如何使用 XStream 序列化/反序列化类型层次结构中的对象？

Question

我正在使用 XStream。目前，用其他东西替换 XStream 并不容易。

我有一个接口 (MyInterface) 和几个实现该接口的子类（在下面的示例代码中，有一个称为 MyImplementation）。

我想序列化和反序列化子类的实例。我发现如果我将类属性放入 XML 中，我可以很好地反序列化：

<myInterfaceElement class="myPackage.MyImplementation">
  <field1>value1</field1>
  <field2>value2</field2>
</myInterfaceElement>

但是，我不知道如何让 XStream 写入类属性。序列化时如何让 XStream 包含类属性？或者是否有另一种方法来序列化/反序列化类层次结构，以便所有实现的元素名称相同，并且每个子类都可以定义自己的字段？

这里有一个 MyInterface、MyImplementation 的示例，这是一个试图使其工作的 JUnit 测试用例。 deserializeWithClassAttribute 测试通过，而 classAttributeSetInResult 失败。

---

package myPackage;

public interface MyInterface {

}

---

package myPackage;

public class MyImplementation implements MyInterface {
    public String field1;
    public String field2;

    public MyImplementation(String field1, String field2) {
        this.field1 = field1;
        this.field2 = field2;
    }
}

---

package myPackage;
import org.junit.Test;

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.DomDriver;

import static org.junit.Assert.*;


public class xstreamTest {
    @Test
    public void classAttributeSetInResult() {
        MyInterface objectToSerialize = new MyImplementation("value1", "value2");

        final XStream xStream = new XStream(new DomDriver());
        xStream.alias("myInterfaceElement", MyInterface.class);

        String xmlResult = xStream.toXML(objectToSerialize).toString();

        String expectedResult = 
"<myInterfaceElement class=\"myPackage.MyImplementation\">\n" +
"  <field1>value1</field1>\n" +
"  <field2>value2</field2>\n" +
"</myInterfaceElement>";

        assertEquals(expectedResult, xmlResult);
    }

    @Test
    public void deserializeWithClassAttribute() {
        String inputXmlString = 
"<myInterfaceElement class=\"myPackage.MyImplementation\">\r\n" +
"  <field1>value1</field1>\r\n" +
"  <field2>value2</field2>\r\n" +
"</myInterfaceElement>";

        final XStream xStream = new XStream(new DomDriver());

        MyInterface result = (MyInterface)xStream.fromXML(inputXmlString);
        assertTrue("Instance of MyImplementation returned", result instanceof MyImplementation);

        MyImplementation resultAsMyImplementation = (MyImplementation)result;
        assertEquals("Field 1 deserialized", "value1", resultAsMyImplementation.field1);
        assertEquals("Field 2 deserialized", "value2", resultAsMyImplementation.field2);
    }
}

Answer 1

我通过以下操作解决了这个问题（感谢 McD 提示使用转换器）：

1. 添加一个扩展 ReflectionConverter 的自定义转换器：

   package myPackage;
   
   import com.thoughtworks.xstream.converters.MarshallingContext;
   import com.thoughtworks.xstream.converters.reflection.ReflectionConverter;
   import com.thoughtworks.xstream.converters.reflection.ReflectionProvider;
   import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
   import com.thoughtworks.xstream.mapper.Mapper;
   
   public class MyInterfaceConverter extends ReflectionConverter {
   
       public MyInterfaceConverter(Mapper mapper, ReflectionProvider reflectionProvider) {
           super(mapper, reflectionProvider);
       }
   
       @Override
       public void marshal(Object original, final HierarchicalStreamWriter writer, final MarshallingContext context) {
           writer.addAttribute("class", original.getClass().getCanonicalName());
           super.marshal(original, writer, context);
       }
   
       @SuppressWarnings("rawtypes")
       @Override
       public boolean canConvert(Class type) {
           return MyInterface.class.isAssignableFrom(type);
       }
   
   }
   

2. 在我设置 xStream 时注册新的转换器：

   @Test
   public void classAttributeSetInResult() {
       MyInterface objectToSerialize = new MyImplementation("value1", "value2");
   
       final XStream xStream = new XStream(new DomDriver());
       xStream.alias("myInterfaceElement", MyImplementation.class);
       xStream.registerConverter(new MyInterfaceConverter(xStream.getMapper(), xStream.getReflectionProvider()));
   
       String xmlResult = xStream.toXML(objectToSerialize).toString();
   
       String expectedResult = 
           "<myInterfaceElement class=\"myPackage.MyImplementation\">\n" +
           "  <field1>value1</field1>\n" +
           "  <field2>value2</field2>\n" +
           "</myInterfaceElement>";
   
       assertEquals(expectedResult, xmlResult);
   }
   

希望这将有助于其他人。如果有人有更好的想法，请告诉我！

Answer 2

我会使用自定义转换器来解决这个问题：

你的类/接口：

public static interface MyInterface {
    public String getField1();
    public String getField2();
}

public static class MyImplementation implements MyInterface {
    public String field1;
    public String field2;

    public MyImplementation(String field1, String field2) {
        this.field1 = field1;
        this.field2 = field2;
    }

    public String getField1() { return field1; }
    public String getField2() { return field2; }
}

相当快速和肮脏的转换器：

public static class MyInterfaceConverter implements Converter {
    private static final String ATTR_NAME_CLASS  = "concrete-class";
    private static final String NODE_NAME_FIELD1 = "field1";
    private static final String NODE_NAME_FIELD2 = "field2";

    public boolean canConvert(Class type)
    {
        return type.equals(MyImplementation.class);
    }

    public void marshal(Object obj, HierarchicalStreamWriter writer, MarshallingContext context)
    {
        if (obj == null)
            // no need to save null-objects
            return;

        final String type = obj.getClass().getSimpleName();
        final MyInterface myInterface;
        if (obj instanceof MyImplementation)
            myInterface = (MyInterface) obj;
        // else if (...)
        //      ...
        else
            throw new IllegalArgumentException("Cannot convert objects of type " + obj.getClass());

        writer.addAttribute(ATTR_NAME_CLASS, type);
        marshalAttribute(writer, context, NODE_NAME_FIELD1, myInterface.getField1());
        marshalAttribute(writer, context, NODE_NAME_FIELD2, myInterface.getField2());
    }

    private static void marshalAttribute(HierarchicalStreamWriter writer, MarshallingContext context, String attrName, Object val)
    {
        if (val != null) {
            writer.startNode(attrName);
            context.convertAnother(val);
            writer.endNode();
        }
    }

    public Object unmarshal(HierarchicalStreamReader reader,
            UnmarshallingContext context)
    {
        final String type = reader.getAttribute(ATTR_NAME_CLASS);
        String field1Value = null, field2Value = null;
        while (reader.hasMoreChildren()) {
            reader.moveDown();
            if (NODE_NAME_FIELD1.equals(reader.getNodeName()))
                field1Value = (String)context.convertAnother(null, String.class);
            else if (NODE_NAME_FIELD2.equals(reader.getNodeName()))
                field2Value = (String)context.convertAnother(null, String.class);
            reader.moveUp();
        }

        if (MyImplementation.class.getSimpleName().equals(type)) {
            return new MyImplementation(field1Value, field2Value);
        }
        throw new IllegalArgumentException("Cannot unmarshal objects of type " + type);
    }
}

测试/使用/XStream-Initialization：

@Test
public void classAttributeSetInResult() {
    MyInterface objectToSerialize = new MyImplementation("value1", "value2");

    final XStream xStream = new XStream(new DomDriver());
    xStream.alias("myInterfaceElement", MyImplementation.class);
    // xStream.alias("myInterfaceElement", OtherImplementation.class);
    xStream.registerConverter(new MyInterfaceConverter());

    String xmlResult = xStream.toXML(objectToSerialize).toString();

    String expectedResult = 
"<myInterfaceElement concrete-class=\"MyImplementation\">\r\n" +
"  <field1>value1</field1>\r\n" +
"  <field2>value2</field2>\r\n" +
"</myInterfaceElement>";

    assertEquals(expectedResult, xmlResult);
}

@Test
public void deserializeWithClassAttribute() {
    String inputXmlString = 
"<myInterfaceElement concrete-class=\"MyImplementation\">\r\n" +
"  <field1>value1</field1>\r\n" +
"  <field2>value2</field2>\r\n" +
"</myInterfaceElement>";

    final XStream xStream = new XStream(new DomDriver());
    xStream.alias("myInterfaceElement", MyImplementation.class);
    // xStream.alias("myInterfaceElement", OtherImplementation.class);
    xStream.registerConverter(new MyInterfaceConverter());

    MyInterface result = (MyInterface)xStream.fromXML(inputXmlString);
    assertTrue("Instance of MyImplementation returned", result instanceof MyImplementation);

    MyImplementation resultAsMyImplementation = (MyImplementation)result;
    assertEquals("Field 1 deserialized", "value1", resultAsMyImplementation.field1);
    assertEquals("Field 2 deserialized", "value2", resultAsMyImplementation.field2);
}