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However, the most expensive operation is setting up a database connection. Not only does it require a TCP/IP connection between client and server, which is notoriously expensive, but the database server also needs to do a lot of work before it s ready to accept requests from the client.
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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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Connection creation becomes especially troublesome under load. The resources of the underlying operating system and hardware must be shared among executing SQL statements and setting up the environments for new client connections. What s more, when connections are closed by the client, the database again must do a lot of work to clean up resources. So JDBC code should never create database connections. Instead, connection creation should be delegated to a DataSource object. We ll discuss the javax.sql.DataSource interface later in this chapter, in the The DataSource Interface and Connection Pooling section.
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Inappropriate Connection Life Cycles
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When you ve studied hard and long, and understand how to avoid resource exhaustion and performance bottlenecks, you re still not out of the woods. You need to design your data-access infrastructure in such a way that your applications remain fully flexible. As an example, consider the addNewsletterSubscription() method on the Newsletter SubscriptionDataAccess class in Listing 5-5. This method saves an e-mail address to the database to subscribe a tennis club member to the monthly newsletter. Listing 5-5. The NewsletterSubscriptionDataAccess Class package com.apress.springbook.chapter05; import javax.sql.DataSource; import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.SQLException; public class NewsletterSubscriptionDataAccess { private DataSource dataSource; public void setDataSource(DataSource dataSource) { this.dataSource = dataSource; } public void addNewsletterSubscription(int memberId, String emailAddress) throws MyDataAccessException { Connection conn = null; PreparedStatement statement = null; try { conn = dataSource.getConnection(); statement = conn.prepareStatement( "INSERT INTO t_newsletter_subscriptions (" + "(subscription_id, member_id, email_address) " + " VALUES (" + "newsletter_subscription_seq.nextVal(), , )" ); statement.setInt(1, memberId); statement.setString(2, emailAddress); statement.executeUpdate(); } catch (SQLException e) { throw new MyDataAccessException(e);
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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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} finally { if (statement != null) { try { statement.close(); } catch (java.sql.SQLException // exception must be caught, e.printStackTrace(); } } if (conn != null) { try { conn.close(); } catch (java.sql.SQLException // exception must be caught, e.printStackTrace(); } } } } }
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While the addNewsletterSubscription() method avoids resource leakage and poor performance, it suffers from another kind of leakage: it obtains its own database connection and closes that connection again. This method is inflexible because the application doesn t get the chance to let this method participate in a database transaction. We ll cover the caveats of database transaction managment shortly. Here, we ll show you the consequence of the way addNewsletterSubscription() implements its connection life cycle. First, let s call this method to add a newsletter subscription for one registered member. The execution of the addNewsletterSubscription() method is the only data-access operation we perform. We just want to add the subscription details to the database when a member enters an e-mail address in a form on our website. Listing 5-6 shows how the addNewsletterSubscription() method is called for this use case. Listing 5-6. Calling the addNewsletterSubscription() Method private NewsletterSubscriptionDataAccess subscriptionDataAccess; public void subscribeMemberToNewsletter(Member member, String email) throws MyDataAccessException { subscriptionDataAccess.addNewsletterSubscription(member.getId(), email); } The addNewsletterSubscription() method performs the use case in Listing 5-6 excellently. It creates its own Connection object and closes it again. As such, the application code that calls it doesn t have to worry about the details of the data-access code. However, things become more complicated when a tennis player registers for membership on our website. We need to add a newsletter subscription to the database, and the obvious way forward is to reuse the addNewsletterSubscription() method. The difficulty of this use case is that adding the membership registration details and the subscription details to the database requires calling two data-access methods: saveMembershipRegistration() and addNewsletterSubscription(). We call both methods as shown in Listing 5-7.
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