Eclipse + JBoss 5 + EJB3 Development Guide (10)

Map individual tables to multiple tables by inheriting entity beans (single table policy, single_table)

If you have previously used an entity bean with ejb1.x or ejb2.x, you will find that you cannot divide a single table into multiple tables by inheriting entity beans. And in EJB3, we can easily implement this function. Let's take a look at the table structure and records shown in Figure 1.

Fig. 1 structure and record of t_accounts table

In the T_accounts table, there is a account_type field. This field is a string type with length 1. Can only take two values: C and S. If the field value is C, the current account (Checkingaccount) is represented, and if the field value is S, the savings Account (SavingsAccount) is represented. The first three fields of the T_accounts table (account_id, balance, and Account_type) are required for both a current account and a savings account, and interestrate is only meaningful for a savings account, Overdraftlimit only has meaning for a current account. Therefore, we can divide the t_accounts table into two tables, when the value of Account_type is C and S is a table.

If you are using EJB3 entity beans, you can first write an account class to encapsulate the first three fields of T_accounts, as follows:

Package entity;

Import Javax.persistence.Column;

Import Javax.persistence.DiscriminatorColumn;

Import javax.persistence.Entity;

Import Javax.persistence.GeneratedValue;

Import Javax.persistence.GenerationType;

Import Javax.persistence.Id;

Import javax.persistence.Inheritance;

Import Javax.persistence.InheritanceType;

Import javax.persistence.Table; @Entity @Table (name= "t_accounts") @Inheritance (strategy=inheritancetype.single_table) @DiscriminatorColumn (name= "

　　　 Account_type ") public class Account {protected String ID;

　　　 protected float balance;

　　　 protected String type;

　　　 @Id @GeneratedValue (strategy=generationtype.identity) @Column (name= "account_id") public String getId ()

　　　 {return ID;

　　　 public void SetId (String id) {this.id = ID;

　　　 public float GetBalance () {return balance;

　　　 public void setbalance (float balance) {this.balance = balance; } @ColuMn (name= "Account_type", Insertable=false, Updatable=false) public String GetType () {return type;

　　　 public void SetType (String type) {this.type = type; }

}