| title | Data Access Object | |
|---|---|---|
| category | Architectural | |
| language | en | |
| 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; } // 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
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.