Database Access - Java - page 725

Chapter 18. Database Access
Introduction
Java can be used to access many kinds of databases. A database can be something as simple
as a text file or a fast key/value pairing on disk (DBM format), as sophisticated as a SQL-
based relational database management system (DBMS), as scalable as a cloud-based
“NoSQL” database, or as exotic as an object database. That said, this chapter focuses on rela-
tional databases.
Regardless of how your data is actually stored, in many applications you'll want to write a
class called an accessor [ 53 ] to mediate between the database and the rest of the application.
For example, if you are using JDBC, the answers to your query come back packaged in an
object called a ResultSet , but it would not make sense to structure the rest of your applica-
tion around the ResultSet because it's JDBC-specific. In a Personal Information Manager
application, the primary classes might be Person , Address , and Meeting . You would prob-
ably write a PersonAccessor class to request the names and addresses from the database
and generate Person and Address objects from them. The DataAccessor objects would also
take updates from the main program and store them into the database.
If this reminds you of Enterprise JavaBeans, you're right. If you're familiar with EJB 2
Entity Beans or JPA Entities (incorrectly but frequently called “EJ3 Entities”), you can think
of simple entity beans as a specialized kind of data accessor. In fact, JPA provides a high-
level mechanism for easily building accessors, and is discussed in Easy Database Access
with JPA and/or Hibernate .
Java DataBase Connectivity (JDBC) consists of classes in package java.sql and some ex-
tensions in package javax.sql . (SQL is the ANSI-standardized Structured Query Language,
used by relational database software for ages to provide a reasonably standard command lan-
guage for creating, modifying, updating, and querying relational databases.)
Why was JDBC invented? Java is highly portable, but many databases previously lacked a
portable interface and were tied to one particular language or platform. JDBC is designed to
fill that gap. JDBC is patterned very loosely on Microsoft's Open DataBase Connectivity
(ODBC). Sun's Java group borrowed several general ideas from Microsoft, which in turn