Java xml出现错误 javax.xml.transform.TransformerException: java.lang.NullPointerException

Java xml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 
TransformerFactory tFactory =TransformerFactory.newInstance(); 
Transformer transformer = tFactory.newTransformer(); 
//需要转换的内存中XML源文件 
DOMSource source = new DOMSource(xmlDoc1); 
//生成的xml文件 
File xmlDoc2 = new File(outPutPath+generateXmlFileName); 
StreamResult result = new StreamResult(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
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
m(TransformerImpl.java:717)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
m(TransformerImpl.java:313)
    at operation.AddNeuronID.addNeuronId(AddNeuronID.java:273)
    at operationUI.AddNeuronIDListener.constructor2Perform(AddNeuronIDListen
er.java:245)
    at operationUI.AddNeuronIDListener.actionPerformed(AddNeuronIDListener.j
ava:85)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:19
95)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.jav
a:2318)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel
.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242
)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL
istener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6041)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
    at java.awt.Component.processEvent(Component.java:5806)
    at java.awt.Container.processEvent(Container.java:2058)
    at java.awt.Component.dispatchEventImpl(Component.java:4413)
    at java.awt.Container.dispatchEventImpl(Container.java:2116)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322
)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
    at java.awt.Container.dispatchEventImpl(Container.java:2102)
    at java.awt.Window.dispatchEventImpl(Window.java:2440)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:273)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:183)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:173)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
Caused by: java.lang.NullPointerException
    at com.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters
(ToUnknownStream.java:317)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:240)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:132)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:94)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
mIdentity(TransformerImpl.java:662)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
m(TransformerImpl.java:708)
    ... 29 more
---------
java.lang.NullPointerException
    at com.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters
(ToUnknownStream.java:317)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:240)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:226)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:132)
    at com.sun.org.apache.xalan.internal.xsltc.trax.DOM2TO.parse(DOM2TO.java
:94)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
mIdentity(TransformerImpl.java:662)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
m(TransformerImpl.java:708)
    at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transfor
m(TransformerImpl.java:313)
    at operation.AddNeuronID.addNeuronId(AddNeuronID.java:273)
    at operationUI.AddNeuronIDListener.constructor2Perform(AddNeuronIDListen
er.java:245)
    at operationUI.AddNeuronIDListener.actionPerformed(AddNeuronIDListener.j
ava:85)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:19
95)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.jav
a:2318)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel
.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242
)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL
istener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6041)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
    at java.awt.Component.processEvent(Component.java:5806)
    at java.awt.Container.processEvent(Container.java:2058)
    at java.awt.Component.dispatchEventImpl(Component.java:4413)
    at java.awt.Container.dispatchEventImpl(Container.java:2116)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322
)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
    at java.awt.Container.dispatchEventImpl(Container.java:2102)
    at java.awt.Window.dispatchEventImpl(Window.java:2440)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:273)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:183)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:173)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
    at java.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
at com.sun.org.apache.xml.internal.serializer.ToUnknownStream.characters(ToUnknownStream.java:317)
at com.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

/** 
  * Converts the String to a character array and calls the SAX method 
  * characters(char[],int,int); 
  * 
  * @see ExtendedContentHandler#characters(String) 
  */ 
  public void characters(String chars) throws SAXException 
  { 
    final int length = chars.length(); 
    if (length > m_charsBuff.length) 
    { 
      m_charsBuff = new char[length*2 + 1]; 
    } 
    chars.getChars(0, length, m_charsBuff, 0); 
    this.characters(m_charsBuff, 0, length);  
  }  

       很明显，注意int length = 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）:

case Node.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(String str)或setTextContent(String str)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.

String neuronIdStr = 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：

String neuronIdStr = deviceAndIDMap.get(nameContent); 
if(neuronIdStr == null) 
{ 
   continue; 
} 
//更新 
neuronIdElem.getFirstChild().setNodeValue(neuronIdStr); 
               

    问题解决！

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