java DocumentBuilder.parse() 线程安全吗？
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Is DocumentBuilder.parse() thread safe?
提问by Avi
Is the standard Java 1.6 javax.xml.parsers.DocumentBuilderclass thread safe? Is it safe to call the parse() method from several threads in parallel?
标准的 Java 1.6 javax.xml.parsers.DocumentBuilder类线程安全吗？从多个线程并行调用 parse() 方法是否安全？
The JavaDoc doesn't mention the issue, but the JavaDoc for the same classin Java 1.4 specifically says that it isn'tmeant to be concurrent; so can I assume that in 1.6 it is?
JavaDoc 没有提到这个问题，但Java 1.4 中同一个类的JavaDoc明确指出它不是并发的；所以我可以假设在 1.6 中是这样吗？
The reason is that I have several million tasks running in an ExecutorService, and it seems expensive to call DocumentBuilderFactory.newDocumentBuilder() every time.
原因是我有几百万个任务在 ExecutorService 中运行，每次调用 DocumentBuilderFactory.newDocumentBuilder() 似乎很昂贵。
采纳答案by Tom Hawtin - tackline
Even though DocumentBuilder.parse appears not to mutate the builder it does on the Sun JDK default implementation (based on Apache Xerces). Eccentric design decision. What can you do? I guess use a ThreadLocal:
尽管 DocumentBuilder.parse 似乎不会改变它在 Sun JDK 默认实现（基于 Apache Xerces）上所做的构建器。古怪的设计决策。你能做什么？我想使用 ThreadLocal：
private static final ThreadLocal<DocumentBuilder> builderLocal =
new ThreadLocal<DocumentBuilder>() {
@Override protected DocumentBuilder initialValue() {
try {
return
DocumentBuilderFactory
.newInstance(
"xx.MyDocumentBuilderFactory",
getClass().getClassLoader()
).newDocumentBuilder();
} catch (ParserConfigurationException exc) {
throw new IllegalArgumentException(exc);
}
}
};
(Disclaimer: Not so much as attempted to compile the code.)
（免责声明：不是试图编译代码。）
回答by Trenton
There's a reset() method on DocumentBuilder which restores it to the state when it was first created. If you're going the ThreadLocal route, don't forget to call this or you're hosed.
DocumentBuilder 上有一个 reset() 方法，可以将其恢复到首次创建时的状态。如果您要使用 ThreadLocal 路线，请不要忘记调用它，否则您将被冲洗掉。
回答by Koray Güclü
You can also check this code to make further optimization https://svn.apache.org/repos/asf/shindig/trunk/java/common/src/main/java/org/apache/shindig/common/xml/XmlUtil.java
您还可以检查此代码以进一步优化https://svn.apache.org/repos/asf/shindig/trunk/java/common/src/main/java/org/apache/shindig/common/xml/XmlUtil。爪哇
