Question

亲爱的java编程人员，我使用了http://www.vogella.com/articles/JAXB/article.html

中的一个例子

用于我的3个类，UserStorage，User和UserTest

的JAXB XML用法

它工作正常，但它只是

的非结合性

JAXBContext context = JAXBContext.newInstance(UserStorage.class);
                Marshaller m = context.createMarshaller();
                m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);

the User class is ABSTRACT!, so it throws an

线程“main”中的异常javax.xml.bind.UnmarshalException：无法使用创建platform.User的实例 - 链接异常：[java.lang.InstantiationException] at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.handleEvent（UnmarshallingContext.java:648）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.Loader.reportError（Loader.java:236）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.createInstance（UnmarshallingContext.java:615）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.StructureLoader.startElement（StructureLoader.java:170）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext._startElement（UnmarshallingContext.java:487）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.startElement（UnmarshallingContext.java:465）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.startElement（SAXConnector.java:135）在 com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement（AbstractSAXParser.java:501）在 com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanStartElement（XMLNSDocumentScannerImpl.java:400）在 com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl $ FragmentContentDriver.next（XMLDocumentFragmentScannerImpl.java:2756）在 com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next（XMLDocumentScannerImpl.java:648）在 com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next（XMLNSDocumentScannerImpl.java:140）在 com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument（XMLDocumentFragmentScannerImpl.java:511）在 com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse（XML11Configuration.java:808）在 com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse（XML11Configuration.java:737）在 com.sun.org.apache.xerces.internal.parsers.XMLParser.parse（XMLParser.java:119）在 com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse（AbstractSAXParser.java:1205）在 com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl $ JAXPSAXParser.parse（SAXParserImpl.java:522）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0（UnmarshallerImpl.java:200）在 com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal（UnmarshallerImpl.java:173）在 javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal（AbstractUnmarshallerImpl.java:137）在 javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal（AbstractUnmarshallerImpl.java:194） at platform.UserTest.main（UserTest.java:77）引起： java.lang.InstantiationException at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance（InstantiationExceptionConstructorAccessorImpl.java:30） at java.lang.reflect.Constructor.newInstance（Constructor.java:513）在 com.sun.xml.internal.bind.v2.ClassFactory.create0（ClassFactory.java:112）在 com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.createInstance（ClassBeanInfoImpl.java:231）at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.createInstance（UnmarshallingContext.java:609） ......还有20个

是否有任何解决方案，所以我可以将User类绑定到XML，因为我可以使用用户详细信息保存XML文件，但是当我想要检索它时，它会认为User类是抽象的，我有一个管理员，经纪人，股东子类，但到目前为止，在我的用户测试类中，我只创建了4个管理员来测试，谢谢，希望你能帮忙。

import platform.UserStorage;
import platform.User;



public class UserTest {

private static final String USER_XML = "user2.xml";

public static void main(String[] args) throws JAXBException, IOException {

    ArrayList<User> userList = new ArrayList<User>();

    // create test users
            User user1 = new Admin();
            user1.setName("Dave");
            user1.setPass("1234");
            user1.setDeleted(true);
            user1.setBan(false);
            userList.add(user1);

            User user2 = new Admin();
            user2.setName("James");
            user2.setPass("1234");
            user2.setDeleted(true);
            user2.setBan(false);
            userList.add(user2);

            User user3 = new Admin();
            user3.setName("Mike");
            user3.setPass("1234");
            user3.setDeleted(true);
            user3.setBan(false);
            userList.add(user3);


            // create bookstore, assigning book
            UserStorage userstore = new UserStorage();
            userstore.setListName("Test List");
            userstore.setUserList(userList);

            // create JAXB context and instantiate marshaller
            JAXBContext context = JAXBContext.newInstance(UserStorage.class);
            Marshaller m = context.createMarshaller();
            m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
            m.marshal(userstore, System.out);

            Writer w = null;
            try {
                w = new FileWriter(USER_XML);
                m.marshal(userstore, w);
            } finally {
                try {
                    w.close();
                } catch (Exception e) {
                }
            }

            // get variables from our xml file, created before
            System.out.println();
            System.out.println("Output from our XML File: ");
            Unmarshaller um = context.createUnmarshaller();
            UserStorage userstore2 = (UserStorage) um.unmarshal(new FileReader(
                    USER_XML));

            for (int i = 0; i < userstore2.getUsersList().toArray().length; i++) {
                System.out.println("User " + (i + 1) + ": "
                        + userstore2.getUsersList().get(i).getName() + " Pass "
                        + userstore2.getUsersList().get(i).getPass());
            }}  }



package platform;


import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;


//If you want you can define the order in which the fields are written
//Optional

/**
 * @author dinesh.kaushish, james.wyche //updated XML properties.
 * 
 */
@XmlRootElement(name = "user")
@XmlType(propOrder = { "name", "pass", "deleted", "ban" })

..

public abstract class User implements UserInterface {

private String name;
private String pass;
private boolean deleted;
private boolean ban;

/**
 * @ author dinesh.kaushish
 * @param String username
 * return void
 */

public void setName(String name)
{
    this.name = name;
}

// If you like the variable name, e.g. "name", you can easily change this
    // name for your XML-Output:

/**
 * @author dinesh.kaushish
 * @param null
 * @return String user;
 */
@XmlElement(name = "user")
public String getName()
{
    return this.name;
}

/**
 * @author dinesh.kaushish
 * @param String pwd
 * @return void
 */
public void setPass(String pass)
{
    this.pass=pass;
}

/**
 * @author dinesh.kaushish
 * @param void
 * @return String password
 */
@XmlElement(name = "pass")
public String getPass()
{
    return pass;
}

/**
 * @author dinesh.kaushish
 * @param dFlag
 * @return void
 */
public void setDeleted(boolean deleted)
{
    this.deleted = deleted;
}

/**
 * @author dinesh.kaushish
 * @return boolean isDeleted
 */
@XmlElement(name = "deleted")
public boolean getDeleted()
{
    return deleted;
}


/**
 * @author dinesh.kaushish
 * @param bFlag
 */
public void setBan(boolean ban)
{
    this.ban = ban;
}

/**
 * @author dinesh.kaushish
 * @return Boolean isBanned
 */
@XmlElement(name = "ban")
public Boolean getBan()
{
    return ban;
}


public abstract void addUser();
public abstract void removeUser();
public abstract void verifyUser();
public abstract void passReset();
public abstract void faultReport();
public abstract void RequestsForAccess();
public abstract void UpdateDetails();
public abstract void BanUser();
public abstract void ChangePermissions();


    }



package platform;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.HashMap;

import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.XMLEvent;

import java.util.ArrayList;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;





/**
 * @author michael.wambeek, james.wyche //added JAXB support.
 *
 */
//This statement means that class "Bookstore.java" is the root-element of our example
@XmlRootElement(namespace = "platform")
public class UserStorage {

// XmLElementWrapper generates a wrapper element around XML representation
    @XmlElementWrapper(name = "userList")
    // XmlElement sets the name of the entities
    @XmlElement(name = "user")
private ArrayList<User> userList;
private String listName = "";

// private static UserStorage实例; // james.wyche JAXB代码的结尾。

public UserStorage(){

}

/**
 * @author michael.wambeek
 * 
 * Searches for a username and returns the password.
 * 
 * @param username The username to search for
 * @return The password of the correct username or null
 * @throws Exception
 */
public String findUser(String username) throws Exception{
    return search(username);
}

public boolean storeUser(String username, String password, UserType type){
    return true;
}

/**
 * @author james.wyche
 * @param userList
 */
public void setUserList(ArrayList<User> userList) {
    this.userList = userList;
}

/**
 * 
 * @return UserList
 */
public ArrayList<User> getUsersList() {
    return userList;
}

Answer 1

为什么失败是因为Jaxb将尝试创建User实例。这是抽象的，因此也是失败的。

在抽象类上添加注释

@XmlTransient //Prevents the mapping of a JavaBean property/type to XML representation
@XmlSeeAlso({Admin.class, <other class>}) //Instructs JAXB to also bind other classes when binding this class

查看每个（XmlTransient，XmlSeeAlso）

的javadoc

这样做可以防止jaxb尝试初始化您的抽象类。

我发现这个方法的唯一缺点是会在创建的xml中添加额外的命名空间信息。

Answer 2

您需要向User类添加一个XmlSeeAlso注释，其属性为Admin，并且所有其他具体类都是User类的子类。

@XmlSeeAlso({Admin.class})

PS，不要忘记将Xml标记@XmlRootElement添加到Admin类。

Answer 3

这不起作用，因为在解组xml时JAXB必须创建类的新实例（对象）。如果xml中的标记绑定到抽象类，则它无法实例化该类中的对象。您必须使User类非抽象或将xml标记绑定到User的具体子类。

Answer 4

您必须指定每个元素的concrete类型：

<user xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Admin">
    ...
</user>

如何使用JAXB使抽象类工作

4 个答案: