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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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} finally { if (conn != null) { try { conn.close(); } catch (java.sql.SQLException e) { // exception must be caught, can't do anything with it. e.printStackTrace(); } } } } As you can see in Listing 5-3, the Statement and Result objects are not explicitly closed. The effects of this depend on the cursor type used by the Statement object. The following resources can be locked longer than they should: Memory used on the client side of the connection by the Statement object is freed only on garbage collection. A call to close() would free this memory as soon as possible. Any server-side cursor may not be released in a timely fashion. A call to close() would free these resources as early as possible. Temporary table space in the database that has been allocated to store the results returned by the query is not cleaned up in a timely fashion. A call to close() would free these resources as early as possible. So by not calling the close() methods on the Statement and ResultSet objects, this code doesn t handle its responsibilities to clean up resources when it knows they can be freed. Remember that we said database resources are scarce and should be managed carefully. Listing 5-4 doesn t close the PreparedStatement object it creates. Listing 5-4. JDBC Code That Doesn t Close the PreparedStatement Object private javax.sql.DataSource dataSource; public List findRegisteredPlayers(int tournamentId) throws MyDataAccessException { java.sql.Connection conn = null; try { conn = dataSource.getConnection(); java.sql.PreparedStatement statement = conn.prepareStatement( "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 = " ); statement.setInt(1, tournamentId); java.sql.ResultSet rs = statement.executeQuery(); java.util.List results = new java.util.ArrayList(); 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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CHAPTER 5 INTRODUCTION TO DATA ACCESS
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} finally { if (conn != null) { try { conn.close(); } catch (java.sql.SQLException e) { // exception must be caught, can't do anything with it. e.printStackTrace(); } } } } PreparedStatement objects are often cached by JDBC drivers or connection pools to avoid having to recompile identical SQL statements over and over. You would need to consult the vendor documentation to find out whether this caching is enabled or disabled for your configuration. These caching mechanisms will reuse a PreparedStatement object after its close() method has been called. But since the code in Listing 5-4 never calls this method, the cache hands out objects that are never returned. This may lead to the creation of large amounts of objects that are not garbage-collected (possibly leading to memory leaks) or exceptions that are thrown, depending on the type and configuration of your caching mechanism. Resource exhaustion by itself is a complicated topic. It can be avoided depending on how you write your JDBC code. And it s just one of three categories where database resource management can fail.
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Three expensive operations typically occur when working with JDBC: Resource creation, such as database connections SQL statement compilation SQL statement execution The execution of SQL statements is always going to be the most expensive operation, since it will occur most often. Some SQL statements make inefficient use of database resources and can be rewritten to become less expensive. It s relatively easy to avoid the recompilation of the same SQL statements by using PreparedStatements and a caching mechanism. Not doing so means the database must recompile the same statements over and over again. This consumes resources, is expensive, and can be easily avoided.
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Some databases, such as Oracle, will cache compiled SQL statements (not prepared statements). If you don t use PreparedStatement and your SQL statements have no variable parts so that they are identical for all executions, this cache may be used. However, configuring this cache inside the database so that it will always be used as expected under load requires strong database performance tuning skills. On top of that, it usually takes time and testing to get the configuration right. Also, not all database vendors have such a cache and not all caches perform equally well. That s why PreparedStatement objects are a far more developer-friendly way to improve data-access performance.
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