I have a system where a number of entities represent the ids os objects which never change and may be cached in an application (imagine state abbreviations or a product catalog).
When I declare these references as many-to-one associations hibernate does a join which is slow. Instead I developed a UserType which converts an id to an object held in application cache.
My declaration looks like this
<class name="com.babel17.fbdraft.impl.hibernate.PlayerImplLite" table="player" polymorphism="explicit">
<id name="playerId" type="long" unsaved-value="null">
<column name="player_id" sql-type="int8" not-null="true"/>
<generator class="increment"/>
</id>
<property name="firstName" column="first_name" type="string" not-null="true" length="40"/>
...
<property name="team" type="com.babel17.fbdraft.hibernate.MLBTeamUserType">
<column name="team_id"/>>
</property>
</class>

The generated SQL is
Hibernate: select this.player_id as player_id0_, this.first_name as first_name0_, this.last_name as last_name0_, this.team_id as team_id0_ from player this where 1=1
Hibernate: update player set first_name=?, last_name=?, team_id=? where player_id=?
The query takes 45 sec to retrieve 800 players
If I comment out the team property or declare mutable=false then the update is not issued and the query takes less than 1 sec
So what is going on? Why is an update issued and how do I cause it to go away without making the object immutable

Fix your implementation of UserType.equals(x, y)

1) Good call the equals function was broken
2) However fixing equals does not alter the behavior described in my first post - I checked and equals always returns true when called.
It is true that DeepCopy simply returns the passes argument - I presume this is OK for a passed immutable instance.
The code still does the right thing when the object is marked in the hbm file with mutable="false" but I still don't understant what hibernate is trying to do or why not why it takes so long