primary-key-class

A database table may have a simple primary key (single column like productID) or a composite primary key (multiple columns like ID + RegionCode). If a primary key changes from simple to composite, entity bean code that assumed a single String key would break. There needs to be a flexible way to represent primary keys in EJB.

A Primary Key Class is a custom Java class that wraps one or more primary key fields. It is used as the return type of ejbCreate() and the parameter type for findByPrimaryKey(). Using a wrapper class makes the bean future-proof against schema changes.

1. Option 1 (Direct Field): Use a simple type like String or int as the primary key, declared in ejb-jar.xml via <primkey-field>productID</primkey-field>
2. Option 2 (Wrapper Class): Create a class like ProductPK.java that holds the key fields as instance variables
3. The wrapper class must implement Serializable and override equals() and hashCode() (so the container can compare keys)
4. If the database schema changes to a composite key, you just add fields to the wrapper class — bean code doesn’t break
5. The container uses the primary key to identify entity bean instances in the pool

• Must be Serializable for EJB container to pass keys between JVMs
• Must override equals() and hashCode() for proper key comparison
• Wrapper class protects against database schema changes (composite key evolution)
• Used by ejbCreate(), ejbPostCreate(), and all finder methods
• Can be shared across beans that have the same key structure

• Built from: Entity Bean — Primary key classes are used exclusively with entity beans
• Builds into: ejbCreate() — create method returns the primary key type
• Builds into: Finder Methods — findByPrimaryKey uses the PK class
• Related: getPrimaryKey() — runtime method to retrieve the current bean’s primary key
• Contrasts with: Direct Field Primary Key — simpler but less flexible approach

• Forgetting to override equals() and hashCode() causes container to fail at finding beans by primary key
• Wrapper class must have a no-arg constructor (container instantiates it via reflection)
• Changing from simple to composite key requires updating ejbPostCreate() and home interface method signatures
• The PK class must be available in the EJB jar’s classpath