Java xml出现错误 javax.xml.transform.TransformerException: java.lang.NullPointerException
Javaxml出现错误javax.xml.transform.TransformerException:java.lang.NullPointerException解决办法：
利用Java操作XML，在操作XML过程中，执行到最后一步，在利用Transformer进行XML转换时出现NullPointerException错误，出问题的部分代码如下：
//转换 TransformerFactorytFactory=TransformerFactory.newInstance(); Transformertransformer=tFactory.newTransformer(); //需要转换的内存中XML源文件 DOMSourcesource=newDOMSource(xmlDoc1); //生成的xml文件 FilexmlDoc2=newFile(outPutPath+generateXmlFileName); StreamResultresult=newStreamResult(xmlDoc2); //转换 transformer.transform(source,result);
运行到transform函数时出现以下错误（比较长，由于我是利用SWING设计图形界面的，所以会有一些图形界面事件调用的错误）：
javax.xml.transform.TransformerException:java.lang.NullPointerException atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor m(TransformerImpl.java:717) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor m(TransformerImpl.java:313) atoperation.AddNeuronID.addNeuronId(AddNeuronID.java:273) atoperationUI.AddNeuronIDListener.constructor2Perform(AddNeuronIDListen er.java:245) atoperationUI.AddNeuronIDListener.actionPerformed(AddNeuronIDListener.j ava:85) atjavax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:19 95) atjavax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.jav a:2318) atjavax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel .java:387) atjavax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242 ) atjavax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL istener.java:236) atjava.awt.Component.processMouseEvent(Component.java:6041) atjavax.swing.JComponent.processMouseEvent(JComponent.java:3265) atjava.awt.Component.processEvent(Component.java:5806) atjava.awt.Container.processEvent(Container.java:2058) atjava.awt.Component.dispatchEventImpl(Component.java:4413) atjava.awt.Container.dispatchEventImpl(Container.java:2116) atjava.awt.Component.dispatchEvent(Component.java:4243) atjava.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322 ) atjava.awt.LightweightDispatcher.processMouseEvent(Container.java:3986) atjava.awt.LightweightDispatcher.dispatchEvent(Container.java:3916) atjava.awt.Container.dispatchEventImpl(Container.java:2102) atjava.awt.Window.dispatchEventImpl(Window.java:2440) atjava.awt.Component.dispatchEvent(Component.java:4243) atjava.awt.EventQueue.dispatchEvent(EventQueue.java:599) atjava.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre ad.java:273) atjava.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread. java:183) atjava.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre ad.java:173) atjava.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168) atjava.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160) atjava.awt.EventDispatchThread.run(EventDispatchThread.java:121) Causedby:java.lang.NullPointerException atcom.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters (ToUnknownStream.java:317) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :240) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :132) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :94) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor mIdentity(TransformerImpl.java:662) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor m(TransformerImpl.java:708) ...29more --------- java.lang.NullPointerException atcom.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters (ToUnknownStream.java:317) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :240) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :226) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :132) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java :94) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor mIdentity(TransformerImpl.java:662) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor m(TransformerImpl.java:708) atcom.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor m(TransformerImpl.java:313) atoperation.AddNeuronID.addNeuronId(AddNeuronID.java:273) atoperationUI.AddNeuronIDListener.constructor2Perform(AddNeuronIDListen er.java:245) atoperationUI.AddNeuronIDListener.actionPerformed(AddNeuronIDListener.j ava:85) atjavax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:19 95) atjavax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.jav a:2318) atjavax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel .java:387) atjavax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242 ) atjavax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL istener.java:236) atjava.awt.Component.processMouseEvent(Component.java:6041) atjavax.swing.JComponent.processMouseEvent(JComponent.java:3265) atjava.awt.Component.processEvent(Component.java:5806) atjava.awt.Container.processEvent(Container.java:2058) atjava.awt.Component.dispatchEventImpl(Component.java:4413) atjava.awt.Container.dispatchEventImpl(Container.java:2116) atjava.awt.Component.dispatchEvent(Component.java:4243) atjava.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322 ) atjava.awt.LightweightDispatcher.processMouseEvent(Container.java:3986) atjava.awt.LightweightDispatcher.dispatchEvent(Container.java:3916) atjava.awt.Container.dispatchEventImpl(Container.java:2102) atjava.awt.Window.dispatchEventImpl(Window.java:2440) atjava.awt.Component.dispatchEvent(Component.java:4243) atjava.awt.EventQueue.dispatchEvent(EventQueue.java:599) atjava.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre ad.java:273) atjava.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread. java:183) atjava.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre ad.java:173) atjava.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168) atjava.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160) atjava.awt.EventDispatchThread.run(EventDispatchThread.java:121)
一开始我也感到奇怪，transform函数只是把在内存中的XML树转换成文件，为何会出现nullPointer错误呢。上网查了一下，没什么人找到答案，最后在（http://blog.awe.cz/post/english/beware-of-null-text-nodes/）这个blog中找到相当有用的答案.
其实认真看看出错的信息，会发现有个信息比较重要：
java.lang.NullPointerException atcom.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters(ToUnknownStream.java:317) atcom.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java:240)
从这里可以看出是由于com.sun.org.apache.xml.internal.serializer.ToUnknownStream.java文件中的characters函数出错，所在行数是317,到jdk安装目录，找到src，到相应的文件夹serializer中找到ToUnknownStream.java，找到以下函数
/**
*ConvertstheStringtoacharacterarrayandcallstheSAXmethod
*characters(char[],int,int);
*
*@seeExtendedContentHandler#characters(String)
*/
publicvoidcharacters(Stringchars)throwsSAXException
{
finalintlength=chars.length();
if(length>m_charsBuff.length)
{
m_charsBuff=newchar[length*2+1];
}
chars.getChars(0,length,m_charsBuff,0);
this.characters(m_charsBuff,0,length);
}
很明显，注意intlength=chars.length();如果参数chars是null的话，调用length函数就会出现nullpoiterException错误。再上一层，com.sun.org.apache.xalan.internal.xsltc.trax下的DOM2TO.java文件的parse函数（240行）:
caseNode.TEXT_NODE: _handler.characters(node.getNodeValue()); break;
在这里，就可以知道characters函数为什么出现null参数了。原因是此结点是文本结点TEXT_NODE,而调用本结点中没有内容(null),当调用getNodeValue时，返回null.
上升到transform函数，出现结点内容为null，那在DOMSource类中，由于它是在内存中建立起来的XML树，所以肯定是这个XML树中的有一个或多个元素或结点的内容为null。
OK，问题原因找到。解决办法很简单，DOMSource中有元素或结点为null，那一定是在操作XML时，或者是修改内容，添加元素等等操作，使内容变为null。所以现在需要做的就是找到修改或添加元素内容的代码，把有可能出现null的情况进行处理，如果检测到为null，则不修改XML或不添加此元素。最简单的方法就是用if语句。即setNodeValue(Stringstr)或setTextContent(Stringstr)之前，先查看参数str是否为空（if(str==null)），如果空则不调用此函数。
而在我的程序中，确实我是把空的内容加入到元素中。代码如下：deviceAndIDMap是一个HashMap，当它调用get时，不存在此主键时，会返回null.
StringneuronIdStr=deviceAndIDMap.get(nameContent); //更新 neuronIdElem.getFirstChild().setNodeValue(neuronIdStr);
因此我加一下if语句，就可以解决问题了，代码（此代码是在for循环中的，因此用coninue来跳过本次的修改）如下：
StringneuronIdStr=deviceAndIDMap.get(nameContent);
if(neuronIdStr==null)
{
continue;
}
//更新
neuronIdElem.getFirstChild().setNodeValue(neuronIdStr);
问题解决！
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