使用XStream需注意的问题

         这些问题是我在实际工作中遇到的，遇到一个问题就记一个问题，因为我这人脑子不太好，过段时间就会想不起来怎么解决了。

(一) 类的继承关系，在转化为xml时，父类节点应添加class属性，指明当前是哪个子类继承了它，否则在由xml转化为Object时会出错，因为XStream并不知道该把当前节点实例化为哪种类型。 

(1)一个抽象类 Animal.java

package xml_chb;

public abstract class Animal ...{
    private int age;

    public int getAge() ...{
        return age;
    }

    public void setAge(int age) ...{
        this.age = age;
    }
}

(2) Pig继承自Animal

package xml_chb;

public class Pig extends Animal...{
    private int weight;

    public int getWeight() ...{
        return weight;
    }

    public void setWeight(int weight) ...{
        this.weight = weight;
    }
}

(3) AnimalContor类中包含Animal类型的变量

package xml_chb;

public class AnimalContor ...{
    private String id;
    private Animal animal;
    public Animal getAnimal() ...{
        return animal;
    }
    public void setAnimal(Animal animal) ...{
        this.animal = animal;
    }
    public String getId() ...{
        return id;
    }
    public void setId(String id) ...{
        this.id = id;
    }
    
}

(4) 将AnimalContor对象转化为xml

        //实例化一个AnimalContor对象
        AnimalContor ac=new AnimalContor();
        ac.setId("0001");
        
        //生成Pig对象，并作为AnimalContor的Animal变量实例
        Pig pig=new Pig();
        pig.setAge(10);
        pig.setWeight(100);
        ac.setAnimal(pig);
        
        //将对象转化为xml
        XStream xstream=new XStream(new DomDriver());
        xstream.alias("animalcontor",AnimalContor.class);
        String strxml=xstream.toXML(ac);
        System.out.println(strxml);

输出：

<animalcontor>
  <id>0001</id>
  <animal class="xml_chb.Pig">
    <weight>100</weight>
    <age>10</age>
  </animal>
</animalcontor>

注意，animal节点有class属性，表明当前类由哪个子类来实现

(5) 将xml转化为AnimalContor对象

animal节点有class属性

String str1="<animalcontor><id>0002</id><animal class='xml_chb.Pig'><weight>100</weight><age>11</age></animal></animalcontor>";
        AnimalContor ac1=(AnimalContor)xstream.fromXML(str1);
        System.out.println(ac1.getId());

输出：0002

animal节点没有class属性，会抛出异常

String str2="<animalcontor><id>0003</id><animal><weight>100</weight><age>11</age></animal></animalcontor>";
        AnimalContor ac2=(AnimalContor)xstream.fromXML(str2);
        System.out.println(ac2.getId());

抛出如下异常：

java.lang.InstantiationError: xml_chb.Animal
 at sun.reflect.GeneratedSerializationConstructorAccessor3.newInstance(Unknown Source)
 at java.lang.reflect.Constructor.newInstance(Constructor.java:274)
 at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.newInstance(Sun14ReflectionProvider.java:54)
 at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.instantiateNewInstance(AbstractReflectionConverter.java:223)
 at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:117)
 at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:56)
 at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:45)
 at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:46)
 at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshallField(AbstractReflectionConverter.java:182)
 at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doUnmarshal(AbstractReflectionConverter.java:159)
 at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:118)
 at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:56)
 at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:45)
 at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:46)
 at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:117)
 at com.thoughtworks.xstream.core.ReferenceByXPathMarshallingStrategy.unmarshal(ReferenceByXPathMarshallingStrategy.java:29)
 at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:826)
 at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:813)
 at com.thoughtworks.xstream.XStream.fromXML(XStream.java:761)
 at com.thoughtworks.xstream.XStream.fromXML(XStream.java:753)
 at xml_chb.AnimalContor.main(AnimalContor.java:41)
Exception in thread "main"