Java XPathFactory线程安全

Question

javax.xml.XPathFactory.newInstance()线程安全吗？

我问,因为我发现文档含糊不清.JDK 5文档根本没有提到线程安全性; 在JDK 6中,他们写了以下内容:

XPathFactory类不是线程安全的.换句话说,应用程序有责任确保在任何给定时刻最多只有一个线程正在使用XPathFactory对象.鼓励实现将方法标记为同步以保护自己免受破坏的客户端的影响.

据我所知,单独实现XPathFactory是不安全的,但做这样的事情应该是安全的:

XPath xPathEvaluator = XPathFactory.newInstance().newXPath();

我错过了什么吗？它取决于扩展它的实际类吗？我需要synchronize包含上述语句的方法吗？

Answer 1

XPath xPathEvaluator = XPathFactory.newInstance().newXPath();

这是安全的,因为每个线程都有自己的工厂(感谢newInstance()).无需在此处同步.

你不能安全做的是只让工厂一次,然后在没有同步的情况下在线程之间共享它,例如作为单例.XPath实例(xPathEvaluator)本身也是如此.

Answer 2

"其中一个是XPathFactory.newInstance()非常昂贵;"

真实的声明!我注意到,对于每个调用newInstance()的线程,jaxp.properties必须位于类路径上并读入:

java.lang.Thread.State: BLOCKED (on object monitor)
        at java.util.zip.ZipFile.getEntry(ZipFile.java:160)
        - locked <0x0000000968dec028> (a sun.net.www.protocol.jar.URLJarFile)
        at java.util.jar.JarFile.getEntry(JarFile.java:208)
        at sun.net.www.protocol.jar.URLJarFile.getEntry(URLJarFile.java:107)
        at sun.net.www.protocol.jar.JarURLConnection.connect(JarURLConnection.java:114)
        at sun.net.www.protocol.jar.JarURLConnection.getInputStream(JarURLConnection.java:132)
        at java.net.URL.openStream(URL.java:1010)
        at javax.xml.xpath.SecuritySupport$4.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.xml.xpath.SecuritySupport.getURLInputStream(Unknown Source)
        at javax.xml.xpath.XPathFactoryFinder._newFactory(Unknown Source)
        at javax.xml.xpath.XPathFactoryFinder.newFactory(Unknown Source)
        at javax.xml.xpath.XPathFactory.newInstance(Unknown Source)
        at javax.xml.xpath.XPathFactory.newInstance(Unknown Source)

ZipFile进行本机调用(我相信zlib)并解压缩jar,这需要磁盘IO和处理器绑定的zip解压缩.在这个例子中,我们有1400多个线程在等待锁定.