[hibernate-dev] 6.0 - Type system

[Vlad Mihalcea]

There's a lot to dig in here. I'll have to get the branch and study the
changes, to come back with some opinions.

Vlad

On Thu, Jan 19, 2017 at 7:34 PM, Steve Ebersole <steve at hibernate.org> wrote:

> We are getting pretty far along on the 6.0 changes and I wanted to start
> a(nother) discussion about Types in 6.0 to get feedback and thoughts on a
> few topics.
>
> First a quick break down of JavaTypeDescriptors, SqlTypeDescriptors, Types
> and "persisters"...
>
> (a lot of this is the same from pre-6.0, just making things more explicit)
>
> JavaTypeDescriptors and SqlTypeDescriptors are the "lowest level", so let's
> start there.  A JavaTypeDescriptor is a descriptor of a given Java type.
> That is, it provides Hibernate with information about the Java type.  Is it
> a numeric type?  How do we compare 2 values of this type?  How do we make a
> deep copy of a value of this type?  Etc.  SqlTypeDescriptor is the same,
> but for a database type (VARCHAR, BLOB, etc).  These 2 work together to
> perform reading and writing at the JDBC-level.
>
> We decided to broadly categorize JavaTypeDescriptors based on the JPA type
> categorizations:
>
>    1. BASIC - BasicJavaDescriptor
>       1. TemporalJavaDescriptor
>       2. NumericJavaDescriptor
>    2. MANAGED - ManagedJavaDescriptor
>       1. EMBEDDABLE - EmbeddableJavaDescriptor
>       2. IDENTIFIABLE - IdentifiableJavaDescriptor
>          1. MAPPED_SUPERCLASS - MappedSupercassJavaDescriptor
>          2. ENTITY - EntityJavaDescriptor
>
>
> Type (org.hibernate.type.spi.Type) represents a combination of a
> JavaTypeDescriptor and one or more SqlTypeDescriptors in relation to a
> specific "non-root Navigable domain value".  Navigable is a query-focused
> contract (SQM/HQL/Criteria) so I wont get too deep into that here.  At a
> high-level t is similar to JPA's Bindable except that it applies to
> Collection indices (or map-keys) and elements (or map-values) as well.
> Navigable essentially represents an named navigation one can perform in a
> query.  The root Navigable is always an entity (EntityPersister).
> EntityPersister is the only Navigable that does not expose a Type.  (There
> is an EntityType, but it represents entity-valued non-root Navigables such
> as a ManyToOne).  All other navigables expose a Type.  That is all a
> long-winded way to say that Types represents that Java/SqlTypeDescriptors
> for a role-based Navigable.
>
> Like the categorization discussed above for JavaTypeDescriptor, Type has a
> similar categorization:
>
>    1. Type
>       1. BasicType
>          1. TemporalType
>       2. AnyType
>       3. ManagedType
>          1. EmbeddedType
>          2. IdentifiableType
>             1. MappedSuperclassType
>             2. EntityType
>
> It is important to keep in mind that these represents a specific reference
> to thse things in regards to a Navigable.  E.g. an EntityType is the "type"
> of a SingularPersistentAttribute that is a ManyToOne - it points to the
> corresponding EntityPersister but it also represents the FK columns to
> refer to the entity.  It is a role-based Navigable.
>
> Historically reads and writes have all routed through the Type (with
> certain Types delegating much of that to persisters).  That will no longer
> be the case in 6.0 as one of the main design goals for 6.0 is to re-write
> how Hibernate reads and writes (mainly reads) values from JDBC.  The major
> shift here is to read all values from JDBC using a "SqlSelectionReader"
> equivalent to a BasicType.  These values are read and held in an array that
> "readers" then know how to access (positionally) and use.  Most of that
> design is beyond the discussion here, but it useful to understand.  It is
> discussed in the design.adoc in my orm-sqm poc repo for those curious.
> Long story, short... Types no longer directly implement JDBC read/write
> which we will come back to later.
>
> PersistentAttribute and the other Navigables now take a role in JDBC
> reads/writes.  AttributeConverters and other read/write-related concerns
> have been moved to these contracts.  Again, most of this is covered in the
> mentioned design doc.
>
> Since Type no longer directly implements JDBC read/write operations I think
> it is important to ask ourselves what exactly we see as "customizable" wrt
> each Type.  Is that different for each category, or the same across all
> Type categories?  E.g. I know of no customization of EntityType as it
> exists in 5.x, and tbh I am not even sure what that would mean.  BasicType
> obviously has some parts that we want to allow users to override, but is
> that really best achieved through a custom BasicType?  Or is it better
> served by allowing custom JavaTypeDescriptor/SqlTypeDescriptor and/or
> SqlSelectionReader?  What about EmbeddedType?  CollectionType?  This would
> affect @TypeDef and Type registration methods specific to customizations.
>
> Persisters for the most part continue to serve the same role they have in
> the past with a few additions and some changes...
>
> One addition was the creation of an EmbeddedPersister.  *Embedded*.  This,
> like CollectionPersister, models a "role" e.g. "Person.name" as opposed to
> the Embeddable Name.class.  Note however that JPA calls it an
> EmbeddableType and expects info about the Embeddable (the Class).
> EmbeddedPersister is role-based (Embedded) instead, which is a mismatch.
> In the case there are more than 1 usage of the Embeddable in different
> Embedded roles then we have to decide which EmbeddedPersister to return.
> It affects the sub-Attributes information.  We could just return "one of
> them" and deal with it for Alpha1, but we should answer what we want to do
> there long term.
>
> Collectively, these persisters now implement the JPA ManagedType model
> directly.  Another addition was the creation of ManagedTypeImplementor,
> IdentifiableTypeImplementor and MappedSuperclassTypeImplementor in the
> persister hierarchy.  Which means we can now directly return them in our
> JPA Metamodel impl.
>
> That also means implementing JPA's notion of Attributes.  I also needed
> something similar for SQM's Navigable contract.  Plus I have been working
> towards changing how Hibernate understands Attributes internally
> (encapsulation - OO ftw!) for some time anyway, so this all dove-tailed
> well.
>
> There are some things we should discuss too in terms of user impact.  We
> know up front that we need to move to reading values from JDBC ResultSets
> positionally, as opposed to nominally which is how it was exposed in
> Hibernate prior to 6.0.  So we know already we will be asking implementors
> and consumers of those contracts to make changes.  Given that, we have
> *some* liberty in changing these contracts some more.  We just want to be
> cognizant of (a) how much we change, (b) who it affects (use cases) and (c)
> whether there are alternatives.  For any use cases we determine to be
> "valid" use cases, I think we need to make certain that there is some way
> to handle that in the new alternatives.
>
> One use case, e.g., is setting Query parameters and being able to specify
> its Type.  To a degree we want to be able to continue to support that.  But
> I think we limit it to just references to org.hibernate.type.Type (though
> "gutted") specifically and remove all others; and temporarily have the new
> org.hibernate.type.spi.Type interface extend the old.  This would allow
> them to continue to get these org.hibernate.type.Type references in some
> fashion and use them as Query parameter type hints.  But I think we should
> look at implementing some other parameter type "hints" like accepting
> PersistentAttribute/Navigable references, JPA (static) metamodel
> references, etc.  These are better, as they would include things like
> AttributeConverter whereas the Type reference would not.
>
> Sorry this got so long.  I've had a lot floating around in my head the last
> few days as I have worked on 6.0 and I wanted to bring them up for
> discussion.  The main thing I wonder about is what we mean by "custom
> types" in terms of what exactly is being customized?  And how does that
> relate specifically to BasicType versus EmbeddedType versus ...?
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