Translation zh (#1646)
* add state and callback pattern * add command and template-method pattern * add iterator pattern * add bridege and DI pattern * fix issue #1600 * add converter,proxy,visitor pattern * add caching,composite,delegation,dirty-flag,interpreter patterns * add dao and producer-consumer * add dto and provate class data pattern * fix #1646 png path problems * fix #1646 composite png path case problem Co-authored-by: Mike <admin@xiaod.info> Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
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Mike Liu
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zh/dao/README.md
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zh/dao/README.md
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---
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layout: pattern
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title: Data Access Object
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folder: dao
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permalink: /patterns/dao/
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categories: Architectural
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tags:
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- Data access
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---
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## 目的
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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。
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## 解释
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真实世界例子
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> 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。
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通俗的说
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> DAO是我们通过基本持久性机制提供的接口。
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维基百科说
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> 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。
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**程序示例**
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通过我们的客户示例，下面是基本的`客户`实体。
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```java
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public class Customer {
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private int id;
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private String firstName;
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private String lastName;
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public Customer(int id, String firstName, String lastName) {
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this.id = id;
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this.firstName = firstName;
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this.lastName = lastName;
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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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这是`CustomerDao`接口及其两个不同的实现。
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Here's the `CustomerDao` interface and two different implementations for it. `InMemoryCustomerDao`
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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。
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```java
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public interface CustomerDao {
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Stream<Customer> getAll() throws Exception;
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Optional<Customer> getById(int id) throws Exception;
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boolean add(Customer customer) throws Exception;
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boolean update(Customer customer) throws Exception;
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boolean delete(Customer customer) throws Exception;
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}
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public class InMemoryCustomerDao implements CustomerDao {
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private final Map<Integer, Customer> idToCustomer = new HashMap<>();
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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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private static final Logger LOGGER = LoggerFactory.getLogger(DbCustomerDao.class);
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private final DataSource dataSource;
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public DbCustomerDao(DataSource dataSource) {
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this.dataSource = dataSource;
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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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最后，这是我们使用DAO管理客户数据的方式。
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```java
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final var dataSource = createDataSource();
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createSchema(dataSource);
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final var customerDao = new DbCustomerDao(dataSource);
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addCustomers(customerDao);
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log.info(ALL_CUSTOMERS);
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try (var customerStream = customerDao.getAll()) {
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customerStream.forEach((customer) -> log.info(customer.toString()));
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}
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log.info("customerDao.getCustomerById(2): " + customerDao.getById(2));
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final var customer = new Customer(4, "Dan", "Danson");
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customerDao.add(customer);
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log.info(ALL_CUSTOMERS + customerDao.getAll());
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customer.setFirstName("Daniel");
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customer.setLastName("Danielson");
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customerDao.update(customer);
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log.info(ALL_CUSTOMERS);
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try (var customerStream = customerDao.getAll()) {
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customerStream.forEach((cust) -> log.info(cust.toString()));
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}
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customerDao.delete(customer);
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log.info(ALL_CUSTOMERS + customerDao.getAll());
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deleteSchema(dataSource);
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```
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程序输出:
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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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## 类图
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## 适用性
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在以下情况下，请使用数据访问对象：:
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* 当您要巩固如何访问数据层时。
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* 当您要避免编写多个数据检索/持久层时。
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## 鸣谢
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* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
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