xml可扩展标记语言，标准通用标记语言的子集，是一种用于标记电子文件使其具有结构性的标记语言。
在web中，今天我本想测试一下用xml做为数据库存储用户信息，但是在查询用户信息的时候一直发生：
java.lang.NoClassDefFoundError: org/jaxen/JaxenException
at org.dom4j.DocumentFactory.createXPath(DocumentFactory.java:222)
at org.dom4j.tree.AbstractNode.createXPath(AbstractNode.java:202)
at org.dom4j.tree.AbstractNode.selectSingleNode(AbstractNode.java:178)
at com.changeyd.user.dao.UserDao.findByUsername(UserDao.java:49)
at test.dao.UserDaoTest.testFindByUsername(UserDaoTest.java:19)
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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: java.lang.ClassNotFoundException: org.jaxen.JaxenException
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 28 more
经过寻找错误，发现在对XML文件进行操作的时候，不止需要导入dom4j.jar，还需要jaxem.1.1-beta.jar包，所以导入就不会找不到异常了
所以附上jar免费下载链接：
http://112.17.28.180/file3.data.weipan.cn/57515002/adbce74e61d68b4f2a205eda5a51e17ccf3de723?ip=1528191886,39.178.39.186&ssig=WZiffPmt%2Bx&Expires=1528192486&KID=sae,l30zoo1wmz&fn=jaxen-1.1-beta-6.jar&skiprd=2&se_ip_debug=39.178.39.186&corp=2&from=1221134&wsrid_tag=5b1656df_iydong72_13487-28782&wsiphost=ipdb
