Explanation of context.xml

Question

When using Tomcat, I've always treated web.xml as a kind of .htaccess or httpd.conf equivalent. It seems natural that there might have to be some way of configuring a web server.

However, I don't quite understand the purpose of context.xml. For instance, when working with JDBC, why do I have to add a resource-ref in web.xml and also a Resource with even more info in context.xml? Could I eliminate the context.xml file and somehow instantiate the DataSource in my code? I am asking because hypothetical examples like that help me understand.

EDIT: I am trying to understand what is happening in configs like this, in /META-INF/context.xml:

<Context>
<Resource name="jdbc/postgres" auth="Container" type="javax.sql.DataSource"
    driverClassName="org.postgresql.Driver" url="jdbc:postgresql://127.0.0.1:5432"
    username="postgres" password="yes" maxActive="20" maxIdle="10"
    maxWait="-1" />
</Context>

and, in WEB-INF/web.xml:

<resource-ref>
    <description>postgreSQL Datasource example</description>
    <res-ref-name>jdbc/postgres</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>

Why do I have to put both of those in there to use JDBC? What are they doing exactly and is there another way of doing the same thing but in the Java code? Like I said, not because I want to, but because I want to understand what they are doing better.

Answer 1

I don't quite understand the purpose of context.xml

context.xml is called from the web.xml through <context-param> tag. As you web.xml loads first when an application is created and it will create the references for the contexts that are configured in it.

I don't quite understand the purpose of context.xml

So, the purpose of context.xml is adding separation of codes. you can have separate contexts for different purposes . for example For Database connectivity, using other frameworks etc..,

Could I eliminate the context.xml file and somehow instantiate the DataSource in my code?

Yes ,you can do that by configuring the datasource in web.xml itself.

Hope this helps !!

Answer 2

To your initial questions:

I don't quite understand the purpose of context.xml

In Tomcat, you'll frequently see the term "Context". When you see this it's just referring to your application (i.e. Context == your web app). Thus /META-INF/context.xml is the configuration file specific to your application (actually, it's one of the configuration files because there are others).

For instance, when working with JDBC, why do I have to add a resource-ref in web.xml and also a Resource with even more info in context.xml?

You don't. You do not need to add anything to web.xml. If you define the resource in /META-INF/context.xml, Tomcat will create your resource and expose it through JNDI. You can then retrieve it as you would any resource from JNDI.

Could I eliminate the context.xml file and somehow instantiate the DataSource in my code?

Possibly. It's common to see Spring users create a DataSource in their Spring bean definitions. If you're not using Spring, you could still do it, but it would be more work.

Regardless of how you do this, if you setup the DataSource in your application you lose some flexibility. For example, you cannot share the DataSource across multiple applications and your application has to know how to set up the DataSource. If you define the DataSource in the server's configuration (i.e. with a Resource tag) then your application does not need this information.

To the response by san krish:

So, the purpose of context.xml is adding separation of codes. you can have separate contexts for different purposes . for example For Database connectivity, using other frameworks etc..,

The purpose of the Context tag is to configure the context (i.e. your application). It might provide you with the ability to pull certain aspects of your application, like the DataSource, out of the code and into configuration, but that's just the benefit of externalizing your configuration. It's not the purpose of this file.

Yes ,you can do that by configuring the datasource in web.xml itself.

No, with Tomcat you cannot configure a DataSource strictly through web.xml. It's just not possible to provide all of the information in web.xml that is required to create a DataSource. Because of this, you need to define a Resource tag for your DataSource and it is redundant to specify your DataSource in web.xml.