hibernate-orm/design/working/6.0-posts.adoc

= Hibernate 6.0 Final
Steve Ebersole
:awestruct-tags: ["Hibernate ORM"]
:awestruct-layout: blog-post
:docs-url: https://docs.jboss.org/hibernate/orm/6.0
:javadocs-url: {docs-url}/javadocs
:migration-guide-url: {docs-url}/migration-guide/migration-guide.html
:user-guide-url: {docs-url}/userguide/html_single/Hibernate_User_Guide.html
:jakarta-transformer-url: https://github.com/eclipse/transformer

It has been years in the making, but ORM 6.0 Final has finally been released!

This announcement will discuss the major changes as well as give insight into why
certain choices were made.

We will also be following up with a series of more focused posts targeting specific
improvements or cool new features.  Stay tuned!


[[api-spi]]
== APIs and SPIs

While many things have changed in 6.0, we strove to minimize changes to APIs to help
mitigate migration costs.

[NOTE]
====
See https://hibernate.org/community/compatibility-policy/ for a discussion of what we consider
an API versus an SPI.
====

Applications which use only the Jakarta Persistence APIs will be source compatible within the
discussion in <<jpa>>.

Applications using Hibernate APIs will generally be bytecode and source compatible, aside
from the removal of deprecated stuff.  There are a few one-off changes that break bytecode and/or
source compatibility; these are covered in the link:{migration-guide-url}[migration guide].

One specific change to note is that many of these contracts have been better defined with type
parameters.  Theses where inconsistently and sometimes poorly defined in previous versions.

Quite a few SPI contracts have changed to support many of the topics discussed here as well as in
the link:{migration-guide-url}[migration guide].  Many will also be the subject of the mentioned
follow-up posts.


[[jpa]]
== Jakarta Persistence

Java Persistence has become Jakarta Persistence as part of the overall move of Java EE
to Jakarta.  Various legal requirements forced the changing of the `javax` namespace -
for persistence, that means changing from `javax.persistence` to `jakarta.persistence`
for package names as well as property and hint names.

This is clearly an unfortunate and invasive change, but beyond our control.  Luckily Jakarta
have developed a link:{jakarta-transformer-url}[transformer] to help with these migrations.  We actually
used this tool to migrate Hibernate's own source code.  It works well-ish.

For those using Maven, you are in luck (well, within the bounds of actually using Maven) in that
Jakarta themselves provide a Maven plugin to integrate this transformer.

For those using Gradle, you can use the tasks we developed to transform Hibernate's source code.

There is also a command-line form.  See the link:{jakarta-transformer-url}[transformer] docs for details.


[[read-by-position]]
== Read-by-position

A few years ago, around the 5.4 timeframe, we worked with the amazing performance team at Red Hat
to squeeze even more great performance out of Hibernate ORM.

This work was part of a larger effort to improve the performance of WildFly.  Ultimately, the limiting
factor to additional improvements within Hibernate was our approach of reading values from a JDBC
`ResultSet` by name rather than by position.  For every JDBC driver out there, reading by name is slower.

It quickly became obvious that minimal changes would not be enough, and so this work led to many changes.
A great analogy is to consider migrating a Map-based solution to List-based.  There is the obvious impact
of changing calls to accept an `int` rather than a `String` as well as internally keeping track of the
positions of each selected value within the `ResultSet`.  There is also the perhaps not-so-obvious
impact of changing the callers and consumers of those contracts to keep track of positions.

These changes have led to improvements on a number of fronts:

1. As mentioned, reading by position is significantly faster than reading by name which leads to
performance improvements.
2. Historically Hibernate generated SQL select queries with a defined pattern of named column aliases
which were later used to access the specific result.  We've all seen these "ugly" aliases.  With these
changes, those select-clause aliases are no longer needed resulting in much more readable generated
SQL.
3. Although we implemented some improved support for limiting needed joins within an entity mapping
(joined inheritance, secondary tables) in 5.x, 6.0 allows even better opportunity for this.  In
fact, the support for this in 5.x was conceptually back-ported from the 6.0 work.
4. (2) and (3) combined results in much smaller SQL needing to be sent to the server which can
have an impact on network communication.  Every bit helps.

This was by far the biggest force behind 6.0 initially.



[[mapping-model]]
== Mapping Model

The mapping model is an SPI and as such will not be seen by all users.  But
it is a major development and impacts many users providing extensions.

The main driving force behind this mapping model work was <<read-by-position>>,
and we had a number of design goals in developing it:

- support positional processing of attributes
- make it object-oriented
- make it user friendly

This model can be accessed though
link:{javadocs-url}/org/hibernate/engine/spi/SessionFactoryImplementor.html#getRuntimeMetamodels()[`RuntimeMetamodelsImplementor`]
which provides access to both:

- The Jakarta Persistence model : link:{javadocs-url}/org/hibernate/metamodel/spi/RuntimeMetamodelsImplementor.html#getJpaMetamodel()[`JpaMetamodelImplementor`]
- Hibernate's mapping model : link:{javadocs-url}/org/hibernate/metamodel/spi/RuntimeMetamodelsImplementor.html#getMappingMetamodel()[`MappingMetamodelImplementor`]


[[annotations]]
== Annotations

- type-safety
- `SqlTypes` - extension to `java.sql.Types`


[[query]]
== Query API

- Parameter binding
- `Session#createSelectionQuery`
- `Session#createMutationQuery`
- others???


[[sqm]]
== Semantic Query Model

Hibernate's Semantic Query Model (SQM) is its semantic representation of HQL
and Criteria queries.  HQL is interpreted into SQM; Hibernate's Criteria
implementations are SQM nodes.

6.0 implements quite a few changes shared between HQL and Criteria.  Most of
these are covered in link:{user-guide-url}#query-language[HQL] and
link:{user-guide-url}#criteria[Criteria] chapters of the User Guide.

Some specific changes include

- Automatic de-duplication of single entity results in a Query.  See the link:{migration-guide-url}#query-sqm-rows[Migration Guide] for details
- Set operations (union, intersect, except)
- Set aggregations (listagg, e.g.)
- Window operations (over, e.g.)
- Vastly improved function support.  See the link:{user-guide-url}#hql-exp-functions[User Guide] for details.
- ILIKE operator
- Improved temporal support (arithmetic, etc)


[[hql]]
== HQL

Previous versions of Hibernate used Antlr 2 for parsing.  6.0 updates to Antlr 4 for a few reasons:

- Antlr 2 is no longer supported, and has not for years
- Antlr 4 is faster than Antlr 2
- Antlr 4 grammars are easier to maintain, while the previous Antlr 2 grammars were poorly defined (largely as a function of Antlr 2 itself) and difficult to maintain.



[[criteria]]
== Criteria

- removal of legacy Criteria
- no longer Criteria -> HQL -> Query
- `hibernate.criteria.copy_tree` (performance)
- improved handling of parameter vs. literal




[[sql-ast]]
== SQL as AST

mention Dialect involvement


[[dialect-init]]
== Dialect initialization

- `Dialect` constructor
- `Dialect#initializeFunctionRegistry`