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Mastering Hibernate
Mastering Hibernate

Mastering Hibernate: Learn how to correctly utilize the most popular Object-Relational Mapping tool for your Enterprise application

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Mastering Hibernate

Chapter 2. Advanced Mapping

In this chapter, we will explore mapping concepts. This is at the core of the Object-Relational Mapping challenge to correctly map Java objects and classes to their persistence representation. As discussed in the previous chapter, the object-oriented world has a lot in common with the relational world, attributes, types, and so on. The ORM solutions, such as Hibernate, were created to address their notable differences, inheritance, composition, object reference and identification, and much more. After reading this chapter, you will be able to correctly map database identification to a class attribute using primitive data types or a more complex Java class. You will also see how to create associations between objects and annotate them correctly so that Hibernate has enough information about the relationships between table data. Furthermore, you will be able to instruct Hibernate to perform cascaded operations and map classes as part of an inheritance...

Mapping concepts

Mapping objects to relations is the core mission of Hibernate, and as we discussed previously, this can be a difficult challenge that can only be overcome by understanding the fundamental concepts.

Let's explore mapping concepts by reviewing some of the fundamentals of relational databases, such as relational algebra, simple and composite IDs, join tables, and foreign keys.

Instances and rows

The origin of relational database concepts is Relational Algebra, which is based on Set Theory. Just as we think of objects as instances of classes, you should also think of a database row (a tuple) as an element of a set (the relation) that is defined by the database tab.

For example, if your object graph contains an instance of the Person entity class that represents John and another instance that represents Sara, you should think of these as two rows in the Person table.

In both cases, by being an instance of a class or a row of a table, they agree to possess certain attributes...

Association cardinality

Just as relational databases define the cardinality of table relationships, ORM solutions also define the cardinality of entity associations. Before we discuss how to map associations, let's review how associations are created in a relational database. The following are the two types of relationship:

  • Foreign key relationship: You can relate a table to another by declaring a column as a foreign key of the other table. This is typically used for a 1:1 (one-to-one) or a M:1 (many-to-one) relationship. If you make the foreign key column UNIQUE, then you are defining a 1:1 relationship; otherwise, this is M:1. If you make it NULLABLE, then the relationship is optional; that is, 1:0, 1:1, M:0, or M:1.
  • Join table relationship: Another way to relate two tables is by creating a join table that will have at least two columns that store the primary keys of the rows that should be related. This is typically used for 1:M and M:N relationships. Both columns should be NON-NULLABLE...

Cascade operations

You may already be familiar with cascade delete and update in a relational database. In the ORM world, this notion is a little more complicated. Hibernate treats this as a transitive property that may or may not propagate to the associated entities, depending on the state of the objects (transient, detached, or persistent) as well as the cascade settings that you choose when you define your association.

Hibernate doesn't automatically propagate the persistence operations to the associated entities. However, you can control this behavior using the cascade attribute of an association.

JPA defines certain cascade types: ALL, DETACH, MERGE, PERSIST, REFRESH, and REMOVE. These correspond directly with the operations provided by the EntityManager interface.

The Hibernate Session interface provides additional operations, such as save, update, or replicate (some names are different in Hibernate and JPA but the underlying operations are the same; for example, detach versus evict...

Inheritance

One of the biggest challenges in mapping objects to relations is inheritance. Relational databases do not support this concept. So, ORM solutions need to get creative when dealing with this issue. JPA specifies several strategies, all of which are implemented by Hibernate and these will be discussed here.

Single table strategy

The default strategy to support class hierarchy, in the case of inheritance, is single table strategy. If you don't specify any strategy, Hibernate will look for (or create) a single table with the name of the parent class. This table has columns for every attribute in all the classes in the inheritance model. Let's consider the following superclass and its subclasses:

@Entity
public class Person {
  @Id
  @GeneratedValue
  private long id;
  
  private String firstname;
  private String lastname;

}
  
@Entity
public class Driver extends Person {

  @Column(name="LIC_NUM")
  private String licenseNumber;

}

@Entity
public class Passenger...

Enumeration and custom data type

In this section, we see how to map "constants" to enumerated types. Constants are considered a small finite set of data elements, such as gender (Male, Female) or days of the week, such as Monday, Tuesday, and so on. Hibernate supports using enumerated types for mapping database columns containing data in a finite closed set. (The definition of a closed set comes straight from set theory.)

If you don't wish to use enumerated types, you can create a custom data type to map your data to a primitive type in Java, and you'll see that after we discuss the enumerated type.

Enumerated type

You can define an enumeration in Java and let Hibernate handle the mapping of "constants". JPA supports two kinds of enum: ORDINAL and STRING. If you use the ORDINAL enumeration type, the data in the mapped column is assumed to be an integer. You can guess how the STRING enumeration type behaves:

public enum WeekDay {
  Monday, Tuesday, Wednesday, Thursday...

Mapping concepts


Mapping objects to relations is the core mission of Hibernate, and as we discussed previously, this can be a difficult challenge that can only be overcome by understanding the fundamental concepts.

Let's explore mapping concepts by reviewing some of the fundamentals of relational databases, such as relational algebra, simple and composite IDs, join tables, and foreign keys.

Instances and rows

The origin of relational database concepts is Relational Algebra, which is based on Set Theory. Just as we think of objects as instances of classes, you should also think of a database row (a tuple) as an element of a set (the relation) that is defined by the database tab.

For example, if your object graph contains an instance of the Person entity class that represents John and another instance that represents Sara, you should think of these as two rows in the Person table.

In both cases, by being an instance of a class or a row of a table, they agree to possess certain attributes, such...

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Key benefits

  • Understand the internals of Hibernate and its architecture, and how it manages Entities, Events, Versioning, Filters, and Cache
  • Observe how Hibernate bridges the gap between object-oriented concepts and relational models
  • Discover how Hibernate can address architectural concerns such as Transaction, Database Multi-tenancy, Clustering, and Database Shards

Description

Hibernate has been so successful since its inception that it even influenced the Java Enterprise Edition specification in that the Java Persistence API was dramatically changed to do it the Hibernate way. Hibernate is the tool that solves the complex problem of Object Relational Mapping. It can be used in both Java Enterprise applications as well as .Net applications. Additionally, it can be used for both SQL and NoSQL data stores. Some developers learn the basics of Hibernate and hit the ground quickly. But when demands go beyond the basics, they take a reactive approach instead of learning the fundamentals and core concepts. However, the secret to success for any good developer is knowing and understanding the tools at your disposal. It’s time to learn about your tool to use it better This book first explores the internals of Hibernate by discussing what occurs inside a Hibernate session and how Entities are managed. Then, we cover core topics such as mapping, querying, caching, and we demonstrate how to use a wide range of very useful annotations. Additionally, you will learn how to create event listeners or interceptors utilizing the improved architecture in the latest version of Hibernate.

Who is this book for?

Mastering Hibernate is intended for those who are already using or considering using Hibernate as the solution to address the problem of Object Relational Mapping. If you are already using Hibernate, this book will help you understand the internals and become a power user of Hibernate.

What you will learn

  • Understand the internals of a Hibernate session and how Entities are managed
  • Declare better mapping between entity classes and database tables
  • Manage entity associations and collections
  • Fetch data not just by entity ID, but also using HQL, Criteria Objects, Filters, and Native SQL
  • Observe the first and second level caches and find out how to manage them
  • Collect statistics and metrics data for further observation
  • Make your application work with multi-tenant databases
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Table of Contents

10 Chapters
1. Entity and Session Chevron down icon Chevron up icon
2. Advanced Mapping Chevron down icon Chevron up icon
3. Working with Annotations Chevron down icon Chevron up icon
4. Advanced Fetching Chevron down icon Chevron up icon
5. Hibernate Cache Chevron down icon Chevron up icon
6. Events, Interceptors, and Envers Chevron down icon Chevron up icon
7. Metrics and Statistics Chevron down icon Chevron up icon
8. Addressing Architecture Chevron down icon Chevron up icon
9. EJB and Spring Context Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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CM Aug 20, 2016
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The book doesn't get into any details of really mastering n Hibernate but is a good book for reference.
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