title: Tomcat 8 - context.xml use Environment Variable in Datasource
body: <p>I have a Tomcat 8 project that uses a datasource (see below)</p>
  
  <pre><code>&lt;Resource auth="Container" 
            name="jdbc/JtmDS"  
            driverClassName="org.apache.derby.jdbc.EmbeddedDriver" 
            type="javax.sql.DataSource" 
            username="xfer"
            password="xfer10" 
            url="jdbc:derby:/home/PUID/tm/control/JtmDB"                    
            initialSize="25"
            maxTotal="100" 
            maxIdle="30" 
            maxWaitMillis="10000"                                      
            removeAbandonedOnBorrow="true"
            removeAbandonedTimeout="20" /&gt;
  </code></pre>
  
  <p>This works perfectly well. </p>
  
  <p>However the url is a hard-coded path <code>/home/PUID/tm/control/JtmDB</code></p>
  
  <p>When this gets into production the PUID part of the path will differ across numerous systems. 
  I have an environment variable set <code>export PUID=abcd</code>
  The rest of the application is able to use things like <code>System.getenv( )</code> or <code>${env:PUID}</code> as and where appropriate.</p>
  
  <p>These all work fine.</p>
  
  <p>My question is very simply: 
  How can I make the PUID value in my context.xml a variable that can be read from an environment variable?</p>