优化 DOM 和 XPath Java 代码
我的代码太慢，但我不知道如何改进它。从磁盘读取 1k 文件到 DOM 大约需要 20 毫秒，这可能还不错，具体取决于磁盘，但是我还有另外 20 毫秒来处理 xpath 语句，这太多了。这是一些带有时间注释的示例代码。我该如何改进代码？
这发生在构造时：
DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = this.dbFactory.newDocumentBuilder();
XPathExpression[] ex = new XPathExpression[]{about 30 different expressions}
XPathExpression mainEx =xPath.compile("/rootElement/firstLevel/secondLevel");
然后代码：
Document doc = this.dBuilder.parse("somefile.xml");
//took 20 ms until here
NodeList nodes = (NodeList) mainEx .evaluate,doc, XPathConstants.NODESET);
//took another 20 ms until here !!!
for (int i = 0; i < nodes.getLength(); i++) {
Node n = nodes.item(i);
for (XPathExpression e:ex) {
String v = (String) e.evaluate(n, XPathConstants.STRING);
if (v != null) {
System.out.println(v);
}
}
}
//this only takes 5 ms
My code is too slow, but I'm not sure how to improve it. Reading from disk into DOM for a 1k-file takes about 20 ms, that might be okay depending on the disk, but then I've got another 20 ms for working on a xpath statement, which is far too much. Here is some sample code with time comments. How can I improve the code?
This happens at construction time:
DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = this.dbFactory.newDocumentBuilder();
XPathExpression[] ex = new XPathExpression[]{about 30 different expressions}
XPathExpression mainEx =xPath.compile("/rootElement/firstLevel/secondLevel");
Then the code:
Document doc = this.dBuilder.parse("somefile.xml");
//took 20 ms until here
NodeList nodes = (NodeList) mainEx .evaluate,doc, XPathConstants.NODESET);
//took another 20 ms until here !!!
for (int i = 0; i < nodes.getLength(); i++) {
Node n = nodes.item(i);
for (XPathExpression e:ex) {
String v = (String) e.evaluate(n, XPathConstants.STRING);
if (v != null) {
System.out.println(v);
}
}
}
//this only takes 5 ms
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评论（2）
您可能遇到了我在这里记录的这个问题：
Java XPath（Apache JAXP实现）性能
本质上，您应该添加这些 JVM 参数来大幅加速 Xalan 的 XPath 实现：
或者
You're probably suffering from this problem that I documented here:
Java XPath (Apache JAXP implementation) performance
Essentially, you should add these JVM arguments to heavily speed up Xalan's XPath implementation:
or
您应该将 XPath 表达式预编译为
XPathExpression，使用XPath.compile。然后调用XPathExpression.evaluate。如果您多次执行它，这将节省您的时间。我假设情况确实如此，或者 20 毫秒应该不重要。
编辑：正如评论中提到的，这个 问题 有更多信息，包括JVM 参数。
You should pre-compile your XPath expression to a
XPathExpression, usingXPath.compile. Then callXPathExpression.evaluate.This will save you time if you're executing it more than once. I'm assuming this is the case, or 20 ms shouldn't matter.
EDIT: As mentioned in the comments, this question has further information, including a JVM parameter.