Java Persistence with Hibernate, Second Edition

Christian Bauer, Gary Gregory

• Editorial: Manning Publications
• Año: 2015
• ISBN:
  9781617290459
• Categoría: Ciencia de la computación
• Categoría: Programación en Java

Java Persistence with Hibernate, Second Edition explores Hibernate by developing an application that ties together hundreds of individual examples. In this revised edition, authors Christian Bauer, Gavin King, and Gary Gregory cover Hibernate 5 in detail with the Java Persistence 2.1 standard (JSR 338). All examples have been updated for the latest Hibernate and Java EE specification versions.

Part 1. Getting started with ORM

In part 1, we’ll show you why object persistence is such a complex topic and what solutions you can apply in practice. Chapter 1 introduces the object/relational paradigm mismatch and several strategies to deal with it, foremost object/relational mapping (ORM). In chapter 2, we’ll guide you step by step through a tutorial with Hibernate and Java Persistence—you’ll implement and test a “Hello World” example. Thus prepared, in chapter 3 you’ll be ready to learn how to design and implement complex business domain models in Java, and which mapping metadata options you have available.

After reading this part of the book, you’ll understand why you need ORM and how Hibernate and Java Persistence work in practice. You’ll have written your first small project, and you’ll be ready to take on more complex problems. You’ll also understand how real-world business entities can be implemented as a Java domain model and in what format you prefer to work with ORM metadata.

Chapter 1. Understanding object/relational persistence

In this chapter

• Persistence with SQL databases in Java applications
• The object/relational paradigm mismatch
• Introducing ORM, JPA, and Hibernate

This book is about Hibernate; our focus is on using Hibernate as a provider of the Java Persistence API. We cover basic and advanced features and describe some ways to develop new applications using Java Persistence. Often, these recommendations aren’t specific to Hibernate. Sometimes they’re our own ideas about the best ways to do things when working with persistent data, explained in the context of Hibernate.

The approach to managing persistent data has been a key design decision in every software project we’ve worked on. Given that persistent data isn’t a new or unusual requirement for Java applications, you’d expect to be able to make a simple choice among similar, well-established persistence solutions. Think of web application frameworks (JavaServer Faces versus Struts versus GWT), GUI component frameworks (Swing versus SWT), or template engines (JSP versus Thymeleaf). Each of the competing solutions has various advantages and disadvantages, but they all share the same scope and overall approach. Unfortunately, this isn’t yet the case with persistence technologies, where we see some wildly differing solutions to the same problem.

Persistence has always been a hot topic of debate in the Java community. Is persistence a problem that is already solved by SQL and extensions such as stored procedures, or is it a more pervasive problem that must be addressed by special Java component models, such as EJBs? Should we hand-code even the most primitive CRUD (create, read, update, delete) operations in SQL and JDBC, or should this work be automated? How do we achieve portability if every database management system has its own SQL dialect? Should we abandon SQL completely and adopt a different database technology, such as object database systems or NoSQL systems? The debate may never end, but a solution called object/relational mapping (ORM) now has wide acceptance, thanks in large part to the innovations of Hibernate, an open source ORM service implementation.

Before we can get started with Hibernate, you need to understand the core problems of object persistence and ORM. This chapter explains why you need tools like Hibernate and specifications such as the Java Persistence API (JPA).

First we define persistent data management in the context of object-oriented applications and discuss the relationship of SQL, JDBC, and Java, the underlying technologies and standards that Hibernate builds on. We then discuss the so-called object/relational paradigm mismatch and the generic problems we encounter in object-oriented software development with SQL databases. These problems make it clear that we need tools and patterns to minimize the time we have to spend on the persistence-related code in our applications.

The best way to learn Hibernate isn’t necessarily linear. We understand that you may want to try Hibernate right away. If this is how you’d like to proceed, skip to the next chapter and set up a project with the “Hello World” example. We recommend that you return here at some point as you go through this book; that way, you’ll be prepared and have all the background concepts you need for the rest of the material.

1.1. What is persistence?

Almost all applications require persistent data. Persistence is one of the fundamental concepts in application development. If an information system didn’t preserve data when it was powered off, the system would be of little practical use. Object persistence means individual objects can outlive the application process; they can be saved to a data store and be re-created at a later point in time. When we talk about persistence in Java, we’re normally talking about mapping and storing object instances in a database using SQL. We start by taking a brief look at the technology and how it’s used in Java. Armed with this information, we then continue our discussion of persistence and how it’s implemented in object-oriented applications.

1.1.1. Relational databases

You, like most other software engineers, have probably worked with SQL and relational databases; many of us handle such systems every day. Relational database management systems have SQL-based application programming interfaces; hence, we call today’s relational database products SQL database management systems (DBMS) or, when we’re talking about particular systems, SQL databases.

Relational technology is a known quantity, and this alone is sufficient reason for many organizations to choose it. But to say only this is to pay less respect than is due. Relational databases are entrenched because they’re an incredibly flexible and robust approach to data management. Due to the well-researched theoretical foundation of the relational data model, relational databases can guarantee and protect the integrity of the stored data, among other desirable characteristics. You may be familiar with E.F. Codd’s four-decades-old introduction of the relational model, A Relational Model of Data for Large Shared Data Banks (Codd, 1970). A more recent compendium worth reading, with a focus on SQL, is C. J. Date’s SQL and Relational Theory (Date, 2009).

Relational DBMSs aren’t specific to Java, nor is an SQL database specific to a particular application. This important principle is known as data independence. In other words, and we can’t stress this important fact enough, data lives longer than any application does. Relational technology provides a way of sharing data among different applications, or among different parts of the same overall system (the data entry application and the reporting application, for example). Relational technology is a common denominator of many disparate systems and technology platforms. Hence, the relational data model is often the foundation for the common enterprise-wide representation of business entities.

Before we go into more detail about the practical aspects of SQL databases, we have to mention an important issue: although marketed as relational, a database system providing only an SQL data language interface isn’t really relational and in many ways isn’t even close to the original concept. Naturally, this has led to confusion. SQL practitioners blame the relational data model for shortcomings in the SQL language, and relational data management experts blame the SQL standard for being a weak implementation of the relational model and ideals. Application engineers are stuck somewhere in the middle, with the burden of delivering something that works. We highlight some important and significant aspects of this issue throughout this book, but generally we focus on the practical aspects. If you’re interested in more background material, we highly recommend Practical Issues in Database Management: A Reference for the Thinking Practitioner by Fabian Pascal (Pascal, 2000) and An Introduction to Database Systems by Chris Date (Date, 2003) for the theory, concepts, and ideals of (relational) database systems. The latter book is an excellent reference (it’s big) for all questions you may possibly have about databases and data management.

1.1.2. Understanding SQL

To use Hibernate effectively, you must start with a solid understanding of the relational model and SQL. You...