Data Access Object

See Core J2EE Patterns, 2nd Edition for full description of this pattern and its strategies.

Problem

You want to encapsulate data access and manipulation in a separate layer.

Forces

• You want to implement data access mechanisms to access and manipulate data in a persistent storage.
• You want to decouple the persistent storage implementation from the rest of your application.
• You want to provide a uniform data access API for a persistent mechanism to various types of data sources, such as RDBMS, LDAP, OODB, XML repositories, flat files, and so on.
• You want to organize data access logic and encapsulate proprietary features to facilitate maintainability and portability.

Solution

Use a Data Access Object to abstract and encapsulate all access to the persistent store. The Data Access Object manages the connection with the data source to obtain and store data.

Class Diagram

Sequence Diagram

Strategies

• Custom Data Access Object Strategy
• Data Access Object Factory Strategies
• Transfer Object Collection Strategy
• Cached RowSet Strategy
• Read Only RowSet Strategy
• RowSet Wrapper List Strategy

Consequences

• Centralizes control with loosely coupled handlers
• Enables transparency
• Provides object-oriented view and encapsulates database schemas
• Enables easier migration
• Reduces code complexity in clients
• Organizes all data access code into a separate layer
• Adds extra layer
• Needs class hierarchy design (Factory Method Strategies)
• Introduces complexity to enable object-oriented design (RowSet Wrapper List Strategy)

Related Patterns

• Transfer Object
  Data access objects in their most basic form use transfer objects to transport data to and from their clients. Transfer Object are used in other strategies of Data Access Objects. The RowSet Wrapper List strategy returns a list as a transfer object.
• Factory Method [GoF] and Abstract Factory [GoF]
  The Data Access Object Factory strategies use the Factory Method pattern to implement the concrete factories and its products (DAOs). For added flexibility, the Abstract Factory pattern is used as described in the Data Access Object Factory strategy.
• Broker [POSA1]
  The DAO pattern is related to the Broker pattern, which describes approaches for decoupling clients and servers in distributed systems. The DAO pattern more specifically applies this pattern to decoupling the resource tier from clients in another tier, such as the business or presentation tier.
• Transfer Object Assembler
  The Transfer Object Assembler uses the Data Access Object to obtain data to build the composite transfer object it needs to assemble and send as the model data to the client.
• Value List Handler
  The value list handler needs a list of results to act upon. Such a list is often obtained using a Data Access Object, which can return a set of results. While the value list handler has an option of obtaining a RowSet from the Data Access Object, it might be better to obtain and use a RowSet Wrapper List instead.

Real World Product / Implementation

• CodeFutures adopts and implements the DAO Pattern in their Firestorm DAO product. Check out their product here.

Excerpted from Core J2EE Patterns, 2nd Edition. See the book for full description of the pattern including class diagrams, sequence diagrams and sample code.