bootstrap executable jar file in classpath when server starts

Question

I have a a library that is bundled as an executable jar file and added to weblogic / tomcat classpath, how can I execute a main method from the jar file when the server is starting and loading the classes from the jar file.

what I want to is to have some initialization code to be executed first thing when the jar file is loaded and server is starting without any user intervention.

Note: I know I can bundle my jar in a war file, but I have some aspectj code in my library that I want to weave all running applications in the jvm, when I bundle my jar in war file, the aspectj code will only weave into the classes in the war file so I added my library jar file in the classpath.

Thanks in advance.

Answer 1

Add a class inside your JAR with the following code:

public class TomcatStartupListener implements org.apache.catalina.LifecycleListener {
  public void lifecycleEvent(org.apache.catalina.LifecycleEvent event) {
    if (event.getType().equals("after_start")) {
      // call your main method here
    }
  }
}

Note: In order to compile this, you need to add <tomcat-dir>/lib/catalina.jar to your classpath. Otherwise when compiling it won't be able to find the necessary interfaces (org.apache.catalina.LifecycleListener and org.apache.catalina.LifecycleEvent). Once you're done with the compiling, put the JAR as usual under <tomcat-dir>/lib.

Now open <tomcat-dir>/conf/server.xml and add the following under the <Server> section:

<Listener className="com.yourpackage.TomcatStartupListener" />

Now whenever your Tomcat server starts, this TomcatStartupListener class inside your JAR will be called, and you can invoke your main method. There are a whole lot of other event types too! You can use any of these event types:

• before_init
• after_init
• before_start
• configure_start
• start
• after_start
• before_stop
• stop
• configure_stop
• after_stop
• before_destroy
• after_destroy

This approach is necessary because of the way the classloaders work in Tomcat (or even most JVMs). Here are the important points from that link:

There are three aspects of a class loader behavior
  Lazy Loading
  Class Caching
  Separate Namespaces

The JVM will get very heavy if all classes inside all JARs get loaded indiscriminately. So the classes inside shared JARs are loaded only on-demand. The only way for you to invoke the main method is to add the above lifecycle listener.

Answer 2

Perhaps the simplest thing to do is to deploy a trivial servlet in a .war file that references your .jar file. The servlet can be configured to start up upon deployment/container start, and then it can invoke the class containing your main() method.