发现如果XStream的API存在一个节点

Question

我有类似以下的XML结构。我有转换器来写每一个对象A，B和C。是否有可能在XStream检查，如果在XML层次结构中存在的一个节点？即在下面的情况下，我想要做的事，如果之前我写B存在C节点。

<A>
    <B>
        <C>
        </C>
    </B>
</A>    

Answer 1

如果我理解正确的话，你要检查在CConverter B节点是否已在层次结构中存在。如果结构是上述这总是true。编组过程从根对象开始，并进入内部的性质。所以，写C节点首先B必须存在。

假设你有简单的POJO结构如下图所示：

class A {
    public B b = new B();
}

class B {
    public C c = new C();
}

class C {

}

现在，我们可以实现对A和B转换器：

class AConverter implements Converter {

    public boolean canConvert(Class clazz) {
        return clazz.equals(A.class);
    }

    public void marshal(Object value, HierarchicalStreamWriter writer, MarshallingContext context) {
        System.out.println("Marshalling A!");
        A a = (A) value;
        if (a.b != null) {
            writer.startNode("B");
            context.convertAnother(a.b);
            writer.endNode();
        }
    }

    public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
        throw new NotImplementedException();
    }
}

class BConverter implements Converter {

    public boolean canConvert(Class clazz) {
        return clazz.equals(B.class);
    }

    public void marshal(Object value, HierarchicalStreamWriter writer, MarshallingContext context) {
        System.out.println("Marshalling B!");
        B b = (B) value;
        if (b.c != null) {
            writer.startNode("C");
            context.convertAnother(b.c);
            writer.endNode();
        }
    }

    public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
        throw new NotImplementedException();
    }
}

Main类：

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.converters.Converter;
import com.thoughtworks.xstream.converters.MarshallingContext;
import com.thoughtworks.xstream.converters.UnmarshallingContext;
import com.thoughtworks.xstream.io.HierarchicalStreamReader;
import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
import sun.reflect.generics.reflectiveObjects.NotImplementedException;

public class XStreamApp {
    public static void main(String[] args) {
        XStream xStream = new XStream();
        xStream.registerConverter(new AConverter());
        xStream.registerConverter(new BConverter());
        xStream.alias("A", A.class);

        System.out.println(xStream.toXML(new A()));
    }
}

打印：

Marshalling A!
Marshalling B!
<A>
  <B>
    <C/>
  </B>
</A>