java 解析xml 遇到问题

        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        Document doc = (Document) db.parse("xml/test.xml");
        NodeList nl = ((org.w3c.dom.Document) doc).getElementsByTagName("output");

解析XML时，执行到parse时，总提示如下错误提示：
java.lang.ClassCastException: com.sun.org.apache.xerces.internal.dom.DeferredDocumentImpl cannot be cast to javax.swing.text.Document
    at popocloudRegTest.testPraseXml(popocloudRegTest.java:69)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at junit.framework.TestCase.runTest(TestCase.java:164)
    at junit.framework.TestCase.runBare(TestCase.java:130)
    at junit.framework.TestResult$1.protect(TestResult.java:106)
    at junit.framework.TestResult.runProtected(TestResult.java:124)
    at junit.framework.TestResult.run(TestResult.java:109)
    at junit.framework.TestCase.run(TestCase.java:120)
    at org.eclipse.jdt.internal.junit.runner.junit3.JUnit3TestReference.run(JUnit3TestReference.java:130)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

原因为在Document时，导入的库不对，如果导入import javax.swing.text.Document; 这个库，则会有以上错误提示，如果导入的库为：import org.w3c.dom.Document;则正常使用。
具体也没去深究两个包不同之处。