HHH-4257 map key type no longer inferred correctly, throws exception at runtime

Related to: http://opensource.atlassian.com/projects/hibernate/browse/HHH-4257

Description from HHH-4257
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after upgrading from annotations 3.3.0GA to 3.3.1GA, my mapped collection (ManyToMany) is broken. i've identified the commit that broke it:

http://fisheye.jboss.com/browse/Hibernate/annotations/trunk/src/java/org/hibernate/cfg/annotations/MapBinder.java?r1=12963&r2=14425

The issue is that unless the type is given now expliticly in the @MapKey's @Type, the code now assumes a 'Serializable' type, and attempts to deserialize an object stream from the database column value. the exact exception is:

java.io.StreamCorruptedException: invalid stream header
at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:764)
at java.io.ObjectInputStream.<init>(ObjectInputStream.java:277)
at org.hibernate.util.SerializationHelper$CustomObjectInputStream.<init>(SerializationHelper.java:252)
at org.hibernate.util.SerializationHelper.deserialize(SerializationHelper.java:209)
at org.hibernate.util.SerializationHelper.deserialize(SerializationHelper.java:240)
at org.hibernate.type.SerializableType.fromBytes(SerializableType.java:82)
at org.hibernate.type.SerializableType.get(SerializableType.java:39)
at org.hibernate.type.NullableType.nullSafeGet(NullableType.java:163)
...

The fix is simple: if @Type is NOT given in the @MapKey annotation, keep the old behavior which at least tries to set the type, even if it may not work in all situations. The attached patch fixes the problem for me.

In my particular situation, the map key type is a java.lang.Enum. Here's what my annotated property looks like:

@ManyToMany
@JoinTable(
name="chargeback_owner"
,joinColumns=@JoinColumn(name="person_id",referencedColumnName="person_id")
,inverseJoinColumns=@JoinColumn(name="owner_person_id")
)
@MapKey(columns=@Column(name="owner_type_cd"),targetElement=ChargebackOwnerType.class)
@Enumerated(EnumType.STRING)
@Sort(type=SortType.NATURAL)
private SortedMap<ChargebackOwnerType,Person> owners = new TreeMap<ChargebackOwnerType, Person>();
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