如果页的输出缓存持续时间长于用户控件的输出缓存持续时间，则页的输出缓存持续时间优先。
If the page output cache duration is longer than the user control output cache duration, the page output cache duration takes precedence.
为了使用基于过期的缓存，portlet必须在部署描述符portlet . xml(对于符合JSR 168规范的标准化portlet)中定义过期缓存的持续时间。
To make use of expiration-based caching, portlets must define the duration of the expiration cache in the deployment descriptor portlet.xml (for standardized portlet following JSR 168 specification).
缓存api的设计目的是为了将数据缓存较长的一段时间，或者缓存至满足某些条件时，但每请求缓存则意味着只将数据缓存为该请求的持续时间。
Whereas the cache API is designed to cache data for a long period or until some condition is met, per-request caching simply means caching the data for the duration of the request.
expiration-cache元素指定了缓存将Portlet片段视为“新”的持续时间（以秒为单位）。
The expiration-cache element specifies the duration, in seconds, that a cache considers the portlet fragment to be "fresh".
获取或设置缓存页或缓存控件过程的持续时间。
Gets or sets the time duration during which the page or control is cached.
获取或设置缓存页或缓存控件过程的持续时间。
Gets or sets the time duration during which the page or control is cached.
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