JAXB Marshall是泛型类型的Map结构 [英] JAXB Marshall a Map structure of generic type

问题描述

按照Blaise Doughan的这个例子（http://blog.bdoughan.com/2010/07/xmladapter-jaxbs-secret-weapon.html），我试图将它推广到使用泛型的任何类型。
我获得：

Following this example (http://blog.bdoughan.com/2010/07/xmladapter-jaxbs-secret-weapon.html) of Blaise Doughan that works, i am trying to generalize it to any type using generics. I obtained :

import java.util.*;
import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class MyMapType<K, V> {

    public List<MyMapEntryType<K, V>> entry;

    public MyMapType() {
        entry = new ArrayList<MyMapEntryType<K, V>>();
    }

}

和

import javax.xml.bind.annotation.*;

public class MyMapEntryType<K, V> {

    @XmlAttribute
    public K key;

    @XmlValue
    public V value;

}

然后，

import java.util.*;
import java.util.Map.Entry;

import javax.xml.bind.annotation.adapters.XmlAdapter;

public final class MyMapAdapter<K, V> extends
        XmlAdapter<MyMapType<K, V>, Map<K, V>> {

    @Override
    public MyMapType<K, V> marshal(Map<K, V> arg0) throws Exception {
        MyMapType<K, V> myMapType = new MyMapType<K, V>();
        for (Entry<K, V> entry : arg0.entrySet()) {
            MyMapEntryType<K, V> myMapEntryType = new MyMapEntryType<K, V>();
            myMapEntryType.key = entry.getKey();
            myMapEntryType.value = entry.getValue();
            myMapType.entry.add(myMapEntryType);
        }
        return myMapType;
    }

    @Override
    public Map<K, V> unmarshal(MyMapType<K, V> arg0) throws Exception {
        HashMap<K, V> hashMap = new HashMap<K, V>();
        for (MyMapEntryType<K, V> myEntryType : arg0.entry) {
            hashMap.put(myEntryType.key, myEntryType.value);
        }
        return hashMap;
    }

}

最后，

package forum832656;

import java.util.*;
import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Foo<K, V> {

    @XmlJavaTypeAdapter(MyMapAdapter.class)
    Map<K, V> map = new HashMap<K, V>();

    public Foo() {
    }

    public Map<K, V> getMap() {
        return map;
    }

    public void setMap(Map<K, V> map) {
        this.map = map;
    }

}

然后我试了这个，

 Foo<Integer,String> f = new Foo<Integer,String>();
    f.getMap().put(1, "HELLO");
    f.getMap().put(2, "WORLD");


    JAXBContext context =   JAXBContext.newInstance(Foo.class,MyMapType.class);
        Marshaller m = context.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
        m.marshal(f, System.out);

但我在JAXBContext创建行上获得了NullPointerException ....
有什么想法？

But i obtained a NullPointerException on JAXBContext creation line.... Any idea?

推荐答案

我用递归适配器解决了这个问题：

I solved this with a recursive Adapter:

import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class MapAdapter<K, V> extends XmlAdapter<Adapter<K, V>, Map<K, V>> {

    @Override
    public Map<K, V> unmarshal(Adapter<K, V> v) throws Exception {
        if (v == null) {
            return null;
        }
        Map<K, V> map = new HashMap<K, V>();
        for (MyEntry<K, V> mapEntryType : v.getEntries()) {
            map.put(rUnmarshal(mapEntryType.getKey()),     rUnmarshal(mapEntryType.getValue()));
        }
        return map;
    }

    @Override
    public Adapter<K, V> marshal(Map<K, V> v) throws Exception {
        if (v == null) {
            return null;
        }
        return new Adapter<K, V>(v);
    }

    @SuppressWarnings("unchecked")
    private static <T> T rUnmarshal(T obj) throws Exception {
        if (obj instanceof Adapter) {
            return (T) new MapAdapter<>().unmarshal((Adapter<Object, Object>) obj);
        }
        return obj;
    }

    @SuppressWarnings("unchecked")
    static <T> T rMarshal(T obj) throws Exception {
        if (obj instanceof Map) {
            return (T) new MapAdapter<>().marshal((Map<Object, Object>) obj);
        }
        return obj;
    }

    @XmlType
    @XmlRootElement
    public final static class Adapter<K, V> {

        @XmlElement
        protected List<MyEntry<K, V>> fEntries = new LinkedList<MyEntry<K, V>>();

        // needed for JAXB
        @SuppressWarnings("unused")
        private Adapter() {
        }

        public Adapter(Map<K, V> original) throws Exception {
            for (Map.Entry<K, V> entry : original.entrySet()) {
                this.fEntries.add(new MyEntry<K, V>(entry));
            }
        }

        public List<MyEntry<K, V>> getEntries() {
            return this.fEntries;
        }

    }

    @XmlType
    @XmlRootElement
    public final static class MyEntry<K, V> {

        @XmlElement
        protected K fKey;

        @XmlElement
        protected V fValue;

        // needed for JAXB
        @SuppressWarnings("unused")
        private MyEntry() {
        }

        public MyEntry(Map.Entry<K, V> original) throws Exception {
            this.fKey = rMarshal(original.getKey());
            this.fValue = rMarshal(original.getValue());
        }

        public K getKey() {
            return this.fKey;
        }

        public V getValue() {
            return this.fValue;
        }

    }

}

这应该避免你的NPE。

This should avoid your NPE.

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