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.