使用传递给该方法的xpath字符串调用evaluate，在类的构造函数中构建的dom树和常量指示将结果作为一组节点返回。

Evaluate is called with the XPath string passed into the method, the DOM tree you built in the class's constructor, and the constant indicating to return results as a list of nodes.

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但是请注意，如果在调用上述方法之前已经建立了树，AXIOM 将模拟 StAX 解析器。

Note, however, that if the tree is built prior to calling either of the above methods, AXIOM will emulate the StAX parser.

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通过StAX API对某个元素调用第一个方法，可以访问该元素的XML Infoset，同时缓冲(如果需要)树中未构造的部分以供将来使用。

The first method allows you to access the XML Infoset of the element on which the method is called through the StAX API, while caching (if necessary) any unbuilt parts of tree for later use.

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