Jaxb marshalling with custom annotations

问题:

I have a requirement, to marshall/unmarshall some elements of java pojo depending upon a custom annotation marked on the field. suppose there are 3 fields in my java pojp

@CustomVersion("v1")
private String field1;
@CustomVersion("v1","v2")
private String field2;
@CustomVersion("v2")
private String field3;

i would like to marshall only the fields with v1 if i pass version="v1" parameter while conversion in jaxb. if i pass v2, all fields with v2 annotation should only be marshalled.

is that even possible using jaxb? i am sure selective marshalling would be supported through some library or way, am not still able to figure it out after quite some searching. any help or advice or pointers are highly appreciated.

回答1:

First of all I would suggest doing such preprocessing before marshalling. It would be much easier. However if it is not possible for some reason then you can create you custom type adapter. Then you can put @XmlJavaTypeAdapter(VersioningAdapter.class) on every type that you want to have versioning enabled. @XmlJavaTypeAdapter can also be specified on package level, but you have to specify to which types it applies. You cannot use XmlAdapter without specifying somewhere @XmlJavaTypeAdapter.

Drawbacks of such solution:

• if you have multiple versioned types then each of them has to be annotated with @XmlJavaTypeAdapter
• @XmlJavaTypeAdapter does not work for root element, only on child elements. You have to call adapter manually on root element before marshalling

AFAIK there is no other option for customizing JAXB marshalling. That's why I think that annotation processing should be performed in separate step before marshalling. Unless you can accept mentioned limitations.

Sample adapter (full code can be found here):

public class VersioningAdapter extends XmlAdapter<Object, Object> {

    @Override
    public Object unmarshal(Object v) throws Exception {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object marshal(Object v) throws Exception {
        if (v == null) {
            return v;
        }
        Field[] fields = v.getClass().getDeclaredFields();
        for (Field field : fields) {
            Annotation[] annotations = field.getDeclaredAnnotations();
            CustomVersion annotation = findCustomVersion(annotations);
            if (annotation != null) {
                if (!contains(annotation, Configuration.getVersion())) {
                    field.setAccessible(true);
                    field.set(v, null);
                }
            }
        }
        return v;
    }

    private CustomVersion findCustomVersion(Annotation[] annotations) {
        for (Annotation annotation : annotations) {
            if (annotation instanceof CustomVersion) {
                return (CustomVersion) annotation;
            }
        }
        return null;
    }

    private boolean contains(CustomVersion annotation, String version) {
        String[] values = annotation.value();
        for (String value : values) {
            if (value.equals(version)) {
                return true;
            }
        }
        return false;
    }

}

回答2:

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

Below is an example of how you could use MOXy's @XmlNamedObjectGraphs extension to map your use case.

Java Model

Foo

The @XmlNamedObjectGraphs extension allows you to specify multiple subsets of mappings identified by a key.

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;

import org.eclipse.persistence.oxm.annotations.XmlNamedAttributeNode;
import org.eclipse.persistence.oxm.annotations.XmlNamedObjectGraph;
import org.eclipse.persistence.oxm.annotations.XmlNamedObjectGraphs;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlNamedObjectGraphs({
    @XmlNamedObjectGraph(
        name="v1",
        attributeNodes = { 
            @XmlNamedAttributeNode("field1"),
            @XmlNamedAttributeNode("field2")}),
    @XmlNamedObjectGraph(
        name="v2",
        attributeNodes = { 
            @XmlNamedAttributeNode("field2"),
            @XmlNamedAttributeNode("field3")})
})
public class Foo {

    private String field1 = "ONE";
    private String field2 = "TWO";
    private String field3 = "THREE";

}

jaxb.properties

To use MOXy as your JAXB provider you need to include a file called jaxb.properties with the following entry (see: http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html).

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

Demo Code

Demo

You can specify the key corresponding to the object graph to have that subset applied to the object you are marshalling.

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import org.eclipse.persistence.jaxb.MarshallerProperties;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Foo.class);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        Foo foo = new Foo();

        // Marshal Everything
        marshaller.marshal(foo, System.out);

        // Marshal "v1" Data
        marshaller.setProperty(MarshallerProperties.OBJECT_GRAPH, "v1");
        marshaller.marshal(foo, System.out);

        // Marshal "v2" Data
        marshaller.setProperty(MarshallerProperties.OBJECT_GRAPH, "v2");
        marshaller.marshal(foo, System.out);
    }

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<foo>
   <field1>ONE</field1>
   <field2>TWO</field2>
   <field3>THREE</field3>
</foo>
<?xml version="1.0" encoding="UTF-8"?>
<foo>
   <field1>ONE</field1>
   <field2>TWO</field2>
</foo>
<?xml version="1.0" encoding="UTF-8"?>
<foo>
   <field2>TWO</field2>
   <field3>THREE</field3>
</foo>

For More Information

• http://blog.bdoughan.com/2013/03/moxys-object-graphs-inputoutput-partial.html