| layout | pattern | |
|---|---|---|
| title | Data Access Object | |
| folder | dao | |
| permalink | /patterns/dao/ | |
| categories | Architectural | |
| tags |
|
Object provides an abstract interface to some type of database or other persistence mechanism.
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;
}
public int getId() {
return id;
}
public void setId(final int id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(final String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(final String lastName) {
this.lastName = lastName;
}
@Override
public String toString() {
return "Customer{" + "id=" + getId() + ", firstName='" + getFirstName() + '\'' + ", lastName='"
+ getLastName() + '\'' + '}';
}
@Override
public boolean equals(final Object that) {
var isEqual = false;
if (this == that) {
isEqual = true;
} else if (that != null && getClass() == that.getClass()) {
final var customer = (Customer) that;
if (getId() == customer.getId()) {
isEqual = true;
}
}
return isEqual;
}
@Override
public int hashCode() {
return getId();
}
}Here's the DAO 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<>();
@Override
public Stream<Customer> getAll() {
return idToCustomer.values().stream();
}
@Override
public Optional<Customer> getById(final int id) {
return Optional.ofNullable(idToCustomer.get(id));
}
@Override
public boolean add(final Customer customer) {
if (getById(customer.getId()).isPresent()) {
return false;
}
idToCustomer.put(customer.getId(), customer);
return true;
}
@Override
public boolean update(final Customer customer) {
return idToCustomer.replace(customer.getId(), customer) != null;
}
@Override
public boolean delete(final Customer customer) {
return idToCustomer.remove(customer.getId()) != null;
}
}
public class DbCustomerDao implements CustomerDao {
private static final Logger LOGGER = LoggerFactory.getLogger(DbCustomerDao.class);
private final DataSource dataSource;
public DbCustomerDao(DataSource dataSource) {
this.dataSource = dataSource;
}
@Override
public Stream<Customer> getAll() throws Exception {
try {
var connection = getConnection();
var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS");
var resultSet = statement.executeQuery();
return StreamSupport.stream(new Spliterators.AbstractSpliterator<Customer>(Long.MAX_VALUE,
Spliterator.ORDERED) {
@Override
public boolean tryAdvance(Consumer<? super Customer> action) {
try {
if (!resultSet.next()) {
return false;
}
action.accept(createCustomer(resultSet));
return true;
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
}, false).onClose(() -> mutedClose(connection, statement, resultSet));
} catch (SQLException e) {
throw new CustomException(e.getMessage(), e);
}
}
private Connection getConnection() throws SQLException {
return dataSource.getConnection();
}
private void mutedClose(Connection connection, PreparedStatement statement, ResultSet resultSet) {
try {
resultSet.close();
statement.close();
connection.close();
} catch (SQLException e) {
LOGGER.info("Exception thrown " + e.getMessage());
}
}
private Customer createCustomer(ResultSet resultSet) throws SQLException {
return new Customer(resultSet.getInt("ID"),
resultSet.getString("FNAME"),
resultSet.getString("LNAME"));
}
@Override
public Optional<Customer> getById(int id) throws Exception {
ResultSet resultSet = null;
try (var connection = getConnection();
var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS WHERE ID = ?")) {
statement.setInt(1, id);
resultSet = statement.executeQuery();
if (resultSet.next()) {
return Optional.of(createCustomer(resultSet));
} else {
return Optional.empty();
}
} catch (SQLException ex) {
throw new CustomException(ex.getMessage(), ex);
} finally {
if (resultSet != null) {
resultSet.close();
}
}
}
@Override
public boolean add(Customer customer) throws Exception {
if (getById(customer.getId()).isPresent()) {
return false;
}
try (var connection = getConnection();
var statement = connection.prepareStatement("INSERT INTO CUSTOMERS VALUES (?,?,?)")) {
statement.setInt(1, customer.getId());
statement.setString(2, customer.getFirstName());
statement.setString(3, customer.getLastName());
statement.execute();
return true;
} catch (SQLException ex) {
throw new CustomException(ex.getMessage(), ex);
}
}
@Override
public boolean update(Customer customer) throws Exception {
try (var connection = getConnection();
var statement =
connection
.prepareStatement("UPDATE CUSTOMERS SET FNAME = ?, LNAME = ? WHERE ID = ?")) {
statement.setString(1, customer.getFirstName());
statement.setString(2, customer.getLastName());
statement.setInt(3, customer.getId());
return statement.executeUpdate() > 0;
} catch (SQLException ex) {
throw new CustomException(ex.getMessage(), ex);
}
}
@Override
public boolean delete(Customer customer) throws Exception {
try (var connection = getConnection();
var statement = connection.prepareStatement("DELETE FROM CUSTOMERS WHERE ID = ?")) {
statement.setInt(1, customer.getId());
return statement.executeUpdate() > 0;
} catch (SQLException ex) {
throw new CustomException(ex.getMessage(), ex);
}
}
}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);
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