java - 查找 XStream api 中是否存在节点

Question

我有一个类似于以下的 XML 结构。我有转换器来编写每个对象 A、B 和 C。是否可以在 XStream 中检查 XML 层次结构中是否存在节点？即在下面的例子中，如果 B 节点在我写 C 之前存在，我想做一些事情。

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

Answer 1

如果我没理解错的话，你想在 CConverter 中检查 B 节点是否已经存在于层次结构中。如果结构如上，则始终为 true。编码过程从根对象开始，进入内部属性。所以，要写入C节点，第一个B必须存在。

假设您有如下简单的 POJO 结构:

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

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

class C {

}

现在，我们可以为 A 和 B 实现转换器:

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();
    }
}

主要类:

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>