Enterprise JavaBeans v3.0 - 1、Introduction

Chapter 1. Introduction

This book is about Enterprise JavaBeans 3.0, the latest version of the Enterprise JavaBeans specification, and its counterpart（n.副本, 极相似的人或物, 配对物）, the new Java Persistence specification. Just as the Java platform has revolutionized（革命化的，被彻底改革的） the way we think about software development, the Enterprise JavaBeans (EJB) and Java Persistence specifications have revolutionized the way we think about developing mission-critical enterprise software. They combine server-side components with distributed object technologies, asynchronous messaging, web services, and persistence to greatly simplify the task of application development. It automatically takes into account many of the requirements of business systems, including security, resource pooling, concurrency, and transactional integrity.

This book shows you how to use Enterprise JavaBeans and Java Persistence to develop scalable, portable business systems. But before we can start talking about EJB itself, we'll need a brief introduction to the technologies addressed by EJB and Java Persistence, such as component models, distributed objects, asynchronous messaging, and web services. In Chapter 2, we'll learn about the overall architecture that EJB and Persistence provide. In Chapters 3 and 4, we'll look at how these APIs are integrated together in a Java enterprise environment. The rest of the book is devoted to developing enterprise and entity beans for an imaginary business, and discussing advanced issues.

It is assumed that you're already familiar with Java; if you're not, Learning Java (O'Reilly) is an excellent introduction, as is Head First Java (O'Reilly). This book also assumes that you're conversant in the JDBC API, or at least in SQL. If you're not familiar with JDBC, see Database Programming with JDBC and Java (O'Reilly).

One of Java's most important features is platform independence. Since it was first released, Java has been marketed as "write once, run anywhere." While the hype has gotten a little heavy-handed at times, code written with Sun's Java programming language is remarkably platform-independent. EJB and Java Persistence aren't just platform-independent; they are also implementation-independent. If you've worked with JDBC, you know a little about what this means. Not only can the JDBC API run on a Windows machine or on a Unix machine, it can also access relational databases of many different vendors (DB2, Oracle, MySQL, MS SQL Server, etc.) by using different JDBC drivers. You don't have to code to a particular database implementation; just change JDBC drivers, and you change databases.[*] It's the same with EJB and Java Persistence. Ideally, an EJB component (an enterprise bean) or a Java Persistence object (an entity bean) can run in any application server that implements these specifications.[ ] This means that you can develop and deploy your EJB business system in one server, such as BEA's WebLogic, and later move it to a different EJB server, such as Pramati, Sybase's EAServer, or IBM's WebSphere, or to an open source project such as JBoss, Apache Geronimo, or JOnAS. Implementation independence means that your business components do not depend on the brand of server, which gives you many more options before, during, and after development and deployment.