为什么在运行 StAX Parser 时出现 NullPointerException?

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问题描述

我正在尝试用 Java 编写 StAX XML 解析器，但总是出现 NullPointerException 错误.请帮我解决这个问题.完整问题:

<块引用>

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 Parser 的一个类:

公共类 STAXParser 扩展 DefaultHandler {私有静态列表<鞋子>parseXMLfile(字符串文件名){列出<鞋子>shoesList = new ArrayList<>();鞋鞋 = null;XMLInputFactory xmlInputFactory = XMLInputFactory.newInstance();尝试 {XMLEventReader reader = xmlInputFactory.createXMLEventReader(new FileInputStream(fileName));while (reader.hasNext()){XMLEvent xmlEvent = reader.nextEvent();如果(xmlEvent.isStartElement()){StartElement startElement = xmlEvent.asStartElement();if(startElement.getName().getLocalPart().equals(鞋")){鞋=新鞋()；属性 idAttr = startElement.getAttributeByName(new QName("id"));如果(idAttr！= null){shoes.setId(Integer.parseInt(idAttr.getValue()));}} else if (startElement.getName().getLocalPart().equals("title")){xmlEvent = reader.nextEvent();shoes.setTitle(xmlEvent.asCharacters().getData());//错误第 68 行} 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()));}}如果(xmlEvent.isEndElement()){EndElement endElement = xmlEvent.asEndElement();if(endElement.getName().getLocalPart().equals(鞋")){shoesList.add(鞋子);}}}} 捕捉(FileNotFoundException | XMLStreamException exc){exc.printStackTrace();} 返回鞋子列表；}公共静态 void main(String[] args) 抛出异常 {System.out.println(STAX 解析器");System.out.println();System.out.println("结果:
");System.out.println();字符串文件名 = ShoesShop.xml";列出<鞋子>shoesList = parseXMLfile(fileName);//错误第101行对于(鞋鞋:shoesList){System.out.println(shoes.toString());}}}

这是一个 XML 文件

还有一个用于鞋子的 Java 类

@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "鞋子", propOrder = {标题"，品牌"，类别"，季节"，性别"，详情"，价格"})公共课鞋扩展实体{@XmlElement(必需 = 真)受保护的字符串标题；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保护的品牌品牌；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保护的类别类别；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保护的季节；@XmlElement(必需 = 真)受保护的鞋子.性别性别；@XmlElement(必需 = 真)受保护的鞋子.详细信息；受保护的双倍价格；@XmlAttribute(name = "stock", required = true)受保护的布尔库存；@XmlAttribute(name = "最想要的")受保护的布尔最想要的；公共字符串 getTitle() {返回标题；}公共无效 setTitle(字符串值){this.title = 值；}公共品牌 getBrand(){回归品牌；}public void setBrand(品牌价值){this.brand = 价值；}公共类别 getCategory(){返回类别；}公共无效 setCategory(类别值){this.category = 值；}公共季节 getSeason(){回归季节；}公共无效setSeason(季节值){this.season = 价值；}public Shoes.Gender getGender() {返回性别；}公共无效 setGender(Shoes.Gender 值){this.gender = 价值；}公共鞋.详细信息 getDetails() {退货详情；}public void setDetails(Shoes.Details 值) {this.details = 价值；}公共双getPrice(){退货价格；}公共无效 setPrice(双值){this.price = 价值；}公共布尔 isStock() {退货；}公共无效 setStock(布尔值){this.stock = 价值；}公共布尔 isMostWanted() {返回最想要的；}公共无效 setMostWanted(布尔值){this.most Wanted = 价值；}@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "", propOrder = {})公共静态类详细信息{@XmlElement(必需 = 真)受保护的字符串亮点；@XmlElement(必需 = 真)受保护的字符串组合；公共字符串 getHighlights() {返回亮点；}公共无效setHighlights(字符串值){this.highlights = 价值；}公共字符串 getComposition() {返回组成；}公共无效setComposition(字符串值){this.composition = 价值；}}@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "", propOrder = {u043cu0443u0436u0441u043au043eu0439或u0416u0435u043du0441u043au0438u0439"})公共静态类性别{@XmlElementRefs({@XmlElementRef(name = "u0436u0435u043du0441u043au0438u0439", 命名空间 = "http://www.example.org/ShoesShop", type = JAXBElement.class, required = false),@XmlElementRef(name = "u043cu0443u0436u0441u043au043eu0439", 命名空间 = "http://www.example.org/ShoesShop", type = JAXBElement.class, required = false)})受保护的列表<JAXBElement<字符串>>男性或女性;公共列表<JAXBElement<字符串>>getMaleOrFemale() {if (maleOrFemale == null) {maleOrFemale = new ArrayList<JAXBElement<String>>();}返回 this.maleOrFemale;}}@覆盖公共字符串 toString(){StringBuilder builder = new StringBuilder();builder.append("[title=");builder.append(title);builder.append(", 品牌=");builder.append(品牌)；builder.append(", category=");builder.append(类别);builder.append(", season=");builder.append(季节)；builder.append(", price=");builder.append(价格);builder.append("]");返回 builder.toString();}}

我还需要知道如何将接收到的数据写入新的 XML 文件.

解决方案

已更新: 对原始答案的评论:

<块引用>它不起作用，它给出了同样的错误
这意味着问题是因为 shoes 变量是 null，这在 debugger 中很容易看到.使用调试器会为我们节省很多时间，所以请开始使用调试器.
为了使 shoes 为 null，看来代码遇到了一个不是鞋子元素.
要修复代码，添加一个空检查，并在处理 Shoes 元素结束时设置 shoes = null:
} else if (startElement.getName().getLocalPart().equals("title")) {if (shoes != null) {//<===== 添加这个shoes.setTitle(reader.getElementText());//<===== 解决这个问题(见原始答案)}
if (xmlEvent.isEndElement()) {EndElement endElement = xmlEvent.asEndElement();if (endElement.getName().getLocalPart().equals(鞋")) {shoesList.add(鞋子);鞋=空；//<===== 添加这个}}
原始答案
你的代码是:
} else if (startElement.getName().getLocalPart().equals("title")){xmlEvent = reader.nextEvent();shoes.setTitle(xmlEvent.asCharacters().getData());
问题是代码没有检查如果事件跟在 START_ELEMENT 事件之后的类型.可能是这样的:

元素很可能是空的，即<title/>或<title><title/>，在这种情况下下一个事件是 END_ELEMENT，asCharacters() 返回 null.

元素有注释，例如<title><title/>，在这种情况下，下一个事件是 COMMENT.p>
元素具有混合内容，例如<title>foo<![CDATA[bar]]><title/>，在这种情况下，下一个事件不是全文.

检索元素的文本内容是如此常见，以至于他们为此添加了一个辅助方法:getElementText():

<块引用>

读取纯文本元素的内容.前提条件:当前事件是START_ELEMENT.后置条件:当前事件是对应的END_ELEMENT.

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

这意味着你的代码应该是:

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

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:

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: 
");
        System.out.println();
        String fileName = "ShoesShop.xml";
        List<Shoes> shoesList = parseXMLfile(fileName); //error line 101
        for (Shoes shoes:shoesList){
            System.out.println(shoes.toString());
        }
    }

}

Here's an XML-file

<?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>

Also here's a Java class for Shoes

@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 = {
        "u043cu0443u0436u0441u043au043eu0439Oru0416u0435u043du0441u043au0438u0439"
    })
    public static class Gender {

        @XmlElementRefs({
            @XmlElementRef(name = "u0436u0435u043du0441u043au0438u0439", namespace = "http://www.example.org/ShoesShop", type = JAXBElement.class, required = false),
            @XmlElementRef(name = "u043cu0443u0436u0441u043au043eu0439", namespace = "http://www.example.org/ShoesShop", type = JAXBElement.class, 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();
    }

}

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

解决方案

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 <title> 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:

} 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
    }
}

ORIGINAL ANSWER

Your code is:

} else if (startElement.getName().getLocalPart().equals("title")){
    xmlEvent = reader.nextEvent();
    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. <title/> or <title><title/>, in which case the next event is an END_ELEMENT, and asCharacters() returned null.
The element has a comment, e.g. <title><title/>, in which case the next event is a COMMENT.
The element has mixed content, e.g. <title>foo<![CDATA[bar]]><title/>, 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:

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

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为什么在运行 StAX Parser 时出现 NullPointerException?

问题描述

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