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The try/catch blocks in Listing 5-1 could be moved to reusable utility methods. We ve chosen to show the full JDBC code in the examples in this chapter to highlight all the responsibilities developers must shoulder.
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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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Concerns of leakage also exist when working with ORM tools. However, they are often much less visible and more complicated, and as such less understood. Next, we ll look at the most important categories of leakages and discuss why they are important to fix. These categories apply whenever you write data-access code. To our knowledge, all data-access tools available today can be affected by any of these categories.
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Database Resources
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Data-access code can show three forms of leakage when dealing with database resources: Resource exhaustion: Occurs when database cursors, result sets, and connections are not closed properly. Resources on both sides of the database connection remain locked indefinitely, until a timeout occurs or until cleaned up by the garbage collector. This can lead to slow performance, unrecoverable errors, and memory leaks. These are bad forms of leakage because memory and other valuable resources, both on the server and the client, remain open for too long. They can be fixed by developers, but detecting them can be hard. One form of resource exhaustion is connection leakage. Poor performance: Occurs when database Connection and Statement objects are created in inefficient ways that affect performance negatively. Typically, Connection and Statement objects are cached and reused by the JDBC driver or connection pools. Reusing these objects improves performance compared to creating them from scratch every time such an object is needed. To enable caching and reuse, you typically need to set some configuration and write specific JDBC code. Inappropriate connection life cycles: Occurs when data-access code can t automatically adapt to one of two connection life cycle scenarios. The first one is obtaining and releasing a Connection object for each execution of data-access code. The second one is reusing a Connection object that was created by another party without closing it. Data-access code that doesn t support both is never going to be flexible. Let s look at JDBC examples of each of these three types of leakage.
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Resource Exhaustion
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Database connections or other resources are represented in JDBC as shown in Table 5-1. These JDBC types typically cause resource exhaustion, as demonstrated by the examples in this section. Table 5-1. JDBC Resource Representation
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Connection to the database Execution and potentially results of SQL statement Execution and potentially results of parameterized and precompiled SQL statement Cursor (client or server side)
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JDBC Interface Type
java.sql.Connection java.sql.Statement java.sql.PreparedStatement java.sql.ResultSet
Listing 5-2 shows JDBC code that doesn t properly close the Connection object when an exception occurs.
CHAPTER 5 INTRODUCTION TO DATA ACCESS
Listing 5-2. JDBC Connection Is Not Properly Closed When Exception Occurs private javax.sql.DataSource dataSource; public int countTournamentRegistrations(int tournamentId) throws MyDataAccessException { try { java.sql.Connection conn = dataSource.getConnection(); java.sql.Statement statement = conn.createStatement(); java.sql.ResultSet rs = statement.executeQuery( "SELECT COUNT(0) FROM t_registrations WHERE " + "tournament_id = " + tournamentId ); rs.next(); int result = rs.getInt(1); rs.close(); statement.close(); conn.close(); return result; } catch (java.sql.SQLException e) { throw new MyDataAccessException(e); } } When a SQLException is thrown, the code will not close the Connection object. Each line between the call to getConnection() and the return statement can potentially throw a SQLException, and with each exception, a database connection hangs indefinitely or until database administrators clean up connections. This typically leads to situations where the database server needs to be restarted every few days to clean up unclosed connections. Listing 5-3 shows JDBC code that doesn t close the Statement and ResultSet objects properly. Listing 5-3. JDBC CodeThat Doesn t Close the Statement and ResultSet Objects Properly private javax.sql.DataSource dataSource; public List findRegisteredPlayers(int tournamentId) throws MyDataAccessException { java.sql.Connection conn = null; try { conn = dataSource.getConnection(); java.sql.Statement statement = conn.createStatement(); java.util.List results = new java.util.ArrayList(); java.sql.ResultSet rs = statement.executeQuery( "SELECT p.player_id, p.first_name, p.last_name " + "FROM t_registrations r, t_players p WHERE " + "r.player_id = p.player_id AND" + "r.tournament_id = " + tournamentId ); while (rs.next()) { int playerId = rs.getInt(1); String firstName = rs.getString(2); String lastName = rs.getString(3); Player player = new Player(playerId, firstName, lastName); results.add(player); } return results; } catch (java.sql.SQLException e) { throw new MyDataAccessException(e);
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