Chapter 12 Using Connector/J with Spring

Spring makes extensive use of the Template method design pattern (see Template Method Pattern). Our immediate focus will be on the JdbcTemplate and related classes, specifically NamedParameterJdbcTemplate. The template classes handle obtaining and releasing a connection for data access when one is needed.

The next example shows how to use NamedParameterJdbcTemplate inside of a DAO (Data Access Object) class to retrieve a random city given a country code.

public class Ex2JdbcDao {
     /**
     * Data source reference which will be provided by Spring.
     */
     private DataSource dataSource;

     /**
     * Our query to find a random city given a country code. Notice
     * the ":country" parameter toward the end. This is called a
     * named parameter.
     */
     private String queryString = "select Name from City " +
        "where CountryCode = :country order by rand() limit 1";

     /**
     * Retrieve a random city using Spring JDBC access classes.
     */
     public String getRandomCityByCountryCode(String cntryCode) {
         // A template that permits using queries with named parameters
         NamedParameterJdbcTemplate template =
         new NamedParameterJdbcTemplate(dataSource);
         // A java.util.Map is used to provide values for the parameters
         Map params = new HashMap();
         params.put("country", cntryCode);
         // We query for an Object and specify what class we are expecting
         return (String)template.queryForObject(queryString, params, String.class);
     }

    /**
    * A JavaBean setter-style method to allow Spring to inject the data source.
    * @param dataSource
    */
    public void setDataSource(DataSource dataSource) {
        this.dataSource = dataSource;
    }
}

The focus in the above code is on the getRandomCityByCountryCode() method. We pass a country code and use the NamedParameterJdbcTemplate to query for a city. The country code is placed in a Map with the key "country", which is the parameter is named in the SQL query.

To access this code, you need to configure it with Spring by providing a reference to the data source.

<bean id="dao" class="code.Ex2JdbcDao">
    <property name="dataSource" ref="dataSource"/>
</bean>

At this point, we can just grab a reference to the DAO from Spring and call getRandomCityByCountryCode().

    // Create the application context
    ApplicationContext ctx =
    new ClassPathXmlApplicationContext("ex2appContext.xml");
    // Obtain a reference to our DAO
    Ex2JdbcDao dao = (Ex2JdbcDao) ctx.getBean("dao");

    String countryCode = "USA";

    // Find a few random cities in the US
    for(int i = 0; i < 4; ++i)
        System.out.printf("A random city in %s is %s%n", countryCode,
            dao.getRandomCityByCountryCode(countryCode));

This example shows how to use Spring's JDBC classes to completely abstract away the use of traditional JDBC classes including Connection and PreparedStatement.

You might be wondering how we can add transactions into our code if we do not deal directly with the JDBC classes. Spring provides a transaction management package that not only replaces JDBC transaction management, but also enables declarative transaction management (configuration instead of code).

To use transactional database access, we will need to change the storage engine of the tables in the world database. The downloaded script explicitly creates MyISAM tables which do not support transactional semantics. The InnoDB storage engine does support transactions and this is what we will be using. We can change the storage engine with the following statements.

ALTER TABLE City ENGINE=InnoDB;
ALTER TABLE Country ENGINE=InnoDB;
ALTER TABLE CountryLanguage ENGINE=InnoDB;

A good programming practice emphasized by Spring is separating interfaces and implementations. What this means is that we can create a Java interface and only use the operations on this interface without any internal knowledge of what the actual implementation is. We will let Spring manage the implementation and with this it will manage the transactions for our implementation.

First you create a simple interface:

public interface Ex3Dao {
    Integer createCity(String name, String countryCode,
    String district, Integer population);
}

This interface contains one method that will create a new city record in the database and return the id of the new record. Next you need to create an implementation of this interface.

public class Ex3DaoImpl implements Ex3Dao {
    protected DataSource dataSource;
    protected SqlUpdate updateQuery;
    protected SqlFunction idQuery;

    public Integer createCity(String name, String countryCode,
        String district, Integer population) {
            updateQuery.update(new Object[] { name, countryCode,
                   district, population });
            return getLastId();
        }

    protected Integer getLastId() {
        return idQuery.run();
    }
}

You can see that we only operate on abstract query objects here and do not deal directly with the JDBC API. Also, this is the complete implementation. All of our transaction management will be dealt with in the configuration. To get the configuration started, we need to create the DAO.

<bean id="dao" class="code.Ex3DaoImpl">
    <property name="dataSource" ref="dataSource"/>
    <property name="updateQuery">...</property>
    <property name="idQuery">...</property>
</bean>

Now you need to set up the transaction configuration. The first thing you must do is create transaction manager to manage the data source and a specification of what transaction properties are required for the dao methods.

<bean id="transactionManager"
  class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
    <property name="dataSource" ref="dataSource"/>
</bean>

<tx:advice id="txAdvice" transaction-manager="transactionManager">
    <tx:attributes>
        <tx:method name="*"/>
    </tx:attributes>
</tx:advice>

The preceding code creates a transaction manager that handles transactions for the data source provided to it. The txAdvice uses this transaction manager and the attributes specify to create a transaction for all methods. Finally you need to apply this advice with an AOP pointcut.

<aop:config>
    <aop:pointcut id="daoMethods"
        expression="execution(* code.Ex3Dao.*(..))"/>
     <aop:advisor advice-ref="txAdvice" pointcut-ref="daoMethods"/>
</aop:config>

This basically says that all methods called on the Ex3Dao interface will be wrapped in a transaction. To make use of this, you only have to retrieve the dao from the application context and call a method on the dao instance.

Ex3Dao dao = (Ex3Dao) ctx.getBean("dao");
Integer id = dao.createCity(name,  countryCode, district, pop);

We can verify from this that there is no transaction management happening in our Java code and it is all configured with Spring. This is a very powerful notion and regarded as one of the most beneficial features of Spring.

In many situations, such as web applications, there will be a large number of small database transactions. When this is the case, it usually makes sense to create a pool of database connections available for web requests as needed. Although MySQL does not spawn an extra process when a connection is made, there is still a small amount of overhead to create and set up the connection. Pooling of connections also alleviates problems such as collecting large amounts of sockets in the TIME_WAIT state.

Setting up pooling of MySQL connections with Spring is as simple as changing the data source configuration in the application context. There are a number of configurations that we can use. The first example is based on the Jakarta Commons DBCP library. The example below replaces the source configuration that was based on DriverManagerDataSource with DBCP's BasicDataSource.

<bean id="dataSource" destroy-method="close"
  class="org.apache.commons.dbcp.BasicDataSource">
    <property name="driverClassName" value="${db.driver}"/>
    <property name="url" value="${db.jdbcurl}"/>
    <property name="username" value="${db.username}"/>
    <property name="password" value="${db.password}"/>
    <property name="initialSize" value="3"/>
</bean>

The configuration of the two solutions is very similar. The difference is that DBCP will pool connections to the database instead of creating a new connection every time one is requested. We have also set a parameter here called initialSize. This tells DBCP that we want three connections in the pool when it is created.

Another way to configure connection pooling is to configure a data source in our J2EE application server. Using JBoss as an example, you can set up the MySQL connection pool by creating a file called mysql-local-ds.xml and placing it in the server/default/deploy directory in JBoss. Once we have this setup, we can use JNDI to look it up. With Spring, this lookup is very simple. The data source configuration looks like this.

<jee:jndi-lookup id="dataSource" jndi-name="java:MySQL_DS"/>