为什么我在运行StAX解析器时会得到空指针异常？

huangapple
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英文:

Why I get NullPointerException when running StAX Parser?

问题

  1. 我试图在Java中编写一个StAX XML解析器，但总是遇到NullPointerException错误。请帮我解决这个问题。完整的问题：
  3. > Exception in thread "main" java.lang.NullPointerException
  4. > 	at org.example.shoesshop.parser.STAXParser.parseXMLfile(STAXParser.java:68)
  5. > 	at org.example.shoesshop.parser.STAXParser.main(STAXParser.java:101)
  7. 这是一个用于StAX解析器的类：
  9. public class STAXParser extends DefaultHandler {
  10.     private static List<Shoes> parseXMLfile(String fileName) {
  11.         // ...
  12.         //（原始代码）
  13.         // ...
  14.     }
  16.     public static void main(String[] args) throws Exception {
  17.         // ...
  18.         //（原始代码）
  19.         // ...
  20.     }
  21. }
  23. 这是一个XML文件的示例：
  25. <?xml version="1.0" encoding="UTF-8"?>
  26. <!--（略）-->
  27. </ss:ShoesShop>
  29. 这是一个用于鞋子的Java类：
  31. @XmlAccessorType(XmlAccessType.FIELD)
  32. @XmlType(name = "Shoes", propOrder = {
  33.     "title",
  34.     "brand",
  35.     //（略）
  36. })
  37. public class Shoes extends Entity {
  38.     // ...
  39.     //（原始代码）
  40.     // ...
  41. }
  43. 我还需要知道如何将接收到的数据写入新的XML文件。

如果您有关于如何将接收到的数据写入新的XML文件的更多问题，请随时提问。

英文:

I'm trying to write a StAX XML Parser in Java, but always get NullPointerException error. Please help me to solve this issue. Full problem:

> Exception in thread "main" java.lang.NullPointerException at
> org.example.shoesshop.parser.STAXParser.parseXMLfile(STAXParser.java:68)
> at org.example.shoesshop.parser.STAXParser.main(STAXParser.java:101)

Here's a class for StAX Parser:

  1. public class STAXParser extends DefaultHandler {
  2. 	private static List&lt;Shoes&gt; parseXMLfile(String fileName){
  3. 		List&lt;Shoes&gt; shoesList = new ArrayList&lt;&gt;();
  4. 		Shoes shoes = null;
  5. 		XMLInputFactory xmlInputFactory = XMLInputFactory.newInstance();
  6. 		try {
  7. 			XMLEventReader reader = xmlInputFactory.createXMLEventReader(new FileInputStream(fileName));
  8. 			while (reader.hasNext()){
  9. 				XMLEvent xmlEvent = reader.nextEvent();
  10. 				if(xmlEvent.isStartElement()){
  11. 					StartElement startElement = xmlEvent.asStartElement();
  12. 					if(startElement.getName().getLocalPart().equals(&quot;Shoes&quot;)){
  13. 						shoes = new Shoes();
  14. 						Attribute idAttr = startElement.getAttributeByName(new QName(&quot;id&quot;));
  15. 						if(idAttr != null){
  16. 							shoes.setId(Integer.parseInt(idAttr.getValue()));
  17. 						}
  18. 					} else if (startElement.getName().getLocalPart().equals(&quot;title&quot;)){
  19. 						xmlEvent = reader.nextEvent();
  20. 						shoes.setTitle(xmlEvent.asCharacters().getData()); // error line 68
  21. 					} else if (startElement.getName().getLocalPart().equals(&quot;brand&quot;)){
  22. 						xmlEvent = reader.nextEvent();
  23. 						shoes.setBrand(Brand.fromValue(xmlEvent.asCharacters().getData()));
  24. 					} else if (startElement.getName().getLocalPart().equals(&quot;category&quot;)){
  25. 						xmlEvent = reader.nextEvent();
  26. 						shoes.setCategory(Category.fromValue(xmlEvent.asCharacters().getData()));
  27. 					} else if (startElement.getName().getLocalPart().equals(&quot;season&quot;)){
  28. 						xmlEvent = reader.nextEvent();
  29. 						shoes.setSeason(Season.fromValue(xmlEvent.asCharacters().getData()));
  30. 					} else if (startElement.getName().getLocalPart().equals(&quot;price&quot;)){
  31. 						xmlEvent = reader.nextEvent();
  32. 						shoes.setPrice(Double.parseDouble(xmlEvent.asCharacters().getData()));
  33. 					}
  34. 				}
  35. 				if(xmlEvent.isEndElement()){
  36. 					EndElement endElement = xmlEvent.asEndElement();
  37. 					if(endElement.getName().getLocalPart().equals(&quot;Shoes&quot;)){
  38. 						shoesList.add(shoes);
  39. 					}
  40. 				}
  41. 			}
  42. 		} catch (FileNotFoundException | XMLStreamException exc) {
  43. 			exc.printStackTrace();
  44. 		} return shoesList;
  45. 	}
  47. 	public static void main(String[] args) throws Exception {
  48. 		System.out.println(&quot;STAX Parser&quot;);
  49. 		System.out.println();
  50. 		System.out.println(&quot;Result: \n&quot;);
  51. 		System.out.println();
  52. 		String fileName = &quot;ShoesShop.xml&quot;;
  53. 		List&lt;Shoes&gt; shoesList = parseXMLfile(fileName); //error line 101
  54. 		for (Shoes shoes:shoesList){
  55. 			System.out.println(shoes.toString());
  56. 		}
  57. 	}
  59. }

Here's an XML-file

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
  2. &lt;?xml-stylesheet type = &quot;text/xsl&quot; href = &quot;ShoesShop.xsl&quot;?&gt;
  4. &lt;ss:ShoesShop xmlns:ss=&quot;http://www.example.org/ShoesShop&quot; 
  5. xmlns:xsi=&quot;http://www.w3.org/2001/XMLSchema-instance&quot; 
  6. xsi:schemaLocation=&quot;http://www.example.org/ShoesShop ShoesShop.xsd &quot;&gt;
  7.   &lt;ss:shoes id=&quot;1&quot; stock=&quot;true&quot;&gt;
  8.     &lt;ss:title&gt;Baltrum&lt;/ss:title&gt;
  9.     &lt;ss:brand&gt;Gucci&lt;/ss:brand&gt;
  10.     &lt;ss:category&gt;Boots&lt;/ss:category&gt;
  11.     &lt;ss:season&gt;fall&lt;/ss:season&gt;
  12.     &lt;ss:gender&gt;
  13.       &lt;ss:male&gt;male&lt;/ss:male&gt;
  14.     &lt;/ss:gender&gt;
  15.     &lt;ss:details&gt;
  16.       &lt;ss:highlights&gt;Highlights text 1&lt;/ss:highlights&gt;
  17.       &lt;ss:composition&gt;Composition text 1&lt;/ss:composition&gt;
  18.     &lt;/ss:details&gt;
  19.     &lt;ss:price&gt;734.0&lt;/ss:price&gt;
  20.   &lt;/ss:shoes&gt;
  21.   
  22.   
  23.   &lt;ss:shoes id=&quot;2&quot; stock=&quot;true&quot; mostWanted = &quot;true&quot;&gt;
  24.     &lt;ss:title&gt;Amalfi&lt;/ss:title&gt;
  25.     &lt;ss:brand&gt;Dior&lt;/ss:brand&gt;
  26.     &lt;ss:category&gt;Mules&lt;/ss:category&gt;
  27.     &lt;ss:season&gt;winter&lt;/ss:season&gt;
  28.     &lt;ss:gender&gt;
  29.       &lt;ss:female&gt;female&lt;/ss:female&gt;
  30.     &lt;/ss:gender&gt;
  31.     &lt;ss:details&gt;
  32.       &lt;ss:highlights&gt;Highlights text 2&lt;/ss:highlights&gt;
  33.       &lt;ss:composition&gt;Composition text 2&lt;/ss:composition&gt;
  34.     &lt;/ss:details&gt;
  35.     &lt;ss:price&gt;364.0&lt;/ss:price&gt;
  36.   &lt;/ss:shoes&gt;
  37.   
  38.   &lt;ss:shoes id=&quot;3&quot; stock=&quot;true&quot; mostWanted = &quot;true&quot;&gt;
  39.     &lt;ss:title&gt;Korfu&lt;/ss:title&gt;
  40.     &lt;ss:brand&gt;Mary Katrantzou&lt;/ss:brand&gt;
  41.     &lt;ss:category&gt;Sneakers&lt;/ss:category&gt;
  42.     &lt;ss:season&gt;spring&lt;/ss:season&gt;
  43.     &lt;ss:gender&gt;
  44.       &lt;ss:female&gt;female&lt;/ss:female&gt;
  45.     &lt;/ss:gender&gt;
  46.     &lt;ss:details&gt;
  47.       &lt;ss:highlights&gt;Highlights text 3&lt;/ss:highlights&gt;
  48.       &lt;ss:composition&gt;Composition text 3&lt;/ss:composition&gt;
  49.     &lt;/ss:details&gt;
  50.     &lt;ss:price&gt;173.0&lt;/ss:price&gt;
  51.   &lt;/ss:shoes&gt;
  52. &lt;/ss:ShoesShop&gt;

Also here's a Java class for Shoes

  1. @XmlAccessorType(XmlAccessType.FIELD)
  2. @XmlType(name = &quot;Shoes&quot;, propOrder = {
  3.     &quot;title&quot;,
  4.     &quot;brand&quot;,
  5.     &quot;category&quot;,
  6.     &quot;season&quot;,
  7.     &quot;gender&quot;,
  8.     &quot;details&quot;,
  9.     &quot;price&quot;
  10. })
  11. public class Shoes
  12.     extends Entity
  13. {
  14.     @XmlElement(required = true)
  15.     protected String title;
  16.     @XmlElement(required = true)
  17.     @XmlSchemaType(name = &quot;string&quot;)
  18.     protected Brand brand;
  19.     @XmlElement(required = true)
  20.     @XmlSchemaType(name = &quot;string&quot;)
  21.     protected Category category;
  22.     @XmlElement(required = true)
  23.     @XmlSchemaType(name = &quot;string&quot;)
  24.     protected Season season;
  25.     @XmlElement(required = true)
  26.     protected Shoes.Gender gender;
  27.     @XmlElement(required = true)
  28.     protected Shoes.Details details;
  29.     protected double price;
  30.     @XmlAttribute(name = &quot;stock&quot;, required = true)
  31.     protected boolean stock;
  32.     @XmlAttribute(name = &quot;mostWanted&quot;)
  33.     protected Boolean mostWanted;
  35.     public String getTitle() {
  36.         return title;
  37.     }
  39.     public void setTitle(String value) {
  40.         this.title = value;
  41.     }
  42.     
  43.     public Brand getBrand(){
  44.     	return brand;
  45.     }
  46.     
  47.     public void setBrand(Brand value){
  48.     	this.brand = value;
  49.     }
  50.     
  51.     public Category getCategory(){
  52.     	return category;
  53.     }
  54.     
  55.     public void setCategory(Category value){
  56.     	this.category = value;
  57.     }
  58.     
  59.     public Season getSeason(){
  60.     	return season;
  61.     }
  62.     
  63.     public void setSeason(Season value) {
  64.     	this.season = value;
  65.     }
  67.     public Shoes.Gender getGender() {
  68.         return gender;
  69.     }
  71.     public void setGender(Shoes.Gender value) {
  72.         this.gender = value;
  73.     }
  75.     public Shoes.Details getDetails() {
  76.         return details;
  77.     }
  79.     public void setDetails(Shoes.Details value) {
  80.         this.details = value;
  81.     }
  83.     public double getPrice() {
  84.         return price;
  85.     }
  87.     public void setPrice(double value) {
  88.         this.price = value;
  89.     }
  91.     public boolean isStock() {
  92.         return stock;
  93.     }
  95.     public void setStock(boolean value) {
  96.         this.stock = value;
  97.     }
  99.     public Boolean isMostWanted() {
  100.         return mostWanted;
  101.     }
  103.     public void setMostWanted(Boolean value) {
  104.         this.mostWanted = value;
  105.     }
  107.     @XmlAccessorType(XmlAccessType.FIELD)
  108.     @XmlType(name = &quot;&quot;, propOrder = {
  110.     })
  111.     public static class Details {
  113.         @XmlElement(required = true)
  114.         protected String highlights;
  115.         @XmlElement(required = true)
  116.         protected String composition;
  118.         public String getHighlights() {
  119.             return highlights;
  120.         }
  122.         public void setHighlights(String value) {
  123.             this.highlights = value;
  124.         }
  126.         public String getComposition() {
  127.             return composition;
  128.         }
  130.         public void setComposition(String value) {
  131.             this.composition = value;
  132.         }
  134.     }
  136.     @XmlAccessorType(XmlAccessType.FIELD)
  137.     @XmlType(name = &quot;&quot;, propOrder = {
  138.         &quot;\u043c\u0443\u0436\u0441\u043a\u043e\u0439Or\u0416\u0435\u043d\u0441\u043a\u0438\u0439&quot;
  139.     })
  140.     public static class Gender {
  142.         @XmlElementRefs({
  143.             @XmlElementRef(name = &quot;\u0436\u0435\u043d\u0441\u043a\u0438\u0439&quot;, namespace = &quot;http://www.example.org/ShoesShop&quot;, type = JAXBElement.class, required = false),
  144.             @XmlElementRef(name = &quot;\u043c\u0443\u0436\u0441\u043a\u043e\u0439&quot;, namespace = &quot;http://www.example.org/ShoesShop&quot;, type = JAXBElement.class, required = false)
  145.         })
  146.         protected List&lt;JAXBElement&lt;String&gt;&gt; maleOrFemale;
  148.         public List&lt;JAXBElement&lt;String&gt;&gt; getMaleOrFemale() {
  149.             if (maleOrFemale == null) {
  150.                 maleOrFemale = new ArrayList&lt;JAXBElement&lt;String&gt;&gt;();
  151.             }
  152.             return this.maleOrFemale;
  153.         }
  155.     }
  156.     
  157.     @Override
  158.     public String toString(){
  159.     	StringBuilder builder = new StringBuilder();
  160.     	builder.append(&quot;[title=&quot;);
  161.     	builder.append(title);
  162.     	builder.append(&quot;, brand=&quot;);
  163.     	builder.append(brand);
  164.     	builder.append(&quot;, category=&quot;);
  165.     	builder.append(category);
  166.     	builder.append(&quot;, season=&quot;);
  167.     	builder.append(season);
  168.     	builder.append(&quot;, price=&quot;);
  169.     	builder.append(price);
  170.     	builder.append(&quot;]&quot;);
  171.     	return builder.toString();
  172.     }
  174. }

Also I need to know how to write a received data into a new XML-file.

答案1

得分: 1

UPDATED: 对原回答的评论：

它不起作用，仍然显示相同的错误

这意味着问题是因为 shoes 变量是 null，很容易通过调试器看到。使用调试器可以节省我们很多时间，所以请开始使用一个。

为了让 shoes 变量为 null，看起来代码遇到了一个不是 Shoes 元素的子元素的 <title> 元素。

要修复代码，在处理完 Shoes 元素后，添加一个空值检查，并且在代码末尾设置 shoes = null：

  1. } else if (startElement.getName().getLocalPart().equals("title")) {
  2.     if (shoes != null) { // <===== 添加这一行
  3.         shoes.setTitle(reader.getElementText()); // <===== 修复这一行（参见原始回答）
  4.     }
  5. }
  1. if (xmlEvent.isEndElement()) {
  2.     EndElement endElement = xmlEvent.asEndElement();
  3.     if (endElement.getName().getLocalPart().equals("Shoes")) {
  4.         shoesList.add(shoes);
  5.         shoes = null; // <===== 添加这一行
  6.     }
  7. }

原始回答

您的代码是：

  1. } else if (startElement.getName().getLocalPart().equals("title")){
  2.     xmlEvent = reader.nextEvent();
  3.     shoes.setTitle(xmlEvent.asCharacters().getData());

问题是代码没有检查 START_ELEMENT 事件之后的事件类型。可能是：

  • 最有可能的情况是元素是空的，即 &lt;title/&gt; 或 &lt;title&gt;&lt;title/&gt;，在这种情况下，下一个事件是 END_ELEMENT，而 asCharacters() 返回了 null。

  • 该元素有注释，例如 &lt;title&gt;&lt;!-- 没有标题 --&gt;&lt;title/&gt;，在这种情况下，下一个事件是 COMMENT。

  • 该元素具有混合内容，例如 &lt;title&gt;foo&lt;![CDATA[bar]]&gt;&lt;title/&gt;，在这种情况下，下一个事件不是完整的文本。

检索元素的文本内容是一种常见的操作，因此他们为此添加了一个辅助方法：getElementText()：

读取仅包含文本的元素的内容。前提条件：当前事件是 START_ELEMENT。后置条件：当前事件是相应的 END_ELEMENT。

抛出：
XMLStreamException - 如果当前事件不是 START_ELEMENT，或者如果遇到非文本元素

这意味着您的代码应该是：

  1. } else if (startElement.getName().getLocalPart().equals("title")) {
  2.     shoes.setTitle(reader.getElementText());
英文:

UPDATED: Comment to original answer:

> It doesn't work, it gives the same error

That means the problem is because the shoes variable is null, as would have easily been seen with a debugger. Using a debugger would have saved us all a lot of time, so please start using one.

In order for shoes to be null, it appears that the code encountered a &lt;title&gt; element that is not a child of a Shoes element.

To fix the code, add a null-check, and also set shoes = null at the end of processing the Shoes element:

  1. } else if (startElement.getName().getLocalPart().equals(&quot;title&quot;)) {
  2.     if (shoes != null) { // &lt;===== ADD THIS
  3.         shoes.setTitle(reader.getElementText()); // &lt;===== Fix this (see original answer)
  4.     }
  1. if (xmlEvent.isEndElement()) {
  2.     EndElement endElement = xmlEvent.asEndElement();
  3.     if (endElement.getName().getLocalPart().equals(&quot;Shoes&quot;)) {
  4.         shoesList.add(shoes);
  5.         shoes = null; // &lt;===== ADD THIS
  6.     }
  7. }

ORIGINAL ANSWER

Your code is:

  1. } else if (startElement.getName().getLocalPart().equals(&quot;title&quot;)){
  2.     xmlEvent = reader.nextEvent();
  3.     shoes.setTitle(xmlEvent.asCharacters().getData());

The problem is that the code isn't checking what type if event follows the START_ELEMENT event. It could be that:

  • Most likely, the element is empty, i.e. &lt;title/&gt; or &lt;title&gt;&lt;title/&gt;, in which case the next event is an END_ELEMENT, and asCharacters() returned null.

  • The element has a comment, e.g. &lt;title&gt;&lt;!-- there is no title --&gt;&lt;title/&gt;, in which case the next event is a COMMENT.

  • The element has mixed content, e.g. &lt;title&gt;foo&lt;![CDATA[bar]]&gt;&lt;title/&gt;, in which case the next event is not the full text.

Retrieving the text content of an element is such a common thing that they added a helper method for that: getElementText():

> Reads the content of a text-only element. Precondition: the current event is START_ELEMENT. Postcondition: The current event is the corresponding END_ELEMENT.
>
> Throws:
> XMLStreamException - if the current event is not a START_ELEMENT or if a non text element is encountered

Which means that your code should be:

  1. } else if (startElement.getName().getLocalPart().equals(&quot;title&quot;)) {
  2.     shoes.setTitle(reader.getElementText());

huangapple
  • 本文由 发表于 2020年10月12日 06:28:08
  • 转载请务必保留本文链接：https://go.coder-hub.com/64309600.html
  • java
  • parsing
  • stax
  • xml
  • xml-parsing
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