将空值表示为 xml jaxb 中的空元素 [英] Represent null value as empty element in xml jaxb
问题描述
我需要在 jaxb 中将空值显示为空元素.我正在使用 jaxb 的 moxy 实现.我找到了这个选项
I need to display null value as empty element in jaxb. I am using moxy implementation of jaxb. I found this option
@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)
Is there any similar extension that can be applied at Class level (for all elements defined in it)
推荐答案
我强烈建议用没有节点或用 xsi:nil="true" 表示
null
属性.这对模式验证最有效(即
或
不是 xsd 类型的有效元素:int
.但是如果你不能,这里是你如何完成你的用例:
I would strongly recommend representing null
with either the absence of the node or with the xsi:nil="true"
attribute. This works best with schema validation (i.e. <age/>
or <age></age>
is not a valid element of type xsd:int
. However if you can't here is how you can accomplish your use case:
标准 JAXB 行为
使用标准 API,您可以控制是否将 null 表示为不存在的节点,或者使用带有 @XmlElement
注释的 xsi:nil="true"
(请参阅:http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html).
Using the standard APIs you can control whether null is represented as an absent node or with xsi:nil="true"
with the @XmlElement
annotation (see: 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;
}
以下是两个字段的值都为空时的 XML 输出.
Below is the XML output if the values of both fields are null.
<?xml version="1.0" encoding="UTF-8"?>
<address>
<city xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</address>
<小时>
MOXy - 覆盖每个班级的这种行为
MOXy 没有提供注释来为类上的所有属性指定空策略.但是,您可以通过 @XmlCustomizer
注释利用 DescriptorCustomizer
并调整本机 MOXy 映射元数据来完成相同的事情.
MOXy does not provide an annotation to specify the null policy for all the properties on a class. However you can leverage a DescriptorCustomizer
via the @XmlCustomizer
annotation and tweak the native MOXy mapping metadata to accomplish the same thing.
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>
<小时>
MOXy - 覆盖所有班级的这种行为
如果您想覆盖所有映射类的空处理,我建议改用 SessionEventListener
.如果您愿意,也可以使用这种方法更新单个类的元数据.
If instead you want to override null handling for all of the mapped classes I would recommend using a SessionEventListener
instead. If you prefer you could also use this approach to update the metadata for a single class.
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);
}
}
输出
<?xml version="1.0" encoding="UTF-8"?>
<address>
<street/>
<city/>
</address>
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