为什么在运行StAX Parser时会出现NullPointerException？

问题描述

我试图用Java编写StAX XML解析器，但始终会收到NullPointerException错误。请帮我解决这个问题。完整问题：

线程“ main”中的异常java.lang.NullPointerException在 org.example.shoesshop.parser.STAXParser.parseXMLfile（STAXParser.java:68） 在org.example.shoesshop.parser.STAXParser.main（STAXParser.java:101）

这是StAX解析器的类：

public class STAXParser extends DefaultHandler {
    private static List<Shoes> parseXMLfile(String fileName){
        List<Shoes> shoesList = new ArrayList<>();
        Shoes shoes = null;
        XMLInputFactory xmlInputFactory = XMLInputFactory.newInstance();
        try {
            XMLEventReader reader = xmlInputFactory.createXMLEventReader(new FileInputStream(fileName));
            while (reader.hasNext()){
                XMLEvent xmlEvent = reader.nextEvent();
                if(xmlEvent.isstartElement()){
                    StartElement startElement = xmlEvent.asstartElement();
                    if(startElement.getName().getLocalPart().equals("Shoes")){
                        shoes = new Shoes();
                        Attribute idAttr = startElement.getAttributeByName(new QName("id"));
                        if(idAttr != null){
                            shoes.setId(Integer.parseInt(idAttr.getValue()));
                        }
                    } else if (startElement.getName().getLocalPart().equals("title")){
                        xmlEvent = reader.nextEvent();
                        shoes.setTitle(xmlEvent.asCharacters().getData()); // error line 68
                    } else if (startElement.getName().getLocalPart().equals("brand")){
                        xmlEvent = reader.nextEvent();
                        shoes.setBrand(Brand.fromValue(xmlEvent.asCharacters().getData()));
                    } else if (startElement.getName().getLocalPart().equals("category")){
                        xmlEvent = reader.nextEvent();
                        shoes.setCategory(Category.fromValue(xmlEvent.asCharacters().getData()));
                    } else if (startElement.getName().getLocalPart().equals("season")){
                        xmlEvent = reader.nextEvent();
                        shoes.setSeason(Season.fromValue(xmlEvent.asCharacters().getData()));
                    } else if (startElement.getName().getLocalPart().equals("price")){
                        xmlEvent = reader.nextEvent();
                        shoes.setPrice(Double.parseDouble(xmlEvent.asCharacters().getData()));
                    }
                }
                if(xmlEvent.isEndElement()){
                    EndElement endElement = xmlEvent.asEndElement();
                    if(endElement.getName().getLocalPart().equals("Shoes")){
                        shoesList.add(shoes);
                    }
                }
            }
        } catch (FileNotFoundException | XMLStreamException exc) {
            exc.printstacktrace();
        } return shoesList;
    }

    public static void main(String[] args) throws Exception {
        System.out.println("STAX Parser");
        System.out.println();
        System.out.println("Result: \n");
        System.out.println();
        String fileName = "ShoesShop.xml";
        List<Shoes> shoesList = parseXMLfile(fileName); //error line 101
        for (Shoes shoes:shoesList){
            System.out.println(shoes.toString());
        }
    }

}

这是一个XML文件

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type = "text/xsl" href = "ShoesShop.xsl"?>

<ss:ShoesShop xmlns:ss="http://www.example.org/ShoesShop" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://www.example.org/ShoesShop ShoesShop.xsd ">
  <ss:shoes id="1" stock="true">
    <ss:title>Baltrum</ss:title>
    <ss:brand>Gucci</ss:brand>
    <ss:category>Boots</ss:category>
    <ss:season>fall</ss:season>
    <ss:gender>
      <ss:male>male</ss:male>
    </ss:gender>
    <ss:details>
      <ss:highlights>Highlights text 1</ss:highlights>
      <ss:composition>Composition text 1</ss:composition>
    </ss:details>
    <ss:price>734.0</ss:price>
  </ss:shoes>
  
  
  <ss:shoes id="2" stock="true" mostWanted = "true">
    <ss:title>Amalfi</ss:title>
    <ss:brand>dior</ss:brand>
    <ss:category>Mules</ss:category>
    <ss:season>winter</ss:season>
    <ss:gender>
      <ss:female>female</ss:female>
    </ss:gender>
    <ss:details>
      <ss:highlights>Highlights text 2</ss:highlights>
      <ss:composition>Composition text 2</ss:composition>
    </ss:details>
    <ss:price>364.0</ss:price>
  </ss:shoes>
  
  <ss:shoes id="3" stock="true" mostWanted = "true">
    <ss:title>Korfu</ss:title>
    <ss:brand>Mary Katrantzou</ss:brand>
    <ss:category>sneakers</ss:category>
    <ss:season>spring</ss:season>
    <ss:gender>
      <ss:female>female</ss:female>
    </ss:gender>
    <ss:details>
      <ss:highlights>Highlights text 3</ss:highlights>
      <ss:composition>Composition text 3</ss:composition>
    </ss:details>
    <ss:price>173.0</ss:price>
  </ss:shoes>
</ss:ShoesShop>

这也是鞋类的java类

@XmlAccessorType(XmlAccesstype.FIELD)
@XmlType(name = "Shoes",propOrder = {
    "title","brand","category","season","gender","details","price"
})
public class Shoes
    extends Entity
{
    @XmlElement(required = true)
    protected String title;
    @XmlElement(required = true)
    @XmlSchemaType(name = "string")
    protected Brand brand;
    @XmlElement(required = true)
    @XmlSchemaType(name = "string")
    protected Category category;
    @XmlElement(required = true)
    @XmlSchemaType(name = "string")
    protected Season season;
    @XmlElement(required = true)
    protected Shoes.Gender gender;
    @XmlElement(required = true)
    protected Shoes.Details details;
    protected double price;
    @XmlAttribute(name = "stock",required = true)
    protected boolean stock;
    @XmlAttribute(name = "mostWanted")
    protected Boolean mostWanted;

    public String getTitle() {
        return title;
    }

    public void setTitle(String value) {
        this.title = value;
    }
    
    public Brand getBrand(){
        return brand;
    }
    
    public void setBrand(Brand value){
        this.brand = value;
    }
    
    public Category getCategory(){
        return category;
    }
    
    public void setCategory(Category value){
        this.category = value;
    }
    
    public Season getSeason(){
        return season;
    }
    
    public void setSeason(Season value) {
        this.season = value;
    }

    public Shoes.Gender getGender() {
        return gender;
    }

    public void setGender(Shoes.Gender value) {
        this.gender = value;
    }

    public Shoes.Details getDetails() {
        return details;
    }

    public void setDetails(Shoes.Details value) {
        this.details = value;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double value) {
        this.price = value;
    }

    public boolean isstock() {
        return stock;
    }

    public void setStock(boolean value) {
        this.stock = value;
    }

    public Boolean isMostWanted() {
        return mostWanted;
    }

    public void setMostWanted(Boolean value) {
        this.mostWanted = value;
    }

    @XmlAccessorType(XmlAccesstype.FIELD)
    @XmlType(name = "",propOrder = {

    })
    public static class Details {

        @XmlElement(required = true)
        protected String highlights;
        @XmlElement(required = true)
        protected String composition;

        public String getHighlights() {
            return highlights;
        }

        public void setHighlights(String value) {
            this.highlights = value;
        }

        public String getComposition() {
            return composition;
        }

        public void setComposition(String value) {
            this.composition = value;
        }

    }

    @XmlAccessorType(XmlAccesstype.FIELD)
    @XmlType(name = "",propOrder = {
        "\u043c\u0443\u0436\u0441\u043a\u043e\u0439Or\u0416\u0435\u043d\u0441\u043a\u0438\u0439"
    })
    public static class Gender {

        @XmlElementRefs({
            @XmlElementRef(name = "\u0436\u0435\u043d\u0441\u043a\u0438\u0439",namespace = "http://www.example.org/ShoesShop",type = JAXBElement.class,required = false),@XmlElementRef(name = "\u043c\u0443\u0436\u0441\u043a\u043e\u0439",required = false)
        })
        protected List<JAXBElement<String>> maleOrFemale;

        public List<JAXBElement<String>> getMaleOrFemale() {
            if (maleOrFemale == null) {
                maleOrFemale = new ArrayList<JAXBElement<String>>();
            }
            return this.maleOrFemale;
        }

    }
    
    @Override
    public String toString(){
        StringBuilder builder = new StringBuilder();
        builder.append("[title=");
        builder.append(title);
        builder.append(",brand=");
        builder.append(brand);
        builder.append(",category=");
        builder.append(category);
        builder.append(",season=");
        builder.append(season);
        builder.append(",price=");
        builder.append(price);
        builder.append("]");
        return builder.toString();
    }

}

我还需要知道如何将接收到的数据写入新的XML文件。

解决方法

已更新： 对原始答案的评论：

它不起作用，给出相同的错误

这意味着问题是因为shoes变量为null，这在 debugger 中很容易看到。使用调试器可以节省很多时间，所以请开始使用调试器。

为了使shoes为null，似乎代码遇到了<title>元素，它不是Shoes元素的子元素。

要修复代码，请添加空检查，并在处理shoes = null元素的最后设置Shoes：

} else if (startElement.getName().getLocalPart().equals("title")) {
    if (shoes != null) { // <===== ADD THIS
        shoes.setTitle(reader.getElementText()); // <===== Fix this (see original answer)
    }

if (xmlEvent.isEndElement()) {
    EndElement endElement = xmlEvent.asEndElement();
    if (endElement.getName().getLocalPart().equals("Shoes")) {
        shoesList.add(shoes);
        shoes = null; // <===== ADD THIS
    }
}

---

原始答案

您的代码是：

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

问题在于，如果事件紧跟START_ELEMENT事件之后，代码没有检查哪种类型。可能是这样：

• 该元素很可能为空，即<title/>或<title><title/>，在这种情况下，下一个事件为END_ELEMENT，并且asCharacters()返回了{{ 1}}。

• 该元素具有注释，例如null，在这种情况下，下一个事件是<title><!-- there is no title --><title/>。

• 元素具有混合的内容，例如COMMENT，在这种情况下，下一个事件不是全文。

检索元素的文本内容非常常见，他们为此添加了一个辅助方法：getElementText()：

读取纯文本元素的内容。前提条件：当前事件为<title>foo<![CDATA[bar]]><title/>。后置条件：当前事件是对应的START_ELEMENT。

抛出：
END_ELEMENT-如果当前事件不是XMLStreamException或遇到非文本元素

这意味着您的代码应为：

START_ELEMENT