Question

我创建了一个删除方法。我的方法有一个索引作为参数来自jlist位置。该方法将从用户单击的jlist中获取位置，并在xml中查找该元素并删除它和所有子项。

因此，如果用户单击TrainCategorySpeed，它具有索引122并将在XML中查找该类型

<type>
   <OBJECT_TYPE>TrainCategorySpeed</OBJECT_TYPE> 
-  <prop>
    <DESCRIPTION>Differential speed limit</DESCRIPTION> 
    <PARENT>NULL</PARENT> 
    <VIRTUAL>0</VIRTUAL> 
    <VISIBLE>0</VISIBLE> 
    <PICTURE>NULL</PICTURE> 
    <HELP>NULL</HELP> 
    <MIN_NO>NULL</MIN_NO> 
    <MAX_NO>NULL</MAX_NO> 
    <NAME_FORMAT>NULL</NAME_FORMAT> 
   </prop>
-  <param>
    <PARAMETER>trainCategory</PARAMETER> 
    <DATA_TYPE>INTEGER</DATA_TYPE> 
    <DESCRIPTION>Train category</DESCRIPTION> 
    <MIN_NO>1</MIN_NO> 
    <MAX_NO>1</MAX_NO> 
    <ORDER1>1</ORDER1> 
    <NESTED>1</NESTED> 
    <DEFAULT1>NULL</DEFAULT1> 
    <FORMAT>0:15</FORMAT> 
   </param>
-  <param>
    <PARAMETER>speed</PARAMETER> 
    <DATA_TYPE>INTEGER</DATA_TYPE> 
    <DESCRIPTION>Speed (km/h (V_DIFF))</DESCRIPTION> 
    <MIN_NO>1</MIN_NO> 
    <MAX_NO>1</MAX_NO> 
    <ORDER1>2</ORDER1> 
    <NESTED>1</NESTED> 
    <DEFAULT1>NULL</DEFAULT1> 
    <FORMAT>0:600:5</FORMAT> 
   </param>
  </type>

但是当我试图实现这一点时，它给了我一个nullpointer异常

在下面一行：Node type = doc.getElementsByTagName("type").item(x); 这里的x确实给出了正确的位置，而type是xml中的元素。但类型为null。

这是方法的其余部分：

public void deleteObjType(int x) {

        System.out.println("index : " + x); // This one will return index : 122 in the console and it does 

        File file = new File("xmlFiles/CoreDatamodel.xml");
        DocumentBuilderFactory builderFactory = DocumentBuilderFactory
                .newInstance();
        try {
            DocumentBuilder builder = builderFactory.newDocumentBuilder();
            builder = builderFactory.newDocumentBuilder();
            Document doc = builder.parse(file);

            Node type = doc.getElementsByTagName("type").item(x);
            System.out.println("type : " + type); 

            NodeList nodes = type.getChildNodes();

            for (int i = 0; i < nodes.getLength(); i++) {

                Node element = nodes.item(x);
                System.out.println("element : " + element);

                // remove node
                if ("OBJECT_TYPE".equals(element.getNodeName())) {
                    type.removeChild(element);
                }

            }

            // Save the new update
            save(file, doc);

        } catch (ParserConfigurationException e) {

            e.printStackTrace();
        } catch (SAXException e) {

            e.printStackTrace();
        } catch (IOException e) {

            e.printStackTrace();
        } catch (XPathExpressionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

完整的堆栈跟踪：

java.lang.NullPointerException
    at cxmleditor.service.XMLEditorService.deleteObjType(XMLEditorService.java:106)
    at xmleditor.gui.XmlEditorMain$deleteObjType.actionPerformed(XmlEditorMain.java:383)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.AbstractButton.doClick(Unknown Source)
    at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
    at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(Unknown Source)
    at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

我认为您的问题如下：

for (int i = 0; i < nodes.getLength(); i++) {

请注意，删除节点时我不应该增加。

Answer 2

问题不一定是您的类型对象为null。当deleteObjType（0）;在你提供的简单xml上调用

System.out.println("type : " + type);

只使用类型对象实例的toString（）方法并打印出来：

type : [type: null]

如果您改为

System.out.println("type : " +type);
if(type == null) {
   System.out.println("type is null!");
}else {
  System.out.println("type is not null but an instance of ["+type.getClass().getCanonicalName()+"]");
 }

它打印在我的情况下：

type is not null but an instance of [com.sun.org.apache.xerces.internal.dom.DeferredElementImpl]

DOM也比具有子元素as you can see here的直接元素节点的元素节点复杂一点。我通过实践了解到，简单地打印节点通常是不够的。你真的想要删除xml的哪一部分？

删除因空值而无效的xml元素

2 个答案: