TransactionTemplate and TransactionManager connection objects - spring

Question

The following code in my DAO works perfectly fine.
public void insert(final Person person) {
transactionTemplate.execute(new TransactionCallback<Void>() {
public Void doInTransaction(TransactionStatus txStatus) {
try {
getJdbcTemplate().execute("insert into person(username, password) values ('" + person.getUsername() + "','" + person.getPassword() + "')");
} catch (RuntimeException e) {
txStatus.setRollbackOnly();
throw e;
}
return null;
}
});
}
Following is my spring config.
<bean id="derbyds" class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName" value="org.apache.derby.jdbc.EmbeddedDriver" />
<property name="username" value="app" />
<property name="password" value="app" />
<property name="url" value="jdbc:derby:mytempdb" />
</bean>
<bean id="persondaojdbc" class="com.napp.dao.impl.PersonDaoJdbcImpl">
<property name="dataSource" ref="derbyds" />
<property name="transactionTemplate">
<bean class="org.springframework.transaction.support.TransactionTemplate">
<property name="transactionManager" ref="transactionManager" />
</bean>
</property>
</bean>
<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="derbyds"/>
</bean>
What i wanted to know is how does the
<bean id="persondaojdbc" class="com.napp.dao.impl.PersonDaoJdbcImpl">
<property name="dataSource" ref="derbyds" />
<property name="transactionTemplate">
<bean class="org.springframework.transaction.support.TransactionTemplate">
<property name="transactionManager" ref="transactionManager" />
</bean>
</property>
</bean>
<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="derbyds"/>
</bean>
Now, its imperative that both the TransactionManager and the code (in my case jdbc template) operate on the same connection. I am assuming both of them are getting the Connection objects from the DataSource. DataSource pools connections and its a chance when you call getConnection multiple times, you will get different Connection obejcts. How does spring make sure that the TransactionManager and JdbcTemplate end up getting the same connection objects. My understanding is that, if that doesn't happen, rollbacks, or commits wont work, correct? Could someone throw more light on this.

Answer 1

If you look at the code for JdbcTemplate (one of the execute(...) methods) you will see
Connection con = DataSourceUtils.getConnection(getDataSource());
Which tries to retrieve a Connection from a ConnectionHolder registered with a TransactionSynchronizationManager.
If there is no such object, it just gets a connection from the DataSource and registers it (if it is in a transactional environment, ie. you have a transaction manager). Otherwise, it immediately returns the registered object.
This is the code (stripped of logs and stuff)
ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
conHolder.requested();
if (!conHolder.hasConnection()) {
conHolder.setConnection(dataSource.getConnection());
}
return conHolder.getConnection();
}
// Else we either got no holder or an empty thread-bound holder here.
Connection con = dataSource.getConnection();
// flag set by the TransactionManager
if (TransactionSynchronizationManager.isSynchronizationActive()) {
// Use same Connection for further JDBC actions within the transaction.
// Thread-bound object will get removed by synchronization at transaction completion.
ConnectionHolder holderToUse = conHolder;
if (holderToUse == null) {
holderToUse = new ConnectionHolder(con);
}
else {
holderToUse.setConnection(con);
}
holderToUse.requested();
TransactionSynchronizationManager.registerSynchronization(
new ConnectionSynchronization(holderToUse, dataSource));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse != conHolder) {
TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
}
}
return con;
You'll notice that the JdbcTemplate tries to
finally {
DataSourceUtils.releaseConnection(con, getDataSource());
}
release the Connection, but this only happens if you're in a non-transactional environment, ie.
if it is not managed externally (that is, not bound to the thread).
Therefore, in a transactional world, the JdbcTemplate will be reusing the same Connection object.