How can I specify system properties in Tomcat configuration on startup?

I understand that I can specify system properties to Tomcat by passing arguments with the -D parameter, for example "-Dmy.prop=value".

I am wondering if there is a cleaner way of doing this by specifying the property values in the context.xml file or some other tomcat configuration file. I would like to do this because, first, it is easier to keep track of my properties, and second, I have multiple contexts running and I don't know how I would specify context-specific properties through the -D parameter.

I am using Tomcat version 5.5.

cliff.meyers's original answer that suggested using <env-entry> will not help when using only System.getProperty()

According to the Tomcat 6.0 docs <env-entry> is for JNDI. So that means it won't have any effect on System.getProperty().

With the <env-entry> from cliff.meyers's example, the following code

System.getProperty("SMTP_PASSWORD");

will return null, not the value "abc123ftw".

According to the Tomcat 6 docs, to use <env-entry> you'd have to write code like this to use <env-entry>:

// Obtain our environment naming context
Context initCtx = new InitialContext();
Context envCtx = (Context) initCtx.lookup("java:comp/env");

// Look up our data source
String s = (String)envCtx.lookup("SMTP_PASSWORD");

Caveat: I have not actually tried the example above. But I have tried <env-entry> with System.getProperty(), and that definitely does not work.

(Update: If I could delete this answer I would, although since it's accepted, I can't. I'm updating the description to provide better guidance and discourage folks from using the poor practice I outlined in the original answer).

You can specify these parameters via context or environment parameters, such as in context.xml. See the sections titled "Context Parameters" and "Environment Entries" on this page:

http://tomcat.apache.org/tomcat-5.5-doc/config/context.html

As @netjeff points out, these values will be available via the Context.lookup(String) method and not as System parameters.

Another way to do specify these values is to define variables inside of the web.xml file of the web application you're deploying (see below). As @Roberto Lo Giacco points out, this is generally considered a poor practice since a deployed artifact should not be environment specific. However, below is the configuration snippet if you really want to do this:

<env-entry>
    <env-entry-name>SMTP_PASSWORD</env-entry-name>
    <env-entry-type>java.lang.String</env-entry-type>
    <env-entry-value>abc123ftw</env-entry-value>
</env-entry>