J2EE Design Patterns

Primary Key Patterns

Primary keys are the unique identifiers that allow one row in a database to be referenced by other rows in other tables. In the example we’ve been using throughout the chapter, there are primary keys on the PATIENT, STAFF, and VISIT tables identifying unique patients, staff members, and office visits. The primary keys allow us to reference a specific entity. Staff members and patients can be associated with multiple visits, and multiple visit notes can be associated with a single visit by referencing the appropriate primary key.

Primary keys are at the heart of relational database design: any J2EE application that directly inserts new records into a relational data structure has to be able to generate primary keys. Schemes for primary key generation vary widely. Many databases have field types that increment with each row, making it easy to generate new keys as needed. Since this kind of field isn’t standard SQL-92, other database types require alternate approaches, and auto-increment fields aren’t always the best approach anyway, particularly when you need to know the new key before writing anything to the database.

The patterns in this section describe approaches for creating primary keys within J2EE applications.

PK Block Generator Pattern

Most Java-based primary key generation mechanisms rely on an object that hands out IDs upon request. These objects are generally shared by an entire application to prevent duplicate assignments. Objects requiring a new primary ...