java - Tomcat: hot deploying new jars

Question

Can you hot deploy JAR files on Tomcat 5? The idea is to avoid restarting Tomcat and still be able to load (via reflection) some class from a newly added JAR. Can it be done? How? Is it unadvisable for a production system? Thanks

Edit: my scenario requires the addition of new JAR files for which the names aren't known in advance. Can the server be "watching" a directory of JARs rather than specific JARs?

Answer 1

Tomcat doesn't provide any mechanism to reload a single JAR. However, the whole context can be reloaded.

You just need to tell Tomcat to watch for your JAR in context.xml, like this,

<?xml version="1.0" encoding="UTF-8"?>
<Context override="true" swallowOutput="true" useNaming="false">
  <WatchedResource>WEB-INF/web.xml</WatchedResource>
  <WatchedResource>WEB-INF/lib/your.jar</WatchedResource>
  <Manager pathname=""/>
</Context>

We do this on production. Tomcat used to have some memory leaks but we haven't found any issues with Tomcat 5.5 or later.

Don't know if it's still necessary. We have to make following calls to avoid memory leak during hot deployment.

   public void contextDestroyed(ServletContextEvent sce) {
        // To fix the known memory leaks during re-deploy
        ClassLoader contextClassLoader = 
            Thread.currentThread().getContextClassLoader();
        LogFactory.release(contextClassLoader);

        java.beans.Introspector.flushCaches();
        ...
   }