java-design-patterns/localization/zh/dao

title, category, language, tags

title	category	language	tags
Data Access Object	Architectural	zh	Data access

目的

对象为某种类型的数据库或其他持久性机制提供了抽象接口。

解释

真实世界例子

有一组客户数据需要持久化到数据库中。 我们需要整个额外的增删改查操作以便操作客户数据。

通俗的说

DAO是我们通过基本持久性机制提供的接口。

维基百科说

在计算机软件中，数据访问对象（DAO）是一种模式，可为某种类型的数据库或其他持久性机制提供抽象接口。

程序示例

通过我们的客户示例，下面是基本的客户实体。

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 ->
  ...
}

这是CustomerDao接口及其两个不同的实现。

Here's the CustomerDao interface and two different implementations for it. InMemoryCustomerDao 将简单的客户数据映射保存在内存中 而DBCustomerDao是真正的RDBMS实现。

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
  ...
}

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;
  }

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

最后，这是我们使用DAO管理客户数据的方式。

    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);

程序输出:

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

类图

适用性

在以下情况下，请使用数据访问对象：:

• 当您要巩固如何访问数据层时。
• 当您要避免编写多个数据检索/持久层时。

鸣谢

• J2EE Design Patterns