Spring Tutorials

Introduction of DAO

DAO stands for (Data access object) which is commonly used for database interaction.The Data Access Object (DAO) support in Spring is aimed at making it easy to work with data access technologies like JDBC, Hibernate, JPA or JDO in a consistent way.

This allows one to switch between the aforementioned persistence technologies fairly easily and it also allows one to code without worrying about catching exceptions that are specific to each technology.

DAOs exist to provide a means to read and write data to the database and they should expose this functionality through an interface by which the rest of the application will access them.

There are a number of ways to configure Spring to use DataSources. A simple method is to use spring beans to create a DataSource and inject it into a DAO / POJO class.

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/Test"/>
<property name="username" value=""/>
<property name="password" value=""/>
</bean>
<bean id="empDao" class="com.r4r.EmployeeDao">
<property name="dataSource" ref="myDataSource" />
</bean>

The basic structure of EmployeeDao as given below.

package com.r4r
public class EmloyeeDao {
    DataSource ds;
    SimpleJdbcTemplate jdbcTemplate;
    public void setDataSource(DataSource ds) 
		{
        this.ds = ds;
        jdbcTemplate = new SimpleJdbcTemplate(ds);
        }
    /* DAO methods go here */
}

The Spring DAO class can be instantiated using the following code

BeanFactory bf = new XmlBeanFactory( new ClassPathResource("config.xml") );
return (EmployeeDao) bf.getBean( "empDao" );

The Spring DAO JDBC Template class provides an graceful method of running SQL queries and mapping the result set to Java objects with a less coding.

public Employee getEmployee(int id) throws SQLException {
return jdbcTemplate.queryForObject(
 "SELECT id, name FROM emp WHERE id = ?",
 new ParameterizedRowMapper<Company>() {
public Employee mapRow(ResultSet rs, int i) throws SQLException {
return new Employee( rs.getInt(1), rs.getString(2) );
}
},
id
);
}

Data Access Objects (DAOs) or repositories provide exception translation is to use the @Repository annotation. This annotation also allows the component scanning support to find and configure your DAOs and repositories without having to provide XML configuration entries for them.

@Repository
public class SomeMovieFinder implements MovieFinder {
// ...
}

Any DAO or repository implementation will need to access to a persistence resource, depending on the persistence technology used, for example, a JDBC-based repository will need access to a JDBC DataSource, a JPA-based repository will need access to an EntityManager. The easiest way to accomplish this is to have this resource dependency injected using one of the @Autowired, @Inject, @Resource or @PersistenceContext annotations.

Below Example show for JDBC support. You would have the DataSource injected into an initialization method where you would create a JdbcTemplate and other data access support classes like SimpleJdbcCall etc using this DataSource.

@Repository
public class JdbcMovieFinder implements MovieFinder {
private JdbcTemplate jdbcTemplate;
@Autowired
public void init(DataSource ds) {
this.jdbcTemplate = new JdbcTemplate(ds);
}
// ...
}