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Java DAO

Data Access Object Design Pattern

The Data Access Object is an important component in applications. Business applications almost always need access to data from relational or object databases and the Java platform offers many techniques for accessing this data. The oldest and most mature technique is to use the Java Database Connectivity (JDBC) API, which provides the capability to execute SQL queries against a database and then fetch the results, one column at a time. Although this API provides everything a developer needs to access data and to persist application state, it is a cumbersome API to develop against - which makes a code generator particularly useful.

Java 2 Enterprise Edition (J2EE) offers a newer persistence framework in the form of Entity Beans, a subset of the Enterprise JavaBean (EJB) framework. Although there have been many improvements in the more recent EJB 2.0 specification, many developers are now looking to alternative persistence frameworks for their Data Access Objects, such as Java Persistence API (JPA) and Hibernate.

The Benefits of Data Access Objects

The Data Access Object design pattern provides a technique for separating object persistence and data access logic from any particular persistence mechanism or API. There are clear benefits to this approach from an architectural perspective. The Data Access Object approach provides flexibility to change an application's persistence mechanism over time without the need to re-engineer application logic that interacts with the Data Access Object tier. For example, there may be performance benefits in changing an application's performance mechanism from using Entity Beans to using direct JDBC calls from a session bean, or even a move to an alternative persistence framework, such as JDO. Without a Data Access Object tier in place, this sort of transition would require extensive re-engineering of existing code.

The Data Access Object design pattern also provides a simple, consistent API for data access that does not require knowledge of JDBC, EJB, Hibernate, or JDO interfaces. A typical Data Access Object interface is shown below.

public interface CustomerDAO
{
  public void insert(Customer customer)
   throws CustomerDAOException;

  public void update(CustomerPK pk, Customer customer)
   throws CustomerDAOException;

  public void delete(CustomerPK pk)
   throws CustomerDAOException;

  public Customer[] findAll()
   throws CustomerDAOException;

  public Customer findByPrimaryKey(String email)
   throws CustomerDAOException;

  public Customer[] findByCompany(int companyId)
   throws CustomerDAOException;
}

It is important to note that Data Access Object does not just apply to simple mappings of one object to one relational table, but also allows complex queries to be performed and allows for stored procedures and database views to be mapped into Java data structures.

Minimizing the Impact of Moving to Data Access Objects

CodeFutures' objective is to minimize the importance of the main argument against using Data Access Objects: the fact that it requires a significant amount of repetitive source code to be produced for no immediate advantage over using JDBC, EJB, or JDO directly. For many developers, this disadvantage is good enough reason to ignore the long-term benefits of using a framework-neutral API, especially where there are strict project deadlines. Without the code generation advantages of FireStorm/DAO, it is not easy to justify to a project manager or project sponsor the time and cost of manually writing DAO code, regardless of any future benefits they may see.

Data Access Object Code Generation

CodeFutures' solution to the manual coding problem is to automate the production of a Data Access Object tier, as well as automating the actual implementation logic for whichever persistence framework is deemed appropriate for an application. This approach is easy to adopt because almost all databases use a standard language for defining their structure (SQL).

Recommended Reading on Data Access Objects

Additional useful information on Data Access Objects is available here:

http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html

http://www.corej2eepatterns.com/Patterns2ndEd/DataAccessObject.htm

The best introduction to the general topic of core J2EE design patterns is Core J2EE Patterns: Best Practices and Design Strategies by Dan Malks, Deepak Alur, and John Crupi.

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"Before we discovered FireStorm/DAO, we were reluctant to write DAO code ourselves as it required a lot of time and effort. Now we can concentrate on the real business logic and application requirements."
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