after using a page object
(e.g., the Result page object), Tapestry will not throw it away. Instead, it will put it into a pool for reuse (see the diagram
below). Later when it needs to use the Result page object again, it will check if the pool has one. If so, just take it out
from the pool and use it. Only when the pool has no such page object, will it create a new page object.
所以如果在一個新開的瀏覽器上面，頁面上顯示的參數不會發生改變，
防止這種情況的發生可以
在page-specification中加入
<property name="XXX"> XXX為頁面上的變量
所以對應的頁面為抽象的類 abstract，
對應的變量的get，set方法為abstract