Question

I’m creating a program in Java which contains a tree-structure of objects. All classes involved implement the same interface, and each class contains a list of children:

public class MyClass1 implements MyInterface {    
    List<MyInterface> children;
}

public class MyClass2 implements MyInterface {    
    List<MyInterface> children;
}

public class MyClass3 implements MyInterface {    
    List<MyInterface> children;
}

....

Now the three structure is defined in an XML-file:

<myclass1>
    <myclass2></myclass2>
    <myclass1>
        <myclass3></myclass3>
    </myclass1>
</myclass1>

The tree structure can be of any type, defined by the user. Of course, each class contains class-specific variables which I omitted for now.

Now I’m trying to use XStream to deserialize the XML-file to Java-objects, preferably using annotations, but I don’t know how to do this. I don’t want to end up making a list for each possible class, e.g. in MyClass1:

@XStreamImplicit(itemFieldName="myclass2")
List<MyClass2> children2;
@XStreamImplicit(itemFieldName="myclass3")
List<MyClass3> children3;
@XStreamImplicit(itemFieldName="myclass4")
List<MyClass4> children4;

Any suggestions on how to solve this case with XStream? Or should I use other technologies?

Answer 1

source.xml (Containing the XML we want to load):

<myclass1>
    <myclass2></myclass2>
    <myclass1>
        <myclass3></myclass3>
    </myclass1>
</myclass1>

Interface:

import java.util.List;

public interface Myinterface {
    List<? extends Myinterface> getChildren();
}    

Abstract bean class (declaring children with @XStreamImplicit) :

import java.util.ArrayList;
import java.util.List;

import com.thoughtworks.xstream.annotations.XStreamImplicit;

public class MyAbstract implements Myinterface {
    @XStreamImplicit
    private List<? extends Myinterface> children = new ArrayList<Myinterface>();


    public List<? extends Myinterface> getChildren() {
        return children;
    }
}

Bean classes (extending MyAbstract, using @XStreamAlias to use the XML string you need):

import com.thoughtworks.xstream.annotations.XStreamAlias;

@XStreamAlias(value="myclass1")
public class Myclass1 extends MyAbstract {

}

@XStreamAlias(value="myclass2")
public class Myclass2 extends MyAbstract {

}

@XStreamAlias(value="myclass3")
public class Myclass3 extends MyAbstract {

}

Main class (reading your XML source file. Don’t forget processAnnotations to load configuration from Annotated classes):

import com.thoughtworks.xstream.XStream;

public class XStreamTest {
    public static void main(String[] args) {
        XStream xStream = new XStream();
        xStream.processAnnotations(Myclass1.class);
        xStream.processAnnotations(Myclass2.class);
        xStream.processAnnotations(Myclass3.class);

        Object fromXML = xStream.fromXML(XStreamTest.class.getResourceAsStream("source.xml"));
        System.out.println(fromXML);
    }
}