How does Tomcat load resources from context.xml

Question

Hoping that someone can clear up a few things -

For a project I need to be able to switch between a Sybase and an Oracle database. Using Spring I have been to come up with a solution using AbstractRoutingDataSource to allow switching between the sybase and oracle data source as needed.

However, within my tomcat context.xml, I have listed all data sources - so for each one I have "jdbc/myDbSybaseDataSource" and "jdbc/myDbOracleDataSource". I was wondering, does tomcat attempt to create all of these data sources on start up, or does it wait until something in the code has invoked it?

I know in spring you can lazy load your beans, but would tomcat still create each resource in the context.xml anyways?

Any help is much appreciated!

Edit

The reason for asking is - if for example, the Sybase database is down, we need to know that we can switch to the Oracle, but Tomcat will not try and load the Sybase resource from the context.xml at start up (unfortunately I am not in a position to turn off one of the databases to test this! :) ).

From what I can tell, Tomcat is loading the resources - but not trying to actually connect to the database until part of the code invokes the data source, at which point Spring kicks in and does its work. It simply loads in the resources and keeps the info stored somewhere, when Spring tries to create the data source it looks up the corresponding resource info and uses this

Answer 1

Yes, Tomcat will create these datasources at startup. I cannot tell you how far this initialization goes but I would try to avoid any overhead that you dont need.

Do you need both datasources at runtime or is it just to be able to decide at startup of your application?

In the second case you can see the cotext.xml file as an (external) configuration file. You can then use springs jndi reference feature to connect to the desired database for the current application.

In the root-context.xml you reference ONE datasource by name:

    <jee:jndi-lookup id="mvcDatasource" jndi-name="jdbc/mvcDatasource"
    expected-type="javax.sql.DataSource" />

And depending on which type should be used you declare the correct implementation in the context.xml:

<Context>
    <Environment name="configDirectory" type="java.lang.String" value="file:///tmp/app1" />
    <Resource name="jdbc/mvcDatasource" type="javax.sql.DataSource" auth="Container" driverClassName="org.h2.Driver" maxActive="8" maxIdle="4" username="user" password="" url="jdbc:h2:file:/tmp/app1/db" />
</Context>

As you can see you can use other declarations too. In my case i inject the external config location here to be able to load properties files for the propertyconfigurer too. So this location will be application specific.