Starting a Hibernate project 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: …
${displaysql} … 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: 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. 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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