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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