我正在使用java附带的SAX解析器，我注意到一些我不理解/觉得很烦人的东西。
我正在使用SAX解析器来运行数百个XML文档，而实际上只需要解析其中的一小部分。如果文档中的第一个元素没有正确的名称空间，我将简单地终止并继续到下一个文档。
但一份文件甚至需要200-600毫秒才能开始阅读。我不知道为什么，之前我在开发时不小心关闭了互联网，并得到了大量的异常：
java.net.UnknownHostException: hibernate.sourceforge.net
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at java.net.Socket.connect(Socket.java:538)
at sun.net.NetworkClient.doConnect(NetworkClient.java:180)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:242)
at sun.net.www.http.HttpClient.New(HttpClient.java:339)
at sun.net.www.http.HttpClient.New(HttpClient.java:357)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1220)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1156)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1050)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:984)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1564)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.setupCurrentEntity(XMLEntityManager.java:647)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startEntity(XMLEntityManager.java:1304)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startDTDEntity(XMLEntityManager.java:1270)
at com.sun.org.apache.xerces.internal.impl.XMLDTDScannerImpl.setInputSource(XMLDTDScannerImpl.java:264)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.dispatch(XMLDocumentScannerImpl.java:1161)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$DTDDriver.next(XMLDocumentScannerImpl.java:1045)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:959)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:602)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:112)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:505)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:842)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:771)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:643)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl.parse(SAXParserImpl.java:327)
at javax.xml.parsers.SAXParser.parse(SAXParser.java:195)
at dk.magenta.alfresco.GenerateJavaMojo$Finder.find(GenerateJavaMojo.java:188)
at dk.magenta.alfresco.GenerateJavaMojo$Finder.visitFile(GenerateJavaMojo.java:215)
at dk.magenta.alfresco.GenerateJavaMojo$Finder.visitFile(GenerateJavaMojo.java:160)
at java.nio.file.Files.walkFileTree(Files.java:2670)
at java.nio.file.Files.walkFileTree(Files.java:2742)
at dk.magenta.alfresco.GenerateJavaMojo.execute(GenerateJavaMojo.java:110)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:320)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:156)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:537)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:196)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:141)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:290)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:230)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:409)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:352)
因此，问题有两个：为什么sax解析器要花那么多时间连接到web URL，我可以关闭它吗？或者有没有可以关闭它的实现？
更新：我尝试过添加这样的EntityResolver。
SAXParserFactory factory = SAXParserFactory.newInstance();
factory.setNamespaceAware(true);
factory.setValidating(false);
SAXParser saxParser = factory.newSAXParser();
saxParser.getXMLReader().setEntityResolver(new DummyEntityResolver());
....
public class DummyEntityResolver implements EntityResolver {
public InputSource resolveEntity(String publicID, String systemID)
throws SAXException {
return new InputSource(new StringReader(""));
}
}
它没有效果。平均通话时间仍然是400毫秒，当互联网不可用时，通话几乎是即时的。
文档可能包含对保存在该位置的外部DTD的引用。如果外部DTD包含需要扩展的实体，那么没有它可能无法进行解析，但如果您知道情况并非如此，或者您在本地有DTD的副本，那么您可以在XML解析器上设置EntityResolver，将引用重定向到本地文件。如果你不想编写自己的EntityResolver，你也可以使用Oasis Catalog将引用重定向到本地副本，但除非你害怕编写任何更复杂的Java。
解析器肯定要花时间连接到互联网来下载外部DTD。由于这些XML来自已经验证过的标准公共库，我建议您通过以下方式禁用验证：
SAXParserFactory saxParserFactory=SAXParserFactory.getInstance();
saxParserFactory.setValidating(false);
SAXParser saxParser=saxParserFactory.newSAXParser();