在xml jaxb中将空值表示为空元素

Question

我需要在jaxb中将空值显示为空元素.我正在使用jxyb的moxy实现.我找到了这个选项

@XmlNullPolicy(emptyNodeRepresentsNull = true, nullRepresentationForXml = XmlMarshalNullRepresentation.EMPTY_NODE)

是否有类似的扩展可以在类级别应用(对于其中定义的所有元素)

Answer 1

我强烈建议null使用缺少节点或xsi:nil="true"属性来表示.这对于模式验证最有效(即,<age/>或者<age></age>不是类型的有效元素xsd:int.但是,如果您不能在这里完成您的用例:

标准的JAXB行为

使用标准的API,你可以控制NULL是表示为不存在的节点或xsi:nil="true"与@XmlElement标注(见:http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html).

import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Address {

    private String street;

    @XmlElement(nillable=true)
    private String city;

}

如果两个字段的值都为null,则下面是XML输出.

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <city xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</address>

MOXy - 超越每个级别的行为

MOXy不提供注释来为类的所有属性指定空策略.但是,您可以利用DescriptorCustomizer通过@XmlCustomizer注释并调整本机MOXy映射元数据来完成相同的操作.

DescriptorCustomizer(AddressCustomizer)

import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.mappings.DatabaseMapping;
import org.eclipse.persistence.oxm.mappings.XMLDirectMapping;
import org.eclipse.persistence.oxm.mappings.nullpolicy.XMLNullRepresentationType;

public class AddressCustomizer implements DescriptorCustomizer {

    @Override
    public void customize(ClassDescriptor descriptor) throws Exception {
        for(DatabaseMapping mapping : descriptor.getMappings()) {
            if(mapping.isAbstractDirectMapping()) {
                XMLDirectMapping xmlDirectMapping = (XMLDirectMapping) mapping;
                xmlDirectMapping.getNullPolicy().setMarshalNullRepresentation(XMLNullRepresentationType.EMPTY_NODE);
                xmlDirectMapping.getNullPolicy().setNullRepresentedByEmptyNode(true);
            }
        }
    }

}

DomainModel(地址)

import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlCustomizer(AddressCustomizer.class)
public class Address {

    private String street;

    @XmlElement(nillable=true)
    private String city;

}

产量

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <street/>
   <city/>
</address>

MOXy - 覆盖所有类别的行为

如果您希望覆盖所有映射类的空值处理,我建议使用SessionEventListener替代.如果您愿意,也可以使用此方法更新单个类的元数据.

SessionEventListener(NullPolicySessionEventListener)

import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.mappings.DatabaseMapping;
import org.eclipse.persistence.oxm.mappings.XMLDirectMapping;
import org.eclipse.persistence.oxm.mappings.nullpolicy.XMLNullRepresentationType;
import org.eclipse.persistence.sessions.*;

public class NullPolicySessionEventListener extends SessionEventAdapter {

    @Override
    public void preLogin(SessionEvent event) {
        Project project = event.getSession().getProject();
        for(ClassDescriptor descriptor : project.getOrderedDescriptors()) {
            for(DatabaseMapping mapping : descriptor.getMappings()) {
                if(mapping.isAbstractDirectMapping()) {
                    XMLDirectMapping xmlDirectMapping = (XMLDirectMapping) mapping;
                    xmlDirectMapping.getNullPolicy().setMarshalNullRepresentation(XMLNullRepresentationType.EMPTY_NODE);
                    xmlDirectMapping.getNullPolicy().setNullRepresentedByEmptyNode(true);
                }
            }
        }
     }

}

演示代码

import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextProperties;
import org.eclipse.persistence.sessions.SessionEventListener;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>(1);
        SessionEventListener sessionEventListener = new NullPolicySessionEventListener();
        properties.put(JAXBContextProperties.SESSION_EVENT_LISTENER, sessionEventListener);
        JAXBContext jc = JAXBContext.newInstance(new Class[] {Address.class}, properties);

        Address address = new Address();

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(address, System.out);
    }

}

产量

<?xml version="1.0" encoding="UTF-8"?>
<address>
   <street/>
   <city/>
</address>