使用 JAXB 解组大 xml 文件

Question

I have 1-2MB xml files and I would like to load them into the list of objects. I wanted to do it through JAXB because I have the appropriate XSD, only the problem appears when loading these files because it throws me errors saying that there is an unclosed tag somewhere, or something like that, when I check the file, there are no errors there. When I loaded files with a size of eg 40KB, there was no problem, everything was loading properly. So I understand there's a problem in jaxb when unmarshaling larger files. Is there any way to eat this? Another way of unmarshalling is unlikely to be included in the game because each xml file has a slightly different structure and objects that I have created with XSD.

The structure of xml files:

<Request>
  <Header>
    <Name> </Name>
      <Id> </Id>
  </Header>
  <RequestItems>
   <Request>
     <Header>
       <Name> </Name>
       <Id> </Id>
     </Header>
     <ObjectName>
       <City> </City>
       <Street> </Street>
     </ObjectName>
     </Request>
    </RequestItems>
 </Request>

Inside the RequestItems tags there is a list of Request objects and in the Request ObjectName objects it is different depending on the file.

I do unmrashal in the simplest way and for smaller files it works fine, and the larger ones have strange syntax errors but I understand that it just cuts lines because it is too long and therefore there are syntactic errors.

JAXBContext jaxbContext = JAXBContext.newInstance (Request.class);
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller ();
Request request = (Request) jaxbUnmarshaller.unmarshal (xmlFile);

Answer 1

If I have understood your question correctly then this should work. I have used Moxy which is an extension of Jaxb and Project Lombok for Getter setter.

Imagine the following is the large XML with different structures request.xml :

<request>
    <person>
        <name>Batman</name>
        <age>29</age>
        <job>IT</job>
    </person>
    <animal>
        <name>Tommy</name>
        <age>5</age>
        <type>Dog</type>
    </animal>
    <person>
        <name>Superman</name>
        <age>30</age>
        <job>HR</job>
    </person>
</request>

Following are the related classes and interface:

import jakarta.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({Person.class, Animal.class})
public interface XmlSupportExtension {
    Object xmlSupport();
}

import jakarta.xml.bind.annotation.XmlAccessType;
import jakarta.xml.bind.annotation.XmlAccessorType;
import jakarta.xml.bind.annotation.XmlTransient;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.io.Serializable;

@Data
@NoArgsConstructor
@AllArgsConstructor
@XmlAccessorType(XmlAccessType.FIELD)
@XmlTransient
public class Common implements Serializable {
    private String name;
    private String age;
}

import lombok.*;

@Data
@EqualsAndHashCode(callSuper = true)
@ToString(callSuper = true)
@NoArgsConstructor
@AllArgsConstructor
public class Person extends Common implements XmlSupportExtension {

    private String job;

    @Override
    public Person xmlSupport() {
        return this;
    }
}

import lombok.*;

@Data
@EqualsAndHashCode(callSuper = true)
@ToString(callSuper = true)
@NoArgsConstructor
@AllArgsConstructor
public class Animal extends Common implements XmlSupportExtension {
    private String type;

    @Override
    public Animal xmlSupport() {
        return this;
    }
}

import jakarta.xml.bind.JAXBContext;
import jakarta.xml.bind.JAXBException;
import jakarta.xml.bind.Unmarshaller;

import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.util.Arrays;


public class MainStack {
    public static void main(String[] args) throws JAXBException, XMLStreamException, FileNotFoundException {
        final String[] EVENT_TYPES = new String[]{"person", "animal"};
        InputStream inputStream = MainStack.class.getResourceAsStream("/request.xml");

        //Create an instance of XMLStreamReader to read the events one-by-one
        final XMLInputFactory inputFactory = XMLInputFactory.newInstance();
        inputFactory.setProperty(XMLInputFactory.SUPPORT_DTD, Boolean.FALSE);
        inputFactory.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, Boolean.FALSE);
        final XMLStreamReader xmlStreamReader = inputFactory.createXMLStreamReader(inputStream);

        //Create an instance of JAXBContext and Unmarshaller for unmarshalling the classes to respective event
        final Unmarshaller unmarshaller = JAXBContext.newInstance(Person.class, Animal.class).createUnmarshaller();

        //Navigate to next and start of the XML Elements
        xmlStreamReader.next();

        //Read Until the end of the file and unmarshall event-by-event
        while (xmlStreamReader.hasNext()) {

            //Check if the initial element is one of the elements from "EVENT_TYPES"
            if (xmlStreamReader.isStartElement() && Arrays.asList(EVENT_TYPES).contains(xmlStreamReader.getLocalName())) {

                //Get the event type
                final String eventType = xmlStreamReader.getLocalName();

                Object event = null;

                System.out.println(eventType);

                // Based on eventType make unmarshaller call to respective event class
                switch (eventType) {
                    case "person":
                        //Unmarshal the Person
                        event = unmarshaller.unmarshal(xmlStreamReader, Person.class).getValue();
                        break;
                    case "animal":
                        //Unmarshal the Animal
                        event = unmarshaller.unmarshal(xmlStreamReader, Animal.class).getValue();
                        break;
                    default:
                        //If NONE of the event type matches then do not convert and make a note
                        System.out.println("XML event does not match any of the required event : " + event);
                        break;
                }

                System.out.println(" After Unmarhsalling : " + event.toString());
            }

            //Move to the next event/element in InputStream
            xmlStreamReader.next();
        }
    }
}

This will produce the following output:

person
 After Unmarhsalling : Person(super=Common(name=Batman, age=29), job=IT)
animal
 After Unmarhsalling : Animal(super=Common(name=Tommy, age=5), type=Dog)