Java Reference
In-Depth Information
Beforeyoucancommunicatewithadatasource,youneedtoestablishaconnection.
JDBC provides the
java.sql.DriverManager
class and the
javax.sql.DataSource
interface for this purpose:
•
DriverManager
letsanapplicationconnecttoadatasourcebyspecifyinga
URL. When this class first attempts to establish a connection, it automatically
loads any JDBC 4.x drivers located via the classpath. (Pre-JDBC 4.x drivers
must be loaded manually.)
•
DataSource
hides connection details from the application to promote data
sourceportabilityandispreferredover
DriverManager
forthisreason.Be-
cause a discussion of
DataSource
is somewhat involved (and is typically
used in a Java EE context), I focus on
DriverManager
in this chapter.
Before letting you obtain a data source connection, early JDBC versions required
you to explicitly load a suitable driver, by specifying
Class.forName()
with the
name of the class that implements the
Driver
interface. For example, the JDBC-
ODBC Bridge driver was loaded via
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
. Later JDBC ver-
sions relaxed this requirement by letting you specify a list of drivers to load via the
jdbc.drivers
systemproperty.
DriverManager
wouldattempttoloadallthese
drivers during its initialization.
Under Java 7,
DriverManager
first loads all drivers identified by the
jd-
bc.drivers
system property. It then uses the
java.util.ServiceLoader
-
based service provider mechanism (discussed in Appendix C) to load all drivers from
accessibledriverJARfilessothatyoudon'thavetoexplicitlyloaddrivers.Thismech-
anism requires a driver to be packaged into a JAR file that includes
META-INF/
services/java.sql.Driver
.The
java.sql.Driver
textfilemustcontaina
single line that names the driver's implementation of the
Driver
interface.
Each loaded driver instantiates and registers itself with
DriverManager
via
DriverManager
's
void registerDriver(Driver driver)
classmethod.
When invoked, a
getConnection()
method walks through registered drivers, re-
turning an implementation of the
java.sql.Connection
interface from the first
driver that recognizes
getConnection()
's JDBC URL. (You might want to check
out
DriverManager
's source code to see how this is done.)
Note
Tomaintaindatasource-independence,muchofJDBCconsistsofinterfaces.
Each driver provides implementations of the various interfaces.
