java xml出错,Java xml出现错误 javax.xml.transform.TransformerException: java.lang.NullPointerException...
Java xml出現錯誤 javax.xml.transform.TransformerException: java.lang.NullPointerException解決辦法：
利用Java操作XML，在操作XML過程中，執行到最后一步，在利用Transformer進行XML轉換時出現NullPointerException錯誤，出問題的部分代碼如下：
//轉換
TransformerFactory tFactory =TransformerFactory.newInstance();
Transformer transformer = tFactory.newTransformer();
//需要轉換的內存中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);
//轉換
transformer.transform(source, result);
運行到transform函數時出現以下錯誤(比較長，由于我是利用SWING設計圖形界面的，所以會有一些圖形界面事件調用的錯誤)：
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)
一開始我也感到奇怪，transform函數只是把在內存中的XML樹轉換成文件，為何會出現nullPointer錯誤呢。上網查了一下，沒什么人找到答案，最后在(http://blog.awe.cz/post/english/beware-of-null-text-nodes/)這個blog中找到相當有用的答案.
其實認真看看出錯的信息，會發現有個信息比較重要：
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)
從這里可以看出是由于com.sun.org.apache.xml.internal.serializer.ToUnknownStream.java文件中的characters函數出錯，所在行數是317,到jdk安裝目錄，找到src，到相應的文件夾serializer中找到ToUnknownStream.java，找到以下函數
/**
* 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);
}
很明顯，注意int length = chars.length(); 如果參數chars是null的話，調用length函數就會出現nullpoiterException錯誤。再上一層，com.sun.org.apache.xalan.internal.xsltc.trax下的DOM2TO.java文件的parse函數(240行):
case Node.TEXT_NODE:
_handler.characters(node.getNodeValue());
break;
在這里，就可以知道characters函數為什么出現null參數了。原因是此結點是文本結點TEXT_NODE,而調用本結點中沒有內容(null),當調用getNodeValue時，返回null.
上升到transform函數，出現結點內容為null，那在DOMSource類中，由于它是在內存中建立起來的XML樹，所以肯定是這個XML樹中的有一個或多個元素或結點的內容為null。
OK，問題原因找到。解決辦法很簡單，DOMSource中有元素或結點為null，那一定是在操作XML時，或者是修改內容，添加元素等等操作，使內容變為null。所以現在需要做的就是找到修改或添加元素內容的代碼，把有可能出現null的情況進行處理，如果檢測到為null，則不修改XML或不添加此元素。最簡單的方法就是用if語句。即setNodeValue(String str)或setTextContent(String str)之前，先查看參數str是否為空(if(str == null))，如果空則不調用此函數。
而在我的程序中，確實我是把空的內容加入到元素中。代碼如下：deviceAndIDMap是一個HashMap，當它調用get時，不存在此主鍵時，會返回null.
String neuronIdStr = deviceAndIDMap.get(nameContent);
//更新
neuronIdElem.getFirstChild().setNodeValue(neuronIdStr);
因此我加一下if語句，就可以解決問題了，代碼(此代碼是在for循環中的，因此用coninue來跳過本次的修改)如下：
String neuronIdStr = deviceAndIDMap.get(nameContent);
if(neuronIdStr == null)
{
continue;
}
//更新
neuronIdElem.getFirstChild().setNodeValue(neuronIdStr);
問題解決！
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