但由于范围问题，我无法使其正常工作。
Or maybe I am just using the wrong approach. Any ideas/suggestions are welcome.
或者也许我只是使用了错误的方法。欢迎任何想法/建议。
回答by BalusC
How do I achieve this, making the constants accessible from both backing beans and XHTML files?
我如何实现这一点，使常量可以从支持 bean 和 XHTML 文件中访问？
In backing beans, it's obviously easy. As they're just Java classes, it's not different from the "normal" Java way. You could use enums or public static finalfields. In views, this is a different story. Until the upcoming version 3.0, EL does namely not support constants in any way.
在支持 bean 中，这显然很容易。由于它们只是 Java 类，因此它与“普通”Java 方式没有什么不同。您可以使用enums 或public static final字段。在视图中，这是一个不同的故事。在即将到来的 3.0 版本之前，EL 不以任何方式支持常量。
I'd suggest using enums as EL has implicit support for them in string comparisons. It does not do any compiletime/runtime type safety checks, but you can use the enum name as a string. E.g.
我建议使用枚举，因为 EL 在字符串比较中隐式支持它们。它不进行任何编译时/运行时类型安全检查，但您可以将枚举名称用作字符串。例如
<h:someComponent ... rendered="#{order.status == 'SHIPPING'}" />
If you prefer more self documenting code and a runtime check (no, a compiletime check is not possible), then you could consider using OmniFaces<o:importConstants>.
如果您更喜欢更多的自记录代码和运行时检查（不，编译时检查是不可能的），那么您可以考虑使用OmniFaces<o:importConstants>。
<o:importConstants type="com.example.OrderStatus" />
<h:someComponent ... rendered="#{order.status == OrderStatus.SHIPPING}" />
This is IMO only a bit more clumsy. The runtime check is however nice during development. A typo is easily overseen.
这只是 IMO 有点笨拙。然而，运行时检查在开发过程中很好。一个错字很容易被发现。
In the upcoming EL 3.0 (JSR-341, part of Java EE 7), it's possible to reference constants the same way. See also How to reference constants in EL?This only requires programmatic import of the constants as there's no standard JSP/Facelets tag for that.
在即将发布的 EL 3.0（JSR-341，Java EE 7 的一部分）中，可以以相同的方式引用常量。另请参阅如何在 EL 中引用常量？这仅需要以编程方式导入常量，因为没有标准的 JSP/Facelets 标记。
