从XML文档Java接收子节点

Question

我是xml解析的新手，但是我必须从特定的URL获取XML，然后从中仅保存一个资源。

我如何获取xml：

public Document getXMLFile(String urlToXml) {
    try {
        URL url = new URL(urlToXml);
        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
        Document document = documentBuilder.parse(url.openStream());
        return document;
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (ParserConfigurationException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (SAXException e) {
        e.printStackTrace();
    }
    return null;
}

这可以正常工作并返回该文件：

<CRates xmlns="****" xmlns:dc="http://purl.org/dc/elements/1.1/"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="******">
    <Date>20161003</Date>
    <Currencies>
        <Currency>
            <ID>AUD</ID>
            <Rate>1.46380</Rate>
        </Currency>
        <Currency>
            <ID>BGN</ID>
            <Rate>1.95580</Rate>
        </Currency>
        <Currency>
            <ID>BRL</ID>
            <Rate>3.64090</Rate>
        </Currency>
        <Currency>
            <ID>CAD</ID>
            <Rate>1.47020</Rate>
        </Currency>
        <Currency>
            <ID>CHF</ID>
            <Rate>1.09180</Rate>
        </Currency>
        <Currency>
            <ID>CNY</ID>
            <Rate>7.49620</Rate>
        </Currency>
        <Currency>
            <ID>CZK</ID>
            <Rate>27.02100</Rate>
        </Currency>
        <Currency>
            <ID>DKK</ID>
            <Rate>7.44630</Rate>
        </Currency>
        <Currency>
            <ID>GBP</ID>
            <Rate>0.87318</Rate>
        </Currency>
        <Currency>
            <ID>HKD</ID>
            <Rate>8.71450</Rate>
        </Currency>
        <Currency>
            <ID>HRK</ID>
            <Rate>7.50530</Rate>
        </Currency>
        <Currency>
            <ID>HUF</ID>
            <Rate>308.18000</Rate>
        </Currency>
        <Currency>
            <ID>IDR</ID>
            <Rate>14587.14000</Rate>
        </Currency>
        <Currency>
            <ID>ILS</ID>
            <Rate>4.22280</Rate>
        </Currency>
        <Currency>
            <ID>INR</ID>
            <Rate>74.76600</Rate>
        </Currency>
        <Currency>
            <ID>JPY</ID>
            <Rate>113.90000</Rate>
        </Currency>
        <Currency>
            <ID>KRW</ID>
            <Rate>1237.21000</Rate>
        </Currency>
        <Currency>
            <ID>MXN</ID>
            <Rate>21.61500</Rate>
        </Currency>
        <Currency>
            <ID>MYR</ID>
            <Rate>4.62720</Rate>
        </Currency>
        <Currency>
            <ID>NOK</ID>
            <Rate>8.96250</Rate>
        </Currency>
        <Currency>
            <ID>NZD</ID>
            <Rate>1.54540</Rate>
        </Currency>
        <Currency>
            <ID>PHP</ID>
            <Rate>54.17900</Rate>
        </Currency>
        <Currency>
            <ID>PLN</ID>
            <Rate>4.29330</Rate>
        </Currency>
        <Currency>
            <ID>RON</ID>
            <Rate>4.45050</Rate>
        </Currency>
        <Currency>
            <ID>RUB</ID>
            <Rate>70.00100</Rate>
        </Currency>
        <Currency>
            <ID>SEK</ID>
            <Rate>9.59300</Rate>
        </Currency>
        <Currency>
            <ID>SGD</ID>
            <Rate>1.53260</Rate>
        </Currency>
        <Currency>
            <ID>THB</ID>
            <Rate>38.91000</Rate>
        </Currency>
        <Currency>
            <ID>TRY</ID>
            <Rate>3.38610</Rate>
        </Currency>
        <Currency>
            <ID>USD</ID>
            <Rate>1.12360</Rate>
        </Currency>
        <Currency>
            <ID>ZAR</ID>
            <Rate>15.26410</Rate>
        </Currency>
    </Currencies>
</CRates>

问题是-我怎么能从所有这些东西中仅获得一个指定的节点（例如USD）并将其另存为新的xml？

输出xml的示例：

<CRates>
    <Date>20160321</Date>
    <ID>USD</ID>
    <Rate>1.12360</Rate>
    </Currency>
</CRates>

感谢帮助，

干杯

Answer 1

一种方法是使用XPath来匹配所需的节点。

import javax.xml.xpath.*;

...

XPathFactory xpathFactory = XPathFactory.newInstance();
XPath xpath = xpathFactory.newXPath();
NodeList nodes = (NodeList) xpath.evaluate("//*[@id='USD']", document, XPathConstants.NODESET);

如果您确定只需要一个节点，则XPath.evaluate也可以返回单个Element对象，但是带有NodeLists的该版本将能够处理返回多个节点的情况。