Jaxb: xs:attribute null values - jaxb

Question

Reg: Jaxb
I'm basically trying to set up a role in JAXB which says that whenever an null field is encountered, instead of ignoring it in the output, set it to an empty value.
For xmlElement I got answer like we need to use nillable="true" but for how we need to set the null value. by googling I found that we need to use use="optional" but its not working in my case.
My xsd's part is below:
<xs:attribute name="RomVersion" type="xs:string" use="required" />
<xs:attribute name="MACAddress" type="xs:string" use="required" />
<xs:attribute name="LargestFreeBlock" type="xs:unsignedInt" use="required" />
<xs:attribute name="TimeSinceLastReset" type="xs:unsignedInt" use="optional" />
<xs:attribute name="ResetReason" type="xs:string" use="optional" />
<xs:attribute name="TimeStamp" type="xs:unsignedInt" use="optional" />
<xs:attribute name="ECOList" type="xs:string" use="optional" />
</xs:complexType>
</xs:element>
Please give me the solution ASAP if anyone knows.

Answer 1

Starting from XML Schema
In a previous answer I described how to solve your use case when starting from Java objects. Based on your comments to that answer, this answer describes how the same thing can be done when the model is generated from an XML schema.
XML Schema (attributeAdapter.xsd)
For this example we will use the following XML schema:
<?xml version="1.0" encoding="utf-8" ?>
<xs:schema
elementFormDefault="qualified"
targetNamespace="http://www.example.com/adapter"
xmlns:nytd="http://www.example.com/adapter"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="root">
<xs:complexType>
<xs:attribute name="foo" type="xs:string"/>
<xs:attribute name="bar" type="xs:string"/>
</xs:complexType>
</xs:element>
</xs:schema>
StringConverter
We will need to define a class to do our special String handling. For this use case we want a null field/property value to be treated as empty String ("") in the XML document:
package com.example.adapter;
public class StringConverter {
public static String parseString(String value) {
if("".equals(value)) {
return null;
}
return value;
}
public static String printString(String value) {
if(null == value) {
return "";
}
return value;
}
}
Binding File (attributeAdapterBinding.xml)
We will need to use a JAXB binding file to customize the class generation. The binding file below will allow us to leverage the StringConverter class that we defined above:
<jaxb:bindings
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
version="2.1">
<jaxb:bindings schemaLocation="attributeAdapter.xsd">
<jaxb:bindings node="//xs:element[#name='root']/xs:complexType">
<jaxb:bindings node="xs:attribute[#name='foo']">
<jaxb:property>
<jaxb:baseType>
<jaxb:javaType name="java.lang.String"
parseMethod="com.example.adapter.StringConverter.parseString"
printMethod="com.example.adapter.StringConverter.printString"/>
</jaxb:baseType>
</jaxb:property>
</jaxb:bindings>
<jaxb:bindings node="xs:attribute[#name='bar']">
<jaxb:property>
<jaxb:baseType>
<jaxb:javaType name="java.lang.String"
parseMethod="com.example.adapter.StringConverter.parseString"
printMethod="com.example.adapter.StringConverter.printString"/>
</jaxb:baseType>
</jaxb:property>
</jaxb:bindings>
</jaxb:bindings>
</jaxb:bindings>
</jaxb:bindings>
XJC call
We will make our XJC call as follows:
xjc -d out -b attributeAdapterBinding.xml attributeAdapter.xsd
Domain Model (Root)
The fields/properties that we customized in the binding file will be annotated with #XmlJavaTypeAdapter;
package com.example.adapter;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name = "")
#XmlRootElement(name = "root")
public class Root {
#XmlAttribute
#XmlJavaTypeAdapter(Adapter1 .class)
protected String foo;
#XmlAttribute
#XmlJavaTypeAdapter(Adapter2 .class)
protected String bar;
public String getFoo() {
return foo;
}
public void setFoo(String value) {
this.foo = value;
}
public String getBar() {
return bar;
}
public void setBar(String value) {
this.bar = value;
}
}
XmlAdapter (Adapter1)
The generated XmlAdapter class will look something like the following. Note how it leverages our StringConverter class:
package com.example.adapter;
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class Adapter1 extends XmlAdapter<String, String> {
public String unmarshal(String value) {
return (com.example.adapter.StringConverter.parseString(value));
}
public String marshal(String value) {
return (com.example.adapter.StringConverter.printString(value));
}
}
Demo
Now if we run the following demo code:
package com.example.adapter;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
public class Demo {
public static void main(String[] args) throws Exception {
JAXBContext jc = JAXBContext.newInstance(Root.class);
Root root = new Root();
root.setFoo(null);
root.setBar(null);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(root, System.out);
}
}
Output
We will get the desired output:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root xmlns="http://www.example.com/adapter" foo="" bar=""/>
UPDATE (Alternate Binding File)
Alternatively, if you wanted the adapter applied to all properties of type xsd:string then you could use an binding file that looked something like;
<jaxb:bindings
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
version="2.1">
<jaxb:globalBindings>
<jaxb:javaType
name="String"
xmlType="xs:string"
parseMethod="com.example.adapter.StringConverter.parseString"
printMethod="com.example.adapter.StringConverter.printString"/>
</jaxb:globalBindings>
</jaxb:bindings>

Answer 2

Starting from Java Objects
For fields/properties mapped as #XmlAttribute, a JAXB implementation (Metro, MOXy, JaxMe, etc) will marshal an empty String ("") value as property="". You can use an XmlAdapter to expose your null values as empty Strings to get the desired behaviour:
NullStringAdapter
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class NullStringAdapter extends XmlAdapter<String, String> {
#Override
public String unmarshal(String v) throws Exception {
if("".equals(v)) {
return null;
}
return v;
}
#Override
public String marshal(String v) throws Exception {
if(null == v) {
return "";
}
return v;
}
}
Root
The following is how you specify the adapter in your domain model. The same adapter can be used on many properties:
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
#XmlRootElement
public class Root {
private String foo;
private String bar;
#XmlAttribute
#XmlJavaTypeAdapter(NullStringAdapter.class)
public String getFoo() {
return foo;
}
public void setFoo(String foo) {
this.foo = foo;
}
#XmlAttribute
#XmlJavaTypeAdapter(NullStringAdapter.class)
public String getBar() {
return bar;
}
public void setBar(String bar) {
this.bar = bar;
}
}
Demo
You can demonstrate the concept, by running the following demo code:
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
public class Demo {
public static void main(String[] args) throws Exception {
JAXBContext jc = JAXBContext.newInstance(Root.class);
Root root = new Root();
root.setFoo(null);
root.setBar(null);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(root, System.out);
}
}
Output
The following is the demo code output:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root bar="" foo=""/>
For More Information on JAXB's XmlAdapter See:
http://bdoughan.blogspot.com/2010/07/xmladapter-jaxbs-secret-weapon.html
http://bdoughan.blogspot.com/2010/12/jaxb-and-immutable-objects.html
http://bdoughan.blogspot.com/2010/12/represent-string-values-as-element.html

Answer 3

You may use default values plugin for that.
Please look that question: JAXB xjc: How to generate code for Strings that returns empty if the value is null?