preface to the first edition

Just because it is possible to push twigs along the ground with one's nose does not necessarily mean that that is the best way to collect firewood.

—Anthony Berglas

Today, many software developers work with Enterprise Information Systems (EIS). This kind of application creates, manages, and stores structured information and shares this information between many users in multiple physical locations.

The storage of EIS data involves massive usage of SQL-based database management systems. Every company we've met during our careers uses at least one SQL database; most are completely dependent on relational database technology at the core of their business.

In the past five years, broad adoption of the Java programming language has brought about the ascendancy of the object-oriented paradigm for software development. Developers are now sold on the benefits of object orientation. However, the vast majority of businesses are also tied to long-term investments in expensive relational database systems. Not only are particular vendor products entrenched, but existing legacy data must be made available to (and via) the shiny new object-oriented web applications.

However, the tabular representation of data in a relational system is fundamentally different than the networks of objects used in object-oriented Java applications. This difference has led to the so-called object/relational paradigm mismatch. Traditionally, the importance and cost of this mismatch have been underestimated, and tools for solving the mismatch have been insufficient. Meanwhile, Java developers blame relational technology for the mismatch; data professionals blame object technology.

Object/relational mapping (ORM) is the name given to automated solutions to the mismatch problem. For developers weary of tedious data access code, the good news is that ORM has come of age. Applications built with ORM middleware can be expected to be cheaper, more performant, less vendor-specific, and more able to cope with changes to the internal object or underlying SQL schema. The astonishing thing is that these benefits are now available to Java developers for free.

Gavin King began developing Hibernate in late 2001 when he found that the popular persistence solution at the time—CMP Entity Beans—didn't scale to nontrivial applications with complex data models. Hibernate began life as an independent, noncommercial open source project.

The Hibernate team (including the authors) has learned ORM the hard way—that is, by listening to user requests and implementing what was needed to satisfy those requests. The result, Hibernate, is a practical solution, emphasizing developer productivity and technical leadership. Hibernate has been used by tens of thousands of users and in many thousands of production applications.

When the demands on their time became overwhelming, the Hibernate team concluded that the future success of the project (and Gavin's continued sanity) demanded professional developers dedicated full-time to Hibernate. Hibernate joined jboss.org in late 2003 and now has a commercial aspect; you can purchase commercial support and training from JBoss Inc. But commercial training shouldn't be the only way to learn about Hibernate.

It's obvious that many, perhaps even most, Java projects benefit from the use of an ORM solution like Hibernate—although this wasn't obvious a couple of years ago! As ORM technology becomes increasingly mainstream, product documentation such as Hibernate's free user manual is no longer sufficient. We realized that the Hibernate community and new Hibernate users needed a full-length book, not only to learn about developing software with Hibernate, but also to understand and appreciate the object/relational mismatch and the motivations behind Hibernate's design.

The book you're holding was an enormous effort that occupied most of our spare time for more than a year. It was also the source of many heated disputes and learning experiences. We hope this book is an excellent guide to Hibernate (or, "the Hibernate bible," as one of our reviewers put it) and also the first comprehensive documentation of the object/relational mismatch and ORM in general. We hope you find it helpful and enjoy working with Hibernate.