英文:
Why I get NullPointerException when running StAX Parser?
问题
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:
> 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)
这是一个用于StAX解析器的类:
public class STAXParser extends DefaultHandler {
private static List<Shoes> parseXMLfile(String fileName) {
// ...
//(原始代码)
// ...
}
public static void main(String[] args) throws Exception {
// ...
//(原始代码)
// ...
}
}
这是一个XML文件的示例:
<?xml version="1.0" encoding="UTF-8"?>
<!--(略)-->
</ss:ShoesShop>
这是一个用于鞋子的Java类:
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Shoes", propOrder = {
"title",
"brand",
//(略)
})
public class Shoes extends Entity {
// ...
//(原始代码)
// ...
}
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:
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());
}
}
}
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 = {
"\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", 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.
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
:
} else if (startElement.getName().getLocalPart().equals("title")) {
if (shoes != null) { // <===== 添加这一行
shoes.setTitle(reader.getElementText()); // <===== 修复这一行(参见原始回答)
}
}
if (xmlEvent.isEndElement()) {
EndElement endElement = xmlEvent.asEndElement();
if (endElement.getName().getLocalPart().equals("Shoes")) {
shoesList.add(shoes);
shoes = null; // <===== 添加这一行
}
}
原始回答
您的代码是:
} else if (startElement.getName().getLocalPart().equals("title")){
xmlEvent = reader.nextEvent();
shoes.setTitle(xmlEvent.asCharacters().getData());
问题是代码没有检查 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
<title/>
或<title><title/>
,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
<title><!-- 没有标题 --><title/>
,在这种情况下,下一个事件是COMMENT
。 -
该元素具有混合内容,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:
} else if (startElement.getName().getLocalPart().equals("title")) {
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 <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 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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