JAXB不解组接口列表 [英] JAXB doesn't unmarshal list of interfaces

问题描述

似乎JAXB无法读取它写的内容。请考虑以下代码：

It seems JAXB can't read what it writes. Consider the following code:

interface IFoo {
    void jump();
}

@XmlRootElement
class Bar implements IFoo {
    @XmlElement
    public String y;

    public Bar() {
        y = "";
    }

    public Bar(String y) {
        this.y = y;
    }

    @Override
    public void jump() {
        System.out.println(y);
    }
}

@XmlRootElement
class Baz implements IFoo {
    @XmlElement
    public int x;

    public Baz() {
        x = 0;
    }

    public Baz(int x) {
        this.x = x;
    }

    @Override
    public void jump() {
        System.out.println(x);
    }
}

@XmlRootElement
public class Holder {
    private List<IFoo> things;

    public Holder() {
        things = new ArrayList<>();
    }

    @XmlElementWrapper
    @XmlAnyElement
    public List<IFoo> getThings() {
        return things;
    }

    public void addThing(IFoo thing) {
        things.add(thing);
    }
}

// ...

try {
    JAXBContext context = JAXBContext.newInstance(Holder.class, Bar.class, Baz.class);

    Holder holder = new Holder();
    holder.addThing(new Bar("1"));
    holder.addThing(new Baz(2));
    holder.addThing(new Baz(3));

    for (IFoo thing : holder.getThings()) {
        thing.jump();
    }

    StringWriter s = new StringWriter();
    context.createMarshaller().marshal(holder, s);

    String data = s.toString();

    System.out.println(data);

    StringReader t = new StringReader(data);
    Holder holder2 = (Holder)context.createUnmarshaller().unmarshal(t);

    for (IFoo thing : holder2.getThings()) {
        thing.jump();
    }
}
catch (Exception e) {
    System.err.println(e.getMessage());
}

当然，这是一个简化的例子。关键是我必须在一个集合中存储两个非常不同的实现类Bar和Baz。好吧，我观察到他们有非常相似的公共接口，所以我创建了一个接口IFoo并使它们实现它。现在，我想要有工具来保存和加载这个集合到XML或从XML加载。不幸的是，这段代码不太有用：集合已保存，但无法加载！预期的输出是

It's a simplified example, of course. The point is that I have to store two very differently implemented classes, Bar and Baz, in one collection. Well, I observed that they have pretty similar public interface, so I created an interface IFoo and made them two to implement it. Now, I want to have tools to save and load this collection to/from XML. Unfortunately, this code doesn't quite work: the collection is saved, but then it cannot be loaded! The intended output is

1
2
3
some xml
1
2
3

但不幸的是，实际产量是

But unfortunately, the actual output is

1
2
3
some xml
com.sun.org.apache.xerces.internal.dom.ElementNSImpl cannot be cast to testapplication1.IFoo

显然，我需要使用注释以不同的方式？或者放弃JAXB并寻找其他东西？我可以为我不想（de）编组的所有类编写XMLNode toXML（）方法，但是......

Apparently, I need to use the annotations in a different way? Or to give up on JAXB and look for something else? I, well, can write "XMLNode toXML()" method for all classes I wan't to (de)marshal, but...

推荐答案

尝试以下 @XmlAnyElement（lax = true）。 lax 标志告诉JAXB（JSR-222）实现根据 @XmlRootElement 和<来匹配域对象的元素code> @XmlElementDecl 注释。没有它，内容将被视为DOM节点。

Try the following @XmlAnyElement(lax=true). The lax flag tells the JAXB (JSR-222) implementation to match elements to domain objects based their @XmlRootElement and @XmlElementDecl annotations. Without it the contents are treated as DOM nodes.

@XmlRootElement
public class Holder {
    private List<IFoo> things;

    public Holder() {
        things = new ArrayList<>();
    }

    @XmlElementWrapper
    @XmlAnyElement(lax=true)
    public List<IFoo> getThings() {
        return things;
    }

    public void addThing(IFoo thing) {
        things.add(thing);
    }
}

更多信息

• http://blog.bdoughan.com/2010/08/using-xmlanyelement-to-build-generic.html

• http://blog.bdoughan.com/2010/08/using-xmlanyelement-to-build-generic.html

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