Chapter 1. Understanding object/relational persistence

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 (Struts versus WebWork), GUI component frameworks (Swing versus SWT), or template engines (JSP versus Velocity). 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.

For several years, persistence has been a hot topic of debate in the Java community. Many developers don't even agree on the scope of the problem. Is persistence a problem that is already solved by relational technology and extensions such as stored procedures, or is it a more pervasive problem that must be addressed by special Java component models, such as EJB entity beans? 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? Debate continues, but a solution called object/relational mapping (ORM) now has wide acceptance. Hibernate is an open source ORM service implementation.

Hibernate is an ambitious project that aims to be a complete solution to the problem of managing persistent data in Java. It mediates the application's interaction with a relational database, leaving the developer free to concentrate on the business problem at hand. Hibernate is a nonintrusive solution. You aren't required to follow many Hibernate-specific rules and design patterns when writing your business logic and persistent classes; thus, Hibernate integrates smoothly with most new and existing applications and doesn't require disruptive changes to the rest of the application.

This book is about Hibernate. We'll cover basic and advanced features and describe some ways to develop new applications using Hibernate. Often, these recommendations won't even be specific to Hibernate. Sometimes they will be our ideas about the best ways to do things when working with persistent data, explained in the context of Hibernate. This book is also about Java Persistence, a new standard for persistence that is part of the also updated EJB 3.0 specification. Hibernate implements Java Persistence and supports all the standardized mappings, queries, and APIs. Before we can get started with Hibernate, however, you need to understand the core problems of object persistence and object/relational mapping. This chapter explains why tools like Hibernate and specifications such as Java Persistence and EJB 3.0 are needed.

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 is built on. We then discuss the so-called object/relational paradigm mismatch and the generic problems we encounter in object-oriented software development with relational 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 of our applications. After we look at alternative tools and persistence mechanisms, you'll see that ORM is the best available solution for many scenarios. Our discussion of the advantages and drawbacks of ORM will give you the full background to make the best decision when picking a persistence solution for your own project.

We also take a look at the various Hibernate software modules, and how you can combine them to either work with Hibernate only, or with Java Persistence and EJB 3.0-compliant features.

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 second chapter of this book and have a look at the "Hello World" example and set up a project. We recommend that you return here at some point as you circle through the book. That way, you'll be prepared and have all the background concepts you need for the rest of the material.