title	category	language	tag
Data Access Object	Architectural	en	Data access

Intent

Object provides an abstract interface to some type of database or other persistence mechanism.

Explanation

Real world example

There's a set of customers that need to be persisted to database. Additionally we need the whole set of CRUD (create/read/update/delete) operations so we can operate on customers easily.

In plain words

DAO is an interface we provide over the base persistence mechanism.

Wikipedia says

In computer software, a data access object (DAO) is a pattern that provides an abstract interface to some type of database or other persistence mechanism.

Programmatic Example

Walking through our customers example, here's the basic Customer entity.

public class Customer {

  private int id;
  private String firstName;
  private String lastName;

  public Customer(int id, String firstName, String lastName) {
    this.id = id;
    this.firstName = firstName;
    this.lastName = lastName;
  }
  // getters and setters ->
  ...
}

Here's the CustomerDao interface and two different implementations for it. InMemoryCustomerDao keeps a simple map of customers in memory while DBCustomerDao is the real RDBMS implementation.

public interface CustomerDao {

  Stream<Customer> getAll() throws Exception;

  Optional<Customer> getById(int id) throws Exception;

  boolean add(Customer customer) throws Exception;

  boolean update(Customer customer) throws Exception;

  boolean delete(Customer customer) throws Exception;
}

public class InMemoryCustomerDao implements CustomerDao {

  private final Map<Integer, Customer> idToCustomer = new HashMap<>();

  // implement the interface using the map
  ...
}

@Slf4j
public class DbCustomerDao implements CustomerDao {

  private final DataSource dataSource;

  public DbCustomerDao(DataSource dataSource) {
    this.dataSource = dataSource;
  }

  // implement the interface using the data source
  ...

Finally here's how we use our DAO to manage customers.

    final var dataSource = createDataSource();
    createSchema(dataSource);
    final var customerDao = new DbCustomerDao(dataSource);
    
    addCustomers(customerDao);
    log.info(ALL_CUSTOMERS);
    try (var customerStream = customerDao.getAll()) {
      customerStream.forEach((customer) -> log.info(customer.toString()));
    }
    log.info("customerDao.getCustomerById(2): " + customerDao.getById(2));
    final var customer = new Customer(4, "Dan", "Danson");
    customerDao.add(customer);
    log.info(ALL_CUSTOMERS + customerDao.getAll());
    customer.setFirstName("Daniel");
    customer.setLastName("Danielson");
    customerDao.update(customer);
    log.info(ALL_CUSTOMERS);
    try (var customerStream = customerDao.getAll()) {
      customerStream.forEach((cust) -> log.info(cust.toString()));
    }
    customerDao.delete(customer);
    log.info(ALL_CUSTOMERS + customerDao.getAll());
    
    deleteSchema(dataSource);

The program output:

customerDao.getAllCustomers(): 
Customer{id=1, firstName='Adam', lastName='Adamson'}
Customer{id=2, firstName='Bob', lastName='Bobson'}
Customer{id=3, firstName='Carl', lastName='Carlson'}
customerDao.getCustomerById(2): Optional[Customer{id=2, firstName='Bob', lastName='Bobson'}]
customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@7cef4e59
customerDao.getAllCustomers(): 
Customer{id=1, firstName='Adam', lastName='Adamson'}
Customer{id=2, firstName='Bob', lastName='Bobson'}
Customer{id=3, firstName='Carl', lastName='Carlson'}
Customer{id=4, firstName='Daniel', lastName='Danielson'}
customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@2db0f6b2
customerDao.getAllCustomers(): 
Customer{id=1, firstName='Adam', lastName='Adamson'}
Customer{id=2, firstName='Bob', lastName='Bobson'}
Customer{id=3, firstName='Carl', lastName='Carlson'}
customerDao.getCustomerById(2): Optional[Customer{id=2, firstName='Bob', lastName='Bobson'}]
customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@12c8a2c0
customerDao.getAllCustomers(): 
Customer{id=1, firstName='Adam', lastName='Adamson'}
Customer{id=2, firstName='Bob', lastName='Bobson'}
Customer{id=3, firstName='Carl', lastName='Carlson'}
Customer{id=4, firstName='Daniel', lastName='Danielson'}
customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@6ec8211c

Class diagram

Applicability

Use the Data Access Object in any of the following situations:

• When you want to consolidate how the data layer is accessed.
• When you want to avoid writing multiple data retrieval/persistence layers.

Tutorials

• The DAO Pattern in Java
• Data Access Object Pattern

Credits

• J2EE Design Patterns