java - 在 xml jaxb 中将 null 值表示为空元素

Question

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

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

是否有任何类似的扩展可以应用于类级别(对于其中定义的所有元素)

Answer 1

我强烈建议代表 null没有节点或有 xsi:nil="true"属性。这最适合模式验证(即 <age/> 或 <age></age> 不是 xsd:int 类型的有效元素。但是，如果您不能，这里是如何完成您的用例:

标准 JAXB 行为

使用标准 API，您可以控制 null 是表示为不存在的节点还是用 xsi:nil="true" 表示与 @XmlElement注释(参见: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空，则以下是 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 - 覆盖每个类的此行为

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 native MOXy 映射元数据以完成相同的事情。

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;

}

输出

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

---

MOXy - 覆盖所有类的此行为

如果您想覆盖所有映射类的空值处理，我建议使用 SessionEventListener反而。如果您愿意，还可以使用这种方法来更新单个类的元数据。

session 事件监听器(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);
    }

}

输出

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