如何使用JAXB使抽象类工作

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,但它只是无关的

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

有没有解决方案,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,它表明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 = "";

//私有静态UserStorage实例; //结束james.wyche JAXB代码.

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

看每个的javadoc(XmlTransient,XmlSeeAlso)

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,其中包含属性Admin和子类User类的所有其他具体类.

@XmlSeeAlso({Admin.class})

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

Answer 3

这是行不通的，因为 JAXB 在解组 xml 时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。您要么必须使 User 类成为非抽象类，要么将 xml 标记绑定到 User 的具体子类。