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我正在尝试通过以下函数将标记及其值动态添加到 xml 文件中。我正在尝试在根标记下添加名为first-name的标记及其值。但是在运行以下代码段时，我遇到了异常。

    public void write(String name) {
    try {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();            
        Document document = db.newDocument();

        Element blobKey_E = document.createElement("first-name");
        blobKey_E.appendChild( document.createTextNode( name ) );
        // The following line produces an exception
        // LINE 27 
        document.getDocumentElement().appendChild(blobKey_E); // append the new tag under the root

        TransformerFactory tf = TransformerFactory.newInstance();
        Transformer transformer = tf.newTransformer();
        DOMSource source = new DOMSource(document);
        StreamResult result = new StreamResult(new File("/home/non-admin/NetBeansProjects/Personal Site_Testers/web/xml/xml_1.xml"));
        transformer.transform(source, result);            
    }catch(Exception exc) {
        exc.printStackTrace();
    }
}

例外：

java.lang.NullPointerException
at Beans.XmlBuilder.write(XmlBuilder.java:27)
at Servlets.tester.doGet(tester.java:26)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

我在上面的函数中突出显示了第 27 行。

4

2 回答 2

1

document.getDocumentElement()在这种情况下返回null

Javadoc forgetDocumentElement()说明了以下关于功能的内容：

This is a convenience attribute that allows direct access to the child node that is the root element of the document.

在您的情况下，没有root element附加到您的 DOM。您可能想要做一个document.appendChild(blobKey_E);附加blobKey_E到 DOM 作为根元素。

理想情况下，当您尝试构建 XML DOM 时，您需要遵循以下基本步骤：

创建文档
创建根元素并将其添加到文档
创建子元素并将其附加到根或另一个现有子节点

public void write(String name) {
    try {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();            
        Document document = db.newDocument();

        Element blobKey_E = document.createElement("first-name");
        blobKey_E.appendChild( document.createTextNode( name ) );

        /*
         * Here blobKey_E is treated as the root element for the document that you've created
         */
        document.appendChild(blobKey_E); 
//            // LINE 27 
//            document.getDocumentElement().appendChild(blobKey_E); // append the new tag under the root

        /*
         * Post this point, if you do a document.getDocumentElement(), it will no longer return 
         * a nullpointerexception because blobKey_E will be treated as the root element.
         */

        TransformerFactory tf = TransformerFactory.newInstance();
        Transformer transformer = tf.newTransformer();
        DOMSource source = new DOMSource(document);
        StreamResult result = new StreamResult(new File("/home/non-admin/NetBeansProjects/Personal Site_Testers/web/xml/xml_1.xml"));
        transformer.transform(source, result);            
    }catch(Exception exc) {
        exc.printStackTrace();
    }
}
于 2012-08-27T19:05:00.423 回答
1

文档元素是您需要设置的根元素。将第 27 行更改为

document.appendChild(blobKey_E);
于 2012-08-27T19:12:51.203 回答