What’s new in JPA 2.2 – Java 8 Date and Time Types

Introduction

Now that the JPA 2.2 Review Ballot was approved, let’s start analyzing some of the new additions to the standard which have been supported by Hibernate for quite some time already.

In this article, we are going to see how Java 8 Date/Time API is supported and which types you need to use depending on your business case requirements.

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How to map Java and SQL arrays with JPA and Hibernate

Introduction

Hibernate custom Types allow you to map all sorts of database specific column types, like IP address, JSON columns, bit sets or SQL arrays.

There are two ways to define a custom Hibernate Type:

The latter option is preferred since it allows you to better split the Java-to-JDBC and the JDBC-to-SQL type handling. In this article, we are going to see how you can map SQL arrays to their Java counterpart.

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The best way to map the @DiscriminatorColumn with JPA and Hibernate

Introduction

As previously explained, the SINGLE_TABLE inheritance is the most efficient entity inheritance strategy.

However, for JPQL query such as this one:

List<Post> posts = entityManager
.createQuery(
    "select p " +
    "from Post p " +
    "where p.board = :board", Post.class)
.setParameter("board", board)
.getResultList();

Hibernate generates a SQL query which filters by the associated discriminator column (e.g. DTYPE by default):

SELECT t.id AS id2_1_,
       t.board_id AS board_id8_1_,
       t.createdOn AS createdO3_1_,
       t.owner AS owner4_1_,
       t.title AS title5_1_,
       t.content AS content6_1_
FROM   topic t
WHERE  t.DTYPE = 'Post'
       AND t.board_id = 1

So, because we are filtering by the discriminator column, we might want to index it or include it to speed up queries.

However, the default STRING DiscriminatorType expects a VARCHAR column that must hold the longest entity subclass name. For the Announcement class, we need at least 12 bytes to store the entity class name while for the Post entity, 4 bytes are required.

If the discriminator type column is indexed and we store 1 million Announcement and 100 million Post entities, the index will require 393 MB (12 + 400 million bytes). On the other hand, if the discriminator column is a TINYINT (only 1 byte is needed to store a discriminator value), we need only 96 MB (1 + 100 million bytes).

In this article, I’m going to explain how you can get the most out of the INTEGER DiscriminatorType while still retaining the descriptiveness of the default STRING DiscriminatorType.

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The best way to use entity inheritance with JPA and Hibernate

Introduction

Recently, my friend Lukas Eder wrote the following message on Twitter:

Just like in any OOP (Object-Oriented Programming) language, entity inheritance is suitable for varying behavior rather than reusing data structures, for which we could composition. The Domain Model compromising both data (e.g. persisted entities) and behavior (business logic), we can still make use of inheritance for implementing behavioral software design pattern.

In this article, I’m going to demonstrate how to use JPA inheritance as a means to implement the Strategy design pattern.

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The best way to map the SINGLE_TABLE inheritance with JPA and Hibernate

Introduction

Java, like any other object-oriented programming language, makes heavy use of inheritance and polymorphism. Inheritance allows defining class hierarchies that offer different implementations of a common interface.

Conceptually, the Domain Model defines both data (e.g. persisted entities) and behavior (business logic). Nevertheless, inheritance is more useful for varying behavior rather than reusing data (composition is much more suitable for sharing structures).

Even if the data (persisted entities) and the business logic (transactional services) are decoupled, inheritance can still help varying business logic (e.g. Visitor pattern).

In this article, we are going to see what is the best way to map the SINGLE_TABLE inheritance, which, not only is the default inheritance strategy, but it’s usually the most efficient way to model entity inheritance.

Continue reading “The best way to map the SINGLE_TABLE inheritance with JPA and Hibernate”