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Listing 2.4 A simple Hibernate XML configuration file
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<!DOCTYPE hibernate-configuration SYSTEM "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="hibernate.connection.driver_class"> org.hsqldb.jdbcDriver </property> <property name="hibernate.connection.url"> jdbc:hsqldb:hsql://localhost </property> <property name="hibernate.connection.username"> sa </property>
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<property name="hibernate.dialect"> org.hibernate.dialect.HSQLDialect </property> <!-- Use the C3P0 connection pool provider --> <property name="hibernate.c3p0.min_size">5</property> <property name="hibernate.c3p0.max_size">20</property> <property name="hibernate.c3p0.timeout">300</property> <property name="hibernate.c3p0.max_statements">50</property> <property name="hibernate.c3p0.idle_test_period">3000</property> <!-- Show and print nice SQL on stdout --> <property name="show_sql">true</property> <property name="format_sql">true</property> <!-- List of XML mapping files --> <mapping resource="hello/Message.hbm.xml"/> </session-factory> </hibernate-configuration>
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The document type declaration is used by the XML parser to validate this document against the Hibernate configuration DTD. Note that this isn t the same DTD as the one for Hibernate XML mapping files. Also note that we added some line breaks in the property values to make this more readable you shouldn t do this in your real configuration file (unless your database username contains a line break). First in the configuration file are the database connection settings. You need to tell Hibernate which database JDBC driver you re using and how to connect to the database with a URL, a username, and a password (the password here is omitted, because HSQLDB by default doesn t require one). You set a Dialect, so that Hibernate knows which SQL variation it has to generate to talk to your database; dozens of dialects are packaged with Hibernate look at the Hibernate API documentation to get a list. In the XML configuration file, Hibernate properties may be specified without the hibernate prefix, so you can write either hibernate.show_sql or just show_sql. Property names and values are otherwise identical to programmatic configuration properties that is, to the constants as defined in org.hibernate.cfg.Environment. The hibernate.connection.driver_class property, for example, has the constant Environment.DRIVER. Before we look at some important configuration options, consider the last line in the configuration that names a Hibernate XML mapping file. The Configuration object needs to know about all your XML mapping files before you build the SessionFactory. A SessionFactory is an object that represents a particular
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Hibernate configuration for a particular set of mapping metadata. You can either list all your XML mapping files in the Hibernate XML configuration file, or you can set their names and paths programmatically on the Configuration object. In any case, if you list them as a resource, the path to the mapping files is the relative location on the classpath, with, in this example, hello being a package in the root of the classpath. You also enabled printing of all SQL executed by Hibernate to the console, and you told Hibernate to format it nicely so that you can check what is going on behind the scenes. We ll come back to logging later in this chapter. Another, sometimes useful, trick is to make configuration options more dynamic with system properties:
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... <property name="show_sql">${displaysql}</property> ...
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You can now specify a system property, such as with java -displaysql=true, on the command line when you start your application, and this will automatically be applied to the Hibernate configuration property. The database connection pool settings deserve extra attention. The database connection pool Generally, it isn t advisable to create a connection each time you want to interact with the database. Instead, Java applications should use a pool of connections. Each application thread that needs to do work on the database requests a connection from the pool and then returns it to the pool when all SQL operations have been executed. The pool maintains the connections and minimizes the cost of opening and closing connections. There are three reasons for using a pool:
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Acquiring a new connection is expensive. Some database management systems even start a completely new server process for each connection. Maintaining many idle connections is expensive for a database management system, and the pool can optimize the usage of idle connections (or disconnect if there are no requests). Creating prepared statements is also expensive for some drivers, and the connection pool can cache statements for a connection across requests.
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Figure 2.2 shows the role of a connection pool in an unmanaged application runtime environment (that is, one without any application server).
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