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---
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## Intent
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Object provides an abstract interface to some type of database or other persistence mechanism.
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## Explanation
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Real world example
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> 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.
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> There's a set of customers that need to be persisted to database. Additionally we need the whole
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> set of CRUD (create/read/update/delete) operations so we can operate on customers easily.
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In plain words
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> DAO is an interface we provide over the base persistence mechanism.
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Wikipedia says
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> 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.
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> In computer software, a data access object (DAO) is a pattern that provides an abstract interface
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> to some type of database or other persistence mechanism.
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**Programmatic Example**
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Walking through our customers example, here's the basic Customer entity.
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Walking through our customers example, here's the basic `Customer` entity.
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```java
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public class Customer {
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this.firstName = firstName;
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this.lastName = lastName;
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}
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public int getId() {
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return id;
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}
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public void setId(final int id) {
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this.id = id;
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}
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public String getFirstName() {
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return firstName;
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}
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public void setFirstName(final String firstName) {
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this.firstName = firstName;
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}
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public String getLastName() {
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return lastName;
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}
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public void setLastName(final String lastName) {
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this.lastName = lastName;
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}
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@Override
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public String toString() {
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return "Customer{" + "id=" + getId() + ", firstName='" + getFirstName() + '\'' + ", lastName='"
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+ getLastName() + '\'' + '}';
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}
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@Override
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public boolean equals(final Object that) {
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var isEqual = false;
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if (this == that) {
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isEqual = true;
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} else if (that != null && getClass() == that.getClass()) {
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final var customer = (Customer) that;
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if (getId() == customer.getId()) {
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isEqual = true;
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}
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}
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return isEqual;
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}
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@Override
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public int hashCode() {
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return getId();
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}
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// getters and setters ->
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...
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}
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```
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Here's the DAO interface and two different implementations for it. InMemoryCustomerDao keeps a simple map of customers
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in memory while DBCustomerDao is the real RDBMS implementation.
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Here's the `CustomerDao` interface and two different implementations for it. `InMemoryCustomerDao`
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keeps a simple map of customers in memory while `DBCustomerDao` is the real RDBMS implementation.
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```java
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public interface CustomerDao {
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private final Map<Integer, Customer> idToCustomer = new HashMap<>();
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@Override
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public Stream<Customer> getAll() {
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return idToCustomer.values().stream();
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}
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@Override
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public Optional<Customer> getById(final int id) {
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return Optional.ofNullable(idToCustomer.get(id));
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}
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@Override
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public boolean add(final Customer customer) {
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if (getById(customer.getId()).isPresent()) {
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return false;
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}
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idToCustomer.put(customer.getId(), customer);
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return true;
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}
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@Override
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public boolean update(final Customer customer) {
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return idToCustomer.replace(customer.getId(), customer) != null;
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}
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@Override
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public boolean delete(final Customer customer) {
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return idToCustomer.remove(customer.getId()) != null;
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}
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// implement the interface using the map
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...
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}
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public class DbCustomerDao implements CustomerDao {
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this.dataSource = dataSource;
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}
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@Override
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public Stream<Customer> getAll() throws Exception {
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try {
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var connection = getConnection();
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var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS");
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var resultSet = statement.executeQuery();
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return StreamSupport.stream(new Spliterators.AbstractSpliterator<Customer>(Long.MAX_VALUE,
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Spliterator.ORDERED) {
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@Override
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public boolean tryAdvance(Consumer<? super Customer> action) {
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try {
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if (!resultSet.next()) {
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return false;
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}
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action.accept(createCustomer(resultSet));
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return true;
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} catch (SQLException e) {
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throw new RuntimeException(e);
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}
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}
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}, false).onClose(() -> mutedClose(connection, statement, resultSet));
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} catch (SQLException e) {
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throw new CustomException(e.getMessage(), e);
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}
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}
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private Connection getConnection() throws SQLException {
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return dataSource.getConnection();
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}
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private void mutedClose(Connection connection, PreparedStatement statement, ResultSet resultSet) {
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try {
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resultSet.close();
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statement.close();
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connection.close();
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} catch (SQLException e) {
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LOGGER.info("Exception thrown " + e.getMessage());
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}
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}
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private Customer createCustomer(ResultSet resultSet) throws SQLException {
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return new Customer(resultSet.getInt("ID"),
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resultSet.getString("FNAME"),
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resultSet.getString("LNAME"));
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}
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@Override
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public Optional<Customer> getById(int id) throws Exception {
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ResultSet resultSet = null;
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try (var connection = getConnection();
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var statement = connection.prepareStatement("SELECT * FROM CUSTOMERS WHERE ID = ?")) {
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statement.setInt(1, id);
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resultSet = statement.executeQuery();
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if (resultSet.next()) {
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return Optional.of(createCustomer(resultSet));
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} else {
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return Optional.empty();
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}
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} catch (SQLException ex) {
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throw new CustomException(ex.getMessage(), ex);
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} finally {
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if (resultSet != null) {
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resultSet.close();
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}
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}
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}
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@Override
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public boolean add(Customer customer) throws Exception {
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if (getById(customer.getId()).isPresent()) {
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return false;
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}
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try (var connection = getConnection();
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var statement = connection.prepareStatement("INSERT INTO CUSTOMERS VALUES (?,?,?)")) {
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statement.setInt(1, customer.getId());
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statement.setString(2, customer.getFirstName());
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statement.setString(3, customer.getLastName());
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statement.execute();
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return true;
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} catch (SQLException ex) {
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throw new CustomException(ex.getMessage(), ex);
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}
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}
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@Override
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public boolean update(Customer customer) throws Exception {
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try (var connection = getConnection();
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var statement =
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connection
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.prepareStatement("UPDATE CUSTOMERS SET FNAME = ?, LNAME = ? WHERE ID = ?")) {
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statement.setString(1, customer.getFirstName());
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statement.setString(2, customer.getLastName());
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statement.setInt(3, customer.getId());
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return statement.executeUpdate() > 0;
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} catch (SQLException ex) {
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throw new CustomException(ex.getMessage(), ex);
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}
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}
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@Override
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public boolean delete(Customer customer) throws Exception {
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try (var connection = getConnection();
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var statement = connection.prepareStatement("DELETE FROM CUSTOMERS WHERE ID = ?")) {
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statement.setInt(1, customer.getId());
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return statement.executeUpdate() > 0;
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} catch (SQLException ex) {
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throw new CustomException(ex.getMessage(), ex);
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}
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}
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}
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// implement the interface using the data source
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...
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```
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Finally here's how we use our DAO to manage customers.
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deleteSchema(dataSource);
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```
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The program output:
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```java
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customerDao.getAllCustomers():
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Customer{id=1, firstName='Adam', lastName='Adamson'}
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Customer{id=2, firstName='Bob', lastName='Bobson'}
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Customer{id=3, firstName='Carl', lastName='Carlson'}
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customerDao.getCustomerById(2): Optional[Customer{id=2, firstName='Bob', lastName='Bobson'}]
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customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@7cef4e59
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customerDao.getAllCustomers():
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Customer{id=1, firstName='Adam', lastName='Adamson'}
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Customer{id=2, firstName='Bob', lastName='Bobson'}
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Customer{id=3, firstName='Carl', lastName='Carlson'}
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Customer{id=4, firstName='Daniel', lastName='Danielson'}
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customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@2db0f6b2
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customerDao.getAllCustomers():
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Customer{id=1, firstName='Adam', lastName='Adamson'}
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Customer{id=2, firstName='Bob', lastName='Bobson'}
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Customer{id=3, firstName='Carl', lastName='Carlson'}
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customerDao.getCustomerById(2): Optional[Customer{id=2, firstName='Bob', lastName='Bobson'}]
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customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@12c8a2c0
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customerDao.getAllCustomers():
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Customer{id=1, firstName='Adam', lastName='Adamson'}
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Customer{id=2, firstName='Bob', lastName='Bobson'}
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Customer{id=3, firstName='Carl', lastName='Carlson'}
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Customer{id=4, firstName='Daniel', lastName='Danielson'}
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customerDao.getAllCustomers(): java.util.stream.ReferencePipeline$Head@6ec8211c
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```
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## Class diagram
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![alt text](./etc/dao.png "Data Access Object")
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## Applicability
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Use the Data Access Object in any of the following situations
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* when you want to consolidate how the data layer is accessed
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* when you want to avoid writing multiple data retrieval/persistence layers
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Use the Data Access Object in any of the following situations:
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* When you want to consolidate how the data layer is accessed.
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* When you want to avoid writing multiple data retrieval/persistence layers.
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## Credits
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