Java SAX XML解析器返回java.lang.NullPointerException错误

如何解决Java SAX XML解析器返回java.lang.NullPointerException错误

我正在学习如何用Java编写解析器，并且在编写新的SAX XML解析器时遇到了问题。这是我得到的错误：

线程“ main”中的异常java.lang.NullPointerException

在 org.example.shoesshop.parser.SAXParser.characters（SAXParser.java:112） 在 com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.characters（AbstractSAXParser.java:546） 在 com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaValidator.characters（XMLSchemaValidator.java:812） 在 com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument（XMLDocumentFragmentScannerImpl.java:455） 在 com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse（XML11Configuration.java:841） 在 com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse（XML11Configuration.java:770） 在 com.sun.org.apache.xerces.internal.parsers.XMLParser.parse（XMLParser.java:141） 在 com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse（AbstractSAXParser.java:1213） 在 com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl $ JAXPSAXParser.parse（SAXParserImpl.java:643） 在 com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl.parse（SAXParserImpl.java:327） 在javax.xml.parsers.SAXParser.parse（SAXParser.java:195）在 org.example.shoesshop.parser.SAXParser.parse（SAXParser.java:61）在 org.example.shoesshop.parser.SAXParser.main（SAXParser.java:136）

这是SAX Parser的代码：

public class SAXParser extends DefaultHandler{
    private static boolean logEnabled = true;
    public static void log(Object o) {
        if (logEnabled) {
            System.out.println(o);
        }
    }
    private String current;
    private Brand brand;
    private Category category;
    private Gender gender;
    private Season season;
    private Shoes shoes;
    private Shoes.Details details;
    private ShoesShop shoesShop;
    private String titleParent;
    
    public ShoesShop getShoesShop(){
        return shoesShop;
    }
    
    @Override
    public void error(org.xml.sax.SAXParseException e) throws SAXException {
        throw e;
    };
    
    public List<ShoesShop> parse(InputStream in) throws ParserConfigurationException,SAXException,IOException{
        List<ShoesShop> shoesShop = new ArrayList<>();
        SAXParserFactory factory = SAXParserFactory.newInstance();
        factory.setNamespaceAware(true);
        
        factory.setFeature("http://xml.org/sax/features/validation",true);
        factory.setFeature("http://apache.org/xml/features/validation/schema",true);
        javax.xml.parsers.SAXParser parser = factory.newSAXParser();
        parser.parse(in,this); //line 61 error
        return shoesShop;
    }
    
    @Override
    public void startElement(String uri,String localName,String qName,Attributes attributes) throws SAXException{
        System.out.println(localName);
        current = localName;
        
        if("ShoesShop".equals(current)){
            shoesShop = new ShoesShop();
            return;
        }
        
        if("Shoes".equals(current)){
            shoes = new Shoes();
            return;
        }
    }
    
    @Override
    public void characters(char[] ch,int start,int length) throws SAXException {
        String elementText = new String(ch,start,length);
        
        if (elementText.isEmpty()){
            System.out.println("empty");
            return;
        }
        
        
        else if ("title".equals(current)) {
            if ("title".equals(titleParent)){
                shoes.setTitle(new String (new String(ch,length)));
                return;
            }
        }
        
        else if("category".equals(current)){
            shoes.setCategory(Category.fromValue(new String(ch,length)));
            return;
        }
        
        
        else if ("brand".equals(current)) {
            shoes.setBrand(Brand.fromValue(new String(ch,length))); // line 112 error
            return;
        }
        else if("season".equals(current)){
            shoes.setSeason(Season.fromValue(new String(ch,length)));
            return;
        }
        
        else if("price".equals(current)) {
            shoes.setPrice(new Double(new String(ch,length)));
            return;
        }
    }
    
    @Override
    public void endElement(String uri,String qName) throws SAXException {
        if("ShoesShop".equals(localName)){
            shoesShop.getShoes().add(shoes);
            return;
        }
    }

    public static void main(String[] args) throws FileNotFoundException,ParserConfigurationException,IOException,TransformerException {
        SAXParser parser = new SAXParser();
        parser.parse(new FileInputStream("ShoesShop.xml")); // line 136 error
        ShoesShop shoesShop = parser.getShoesShop();
        String outputXmlFile = "sax.xml";
        System.out.println("SAX Parser");
        System.out.println();
        System.out.println("Result: \n" + shoesShop);
        System.out.println();
    }
}

这是XML文件

<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:ShoesShop>

这是鞋类

@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();
    }
}