在https://xebia.com/blog/mapping-multimaps-with-
hibernate/找到了答案
这是11年前的一篇长博客文章。关键代码是：
public class MultiMapType implements UserCollectionType {public boolean contains(Object collection, Object entity) { return ((MultiMap) collection).containsValue(entity);}public Iterator getElementsIterator(Object collection) { return ((MultiMap) collection).values().iterator();}public Object indexOf(Object collection, Object entity) { for (Iterator i = ((MultiMap) collection).entrySet().iterator(); i.hasNext();) { Map.Entry entry = (Map.Entry) i.next(); Collection value = (Collection) entry.getValue(); if (value.contains(entity)) { return entry.getKey(); } } return null;}public Object instantiate() { return new MultiHashMap();}public PersistentCollection instantiate(SessionImplementor session, CollectionPersister persister) throws HibernateException { return new PersistentMultiMap(session);}public PersistentCollection wrap(SessionImplementor session, Object collection) { return new PersistentMultiMap(session, (MultiMap) collection);}public Object replaceElements(Object original, Object target, CollectionPersister persister, Object owner, Map copyCache, SessionImplementor session) throws HibernateException { MultiMap result = (MultiMap) target; result.clear(); Iterator iter = ( (java.util.Map) original ).entrySet().iterator(); while ( iter.hasNext() ) { java.util.Map.Entry me = (java.util.Map.Entry) iter.next(); Object key = persister.getIndexType().replace( me.getKey(), null, session, owner, copyCache ); Collection collection = (Collection) me.getValue(); for (Iterator iterator = collection.iterator(); iterator.hasNext();) { Object value = persister.getElementType().replace( iterator.next(), null, session, owner, copyCache ); result.put(key, value); } } return result;}}
