我强烈建议
null使用不存在节点或使用
xsi:nil="true"属性来表示。这与模式验证（即
<age/>或
<age></age>不是有效的type元素）一起使用时效果最佳
xsd:int。但是，如果不能这样做，那么可以完成用例：
标准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;}如果两个字段的值均为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-覆盖每个班级的这种行为
MOXy不提供注释来为类的所有属性指定空策略。但是，您可以
DescriptorCustomizer通过
@XmlCustomizer注释利用并调整本机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;}输出量
<?xml version="1.0" encoding="UTF-8"?><address> <street/> <city/></address>
MOXy-覆盖所有类别的这种行为
相反，如果您想覆盖所有映射类的空处理，我建议使用a
SessionEventListener代替。如果您愿意，也可以使用这种方法来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); } } } }}示范代码
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); }}输出量
<?xml version="1.0" encoding="UTF-8"?><address> <street/> <city/></address>
