本文介紹了為什么在運行 StAX Parser 時出現 NullPointerException?的處理方法，對大家解決問題具有一定的參考價值，需要的朋友們下面隨著小編來一起學習吧！
問題描述
我正在嘗試用 Java 編寫 StAX XML 解析器，但總是出現 NullPointerException 錯誤.請幫我解決這個問題.完整問題:
<塊引用>線程main"中的異常；java.lang.NullPointerException 在org.example.shoesshop.parser.STAXParser.parseXMLfile(STAXParser.java:68)在 org.example.shoesshop.parser.STAXParser.main(STAXParser.java:101)
這是 StAX Parser 的一個類:
公共類 STAXParser 擴展 DefaultHandler {私有靜態列表<鞋子>parseXMLfile(字符串文件名){列出<鞋子>shoesList = new ArrayList<>();鞋鞋 = null;XMLInputFactory xmlInputFactory = XMLInputFactory.newInstance();嘗試 {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()；屬性 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());//錯誤第 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；}公共靜態 void main(String[] args) 拋出異常 {System.out.println(STAX 解析器");System.out.println();System.out.println("結果:
");System.out.println();字符串文件名 = ShoesShop.xml";列出<鞋子>shoesList = parseXMLfile(fileName);//錯誤第101行對于(鞋鞋:shoesList){System.out.println(shoes.toString());}}}這是一個 XML 文件
還有一個用于鞋子的 Java 類
@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "鞋子", propOrder = {標題"，品牌"，類別"，季節"，性別"，詳情"，價格"})公共課鞋擴展實體{@XmlElement(必需 = 真)受保護的字符串標題；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保護的品牌品牌；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保護的類別類別；@XmlElement(必需 = 真)@XmlSchemaType(name = "字符串")受保護的季節；@XmlElement(必需 = 真)受保護的鞋子.性別性別；@XmlElement(必需 = 真)受保護的鞋子.詳細信息；受保護的雙倍價格；@XmlAttribute(name = "stock", required = true)受保護的布爾庫存；@XmlAttribute(name = "最想要的")受保護的布爾最想要的；公共字符串 getTitle() {返回標題；}公共無效 setTitle(字符串值){this.title = 值；}公共品牌 getBrand(){回歸品牌；}public void setBrand(品牌價值){this.brand = 價值；}公共類別 getCategory(){返回類別；}公共無效 setCategory(類別值){this.category = 值；}公共季節 getSeason(){回歸季節；}公共無效setSeason(季節值){this.season = 價值；}public Shoes.Gender getGender() {返回性別；}公共無效 setGender(Shoes.Gender 值){this.gender = 價值；}公共鞋.詳細信息 getDetails() {退貨詳情；}public void setDetails(Shoes.Details 值) {this.details = 價值；}公共雙getPrice(){退貨價格；}公共無效 setPrice(雙值){this.price = 價值；}公共布爾 isStock() {退貨；}公共無效 setStock(布爾值){this.stock = 價值；}公共布爾 isMostWanted() {返回最想要的；}公共無效 setMostWanted(布爾值){this.most Wanted = 價值；}@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "", propOrder = {})公共靜態類詳細信息{@XmlElement(必需 = 真)受保護的字符串亮點；@XmlElement(必需 = 真)受保護的字符串組合；公共字符串 getHighlights() {返回亮點；}公共無效setHighlights(字符串值){this.highlights = 價值；}公共字符串 getComposition() {返回組成；}公共無效setComposition(字符串值){this.composition = 價值；}}@XmlAccessorType(XmlAccessType.FIELD)@XmlType(name = "", propOrder = {u043cu0443u0436u0441u043au043eu0439或u0416u0435u043du0441u043au0438u0439"})公共靜態類性別{@XmlElementRefs({@XmlElementRef(name = "u0436u0435u043du0441u043au0438u0439", 命名空間 = "http://www.example.org/ShoesShop", type = JAXBElement.class, required = false),@XmlElementRef(name = "u043cu0443u0436u0441u043au043eu0439", 命名空間 = "http://www.example.org/ShoesShop", type = JAXBElement.class, required = false)})受保護的列表<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;}}@覆蓋公共字符串 toString(){StringBuilder builder = new StringBuilder();builder.append("[title=");builder.append(title);builder.append(", 品牌=");builder.append(品牌)；builder.append(", category=");builder.append(類別);builder.append(", season=");builder.append(季節)；builder.append(", price=");builder.append(價格);builder.append("]");返回 builder.toString();}}我還需要知道如何將接收到的數據寫入新的 XML 文件.
解決方案
已更新: 對原始答案的評論:
<塊引用>它不起作用，它給出了同樣的錯誤
這意味著問題是因為 shoes 變量是 null，這在 debugger 中很容易看到.使用調試器會為我們節省很多時間，所以請開始使用調試器.
為了使 shoes 為 null，看來代碼遇到了一個不是鞋子元素.
要修復代碼，添加一個空檢查，并在處理 Shoes 元素結束時設置 shoes = null:
} else if (startElement.getName().getLocalPart().equals("title")) {if (shoes != null) {//<===== 添加這個shoes.setTitle(reader.getElementText());//<===== 解決這個問題(見原始答案)}if (xmlEvent.isEndElement()) {EndElement endElement = xmlEvent.asEndElement();if (endElement.getName().getLocalPart().equals(鞋")) {shoesList.add(鞋子);鞋=空；//<===== 添加這個}}原始答案
你的代碼是:
} else if (startElement.getName().getLocalPart().equals("title")){xmlEvent = reader.nextEvent();shoes.setTitle(xmlEvent.asCharacters().getData());問題是代碼沒有檢查如果事件跟在 START_ELEMENT 事件之后的類型.可能是這樣的:
元素很可能是空的，即
<title/>或<title><title/>，在這種情況下下一個事件是END_ELEMENT，asCharacters()返回null.元素有注釋，例如
<title><!-- 沒有標題--><title/>，在這種情況下，下一個事件是COMMENT.p>元素具有混合內容，例如
<title>foo<![CDATA[bar]]><title/>，在這種情況下，下一個事件不是全文.
檢索元素的文本內容是如此常見，以至于他們為此添加了一個輔助方法:getElementText():
讀取純文本元素的內容.前提條件:當前事件是START_ELEMENT.后置條件:當前事件是對應的END_ELEMENT.
拋出:XMLStreamException - 如果當前事件不是 START_ELEMENT 或者遇到非文本元素
這意味著你的代碼應該是:
} 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.
解決方案
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 anEND_ELEMENT, andasCharacters()returnednull.The element has a comment, e.g.
<title><!-- there is no title --><title/>, in which case the next event is aCOMMENT.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 correspondingEND_ELEMENT.Throws:
XMLStreamException- if the current event is not aSTART_ELEMENTor 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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