Blaise的回答不错,但已经过时了。有一种更好、更简单的方法可以达到相同的效果。我在许多论坛上搜索并结合不同的解决方案得出了这个方法。我在这里分享,希望对他人有所帮助。
注意:下面的解决方案比仅适用于日期更为通用。
方法一:如果您想在XML中将所有空值替换为空字符串
会话事件适配器类
将下面的类添加到代码中一个方便的包中。
package com.dev
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);
}
}
}
}
}
Entity Class
package com.dev;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.*;
@XmlRootElement(name = "Entity")
public class Entity {
@XmlElement(name = "First_Name", required=true, nillable = true)
private String firstName;
@XmlElement(name = "Last_Name" , required=true, nillable = true)
private String lastName;
public Entity(){}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
DemoApp类
package com.dev;
import javax.xml.bind.*;
import org.eclipse.persistence.*;
import java.util.Map;
import java.util.HashMap;
public class DemoApp {
public static void main(String[] args) throws Exception {
Map<String, Object> properties = new HashMap<String,Object>(1);
SessionEventListener sessionEventListener = new NullSessionEventListener();
properties.put(JAXBContextProperties.SESSION_EVENT_LISTENER, sessionEventListener);
JAXBContext context = JAXBContextFactory.createContext(new Class[] {ListofEntities.class, list.get(0).getClass()},properties);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
Entity entity = new Entity();
entity.setfirstName(null);
entity.setLastName(null);
marshaller.marshal(entity, System.out);
entity.setfirstName("Ramu");
entity.setLastName("K");
marshaller.marshal(entity, System.out);
}
}
输出:
<?xml version="1.0" encoding="UTF-8"?>
<root>
<First_Name/>
<Last_Name/>
</root>
<?xml version="1.0" encoding="UTF-8"?>
<root>
<First_Name>Ramu</First_Name>
<Last_Name>Ramu</Last_Name>
</root>
方法二:如果您只想在xml中替换选定的空值为空字符串
实体类
package com.dev;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.*;
@XmlRootElement(name = "Entity")
public class Entity {
@XmlElement(name = "First_Name", required=true, nillable = true)
@XmlNullPolicy(emptyNodeRepresentsNull = true, nullRepresentationForXml = XmlMarshalNullRepresentation.EMPTY_NODE)
private String firstName;
@XmlElement(name = "Last_Name" , required=true, nillable = true)
private String lastName;
public Entity(){}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
DemoApp类
package com.dev;
import javax.xml.bind.*;
import org.eclipse.persistence.*;
public class DemoApp {
public static void main(String[] args) throws Exception {
JAXBContext context = JAXBContextFactory.createContext(new Class[] {ListofEntities.class, list.get(0).getClass()},null);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
Entity entity = new Entity();
entity.setfirstName(null);
entity.setLastName(null);
marshaller.marshal(entity, System.out);
entity.setfirstName("Ramu");
entity.setLastName("K");
marshaller.marshal(entity, System.out);
}
输出:
在这个输出中,我们只看到带有XmlNullPolicy注释的元素在值为null时被显示。其他元素由于jaxb的默认行为而被省略。
<?xml version="1.0" encoding="UTF-8"?>
<root>
<First_Name/>
</root>
<?xml version="1.0" encoding="UTF-8"?>
<root>
<First_Name>Ramu</First_Name>
<Last_Name>Ramu</Last_Name>
</root>
参考文献:
jaxb.properties文件应该放在哪里?
如何在XML JAXB中将null值表示为空元素