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Tables are the most basic logical unit in a relational database. A table stores conceptually related data into rows and columns. Essentially, tables are the objectoriented (OO) counterparts of objects. Hence, we might imagine that the ActionBazaar database contains tables like CATEGORIES, ITEMS, ORDERS, and so forth. A column is a particular domain of data, and a table is a set of related columns. If tables are the equivalent of objects, columns are the equivalent of object attributes. Consequently, the CATEGORIES table probably has columns such as CATEGORY_ID, CATEGORY_NAME, MODIFICATION_DATE, and CREATION_DATE, as seen in figure B.1. As with object attributes in Java, each relational table column has a data type. Table B.1 lists some column data types commonly used across various databases and their Java equivalents.

In order for clients to access this entity data to do a password comparison, they must go through a session bean. (Actually, if the client were in the local JVM, it could look up the entity bean, but you should enforce otherwise.) We developed a session bean, HelperBean, that provides a login() method for clients to use to access the entity data for password comparison. Here is the HelperBean EJB partial source:

Table B.1 Common column data types and their Java equivalents Java Type java.lang.String char, Char continued on next page

Table B.1 Common column data types and their Java equivalents (continued) Java Type int, Integer, BigInteger double, float, BigDecimal, Double, Float java.sql.Blob, byte[] java.sql.Clob, char[],java.lang.String

public class HelperBean implements SessionBean { private SessionContext ctx; private PasswordLocalHome passwordHome; public boolean login(String userName, String password) throws RemoteException { Password passwd = null; try{ //use the previously looked up PasswordBean localHome interface passwd = passwordHome.findByPrimaryKey(userName); return passwd.isPasswordValid(password); } catch(Exception e){ throw new RemoteException("Finding Password object failed"); } } //other bean methods not shown }

A row is a record saved in the database composed of related data in each column of a table. A row, in effect, is equivalent to an instance of a particular object, in contrast to the class definition. For most OO developers it s not a big leap to imagine an instance of the Category object being saved into a row of the CATEGORIES table. A schema can be compared to a Java package. In other words, a schema is a collection of related tables, similar to how a Java package contains a set of related classes. Usually, all of the tables used in a particular application are organized under a single schema. All the tables used in our example application might be stored under a schema called ACTIONBAZAAR. Typically, a schema stores much more than just tables. It might also have views, triggers, and stored procedures. A detailed discussion of these database features is beyond the scope of this appendix. For coverage of these and other database topics, feel free to investigate a good reference book such as An Introduction to Database Systems, 7th edition, by C. J. Date (Addison Wesley Longman, 1999).

Instead of considering the beginning and end as two separate points, the execution as a whole is a type of joinpoint Constructors: Constructors are the main entities used to create the objects of an application As with methods, the calls and executions of a constructor correspond to joinpoint types Exceptions: An exception is thrown to signal an abnormal run-time situation, and it is caught to execute a particular treatment These two events are major points in the execution of an application They can be both considered joinpoint types Fields: Many aspects, such as the persistence aspect, need to deal with the application s data Fields are the main code elements that implement this data Hence, aspect-oriented languages consider read and write operations on fields as joinpoint types Method calls and executions are clearly the most widely used joinpoint types in AOP.

Figure B.1 Rows and columns in the CATEGORIES table. While columns store a domain of data, rows contain a record composed of a set of related columns in a table.

The PasswordBean entity EJB in the recipe implements only a local and a local home interface. Using only the local interface prevents any remote creation or

We will, however, cover a few more database concepts essential in understanding EJB 3 Persistence next, namely database constraints such as primary and foreign keys.