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Custom startup in servlet

I'm creating an atmosphere servlet application, which uses the following servlet in web.xml https://atmosphere.java.net/nonav/apidocs/org/atmosphere/cpr/AtmosphereServlet.html.
When this service starts up I would like to start to an Akka actor system and have it available throughout the life of the application.
How is this done?

I believe what you want to do is initialize an actor system within the servlet context. This question/answer should help. Don't forget to include the ContextListener in your web.xml.

How do I run an action on server startup using Struts2?

I have to execute a struts2 action on server startup rather than on the first request.

Loading data on startup of an application is a common task, you will find several examples on the web. As said in other answers, you should implement a ServletContextListener (that is not Struts2 specific)... you can read a great example here.
The important thing here is understanding the Action concept:
In Struts2 MVC (Model View Controller) Framework, an Action is the Controller (and part of the Model).
Actions are invoked by Requests coming from the Clients (and one Action is created on every request, so they're thread-safe).
This means that you need a Client, that usually means a guy in front of a pc, clicking on a browser... then, a client call is not the right trigger to perform automated, server-side operation on shared objects.
Of course you could implement some form of lazy-initialitazion (eg. with the help of a custom Interceptor) so that the first user would set-up something in the Application scope, and the other users would retrieve the object already populated, but this is not the best way to do it (you should handle the concurrency on the initialitaion, and you would have one user, the first, waiting for operations that the server could have done in the night on startup...).

Write a ServletContextListener, this will be available only one per web application and will get instatiated when the application is deployed.
Here is the post

If you want to some code to run when your web application, aka Servlet Context, starts for the first time, then you should leverage the hooks provided by the technology. The Servlet API provides lifecycle hooks for you to use to fire code at various lifecycle stages of a web application. Since all Struts 2 applications are Servlet API web applications, then you can leverage this yourself.
The ServletContextListener interface provides an init hook method. You simply implement this interface and register your implementation in the web.xml.
Note, if what you need to do is more Struts 2 specific, then you could consider utilizing something from within the Struts 2 API itself.

Load on start-up in servlet and jsp is present as below
You can ask the page to be loaded when the server starts. This is done via the web.xml file
<servlet>
<servlet-name>login</servlet-name>
<jsp-file>/login.jsp</jsp-file>
<load-on-startup>1</load-on-startup>
</servlet>
Normally jsp file is compiles on first hit.
Now the code says precompile a jsp file without waiting for the first hit.
For struts2 you can change programatically in web.xml
<listener>
<listener-class>your listener class</listener-class>
</listener>
refer this link it might be helpful to you
Loadonstart up.

Jersey initialization code after webapp is fully started

I asked this question earlier:
Jersey app-run initialization code on startup to initialize application
Is there any way to run this initialization code after the server has initialized the web app fully ? I am running into a catch-22 problem in that I need the web app fully started by Tomcat before I can run my login call once ?

There are a few considerations. First, Jersey is implemented as a servlet, and that's how it gets loaded. The simplest way to decouple servlet startup from servlet context startup is to not load it on startup. Accomplish this by omitting the load-on-startup element from the servlet descriptor in the web.xml. Setting it to a non-positive should also work.
If you must load the servlet on startup but still want to decouple it, which was my case, you'll probably have to write some custom code, which I did. I couldn't think of any other way. Use a listener of some type--a ServletContextListener or Spring ApplicationListener, depending on exactly how your app is set up--to kick off a new thread that will initialize your Jersey servlet by making an HTTP request to it. It feels a little dirty, but like I said, it's the only thing I and my team could come up with.

Is there a means of executing code during the startup of Apache Tomcat 6.x to perform initialization routines, etc.?

I want to initialize a global instance of a class before my Tomcat server completes startup and begins to offer my servlets. If this service somehow fails initialization, I'd like the entire startup sequence to fail as well (or as close to this as possible; it would be pointless for the server to be running otherwise). What's the best way I can accomplish this?

Each web application has a ServletContext associated with it. The ServletContext object is created when the application is started and is destroyed when the application is shut down. A ServletContext has a global scope and is similar to a global variable in an application.
http://www.javabeat.net/tips/178-servletcontextlistener-example.html
complete explanation here
http://onjava.com/pub/a/onjava/2001/04/12/listeners.html

One thing you can do portably is to implement a servlet that initializes everything you need in its init() method (and maybe call System.exit() if it fails, i do not know if you have permission to do this in Tomcat). Then you would load it using <load-on-startup> in your web.xml to specify initialization order.

How do servlets work?

I write couples of servlet program, however I dont think I fully understand how servlet run. So here is couples question that I have:
All the code I wrote about Servlet only run on Netbeans with apache tomcat run behind. When I run my html file that make request to the servlet, it usually give error saying that it could not find the servlet, I then have to redeploy the servlet then everything would run fine. Seem like my servlet timeout after a while or something.
Does the servlet run all the time? Servlet has init() and destroy(), so I guess it wont run all the time. So then when does it start and when does it end? Does it start when there is a request from the client, and end when it is time out? And how does I fixed my problem that I have to constantly redeploy the servlet. Thank you very much.

Under normal circumstances, a servlet is only destroyed at shutdown (ie when the application container, such as Tomcat, is shut down). Otherwise it remains in memory for the duration of the application. I couldn't say what's going on with your Netbeans setup, but try deploying your WAR file to a standalone Tomcat installation and see if the problem doesn't go away.
Another time that the application container will call destroy on a servlet is if it is running out of memory, but this is far less common.
Regarding your question about requests, a servlet is designed to handle many requests. It is said that the servlet is application-scoped, whereas the request has its own scope.

You're apparently in middle of developing with servlets. You need to ensure that the webapp is fully published whenever you have made changes to web.xml or any of the Servlet classes. Otherwise you may risk that the resource cannot be found.
In easy terms, a "resource not found" error is basically exactly the same as a "404 page not found". The servlet container can't seem to find a resource which matches the URL or url-pattern. That's all.
As to the Servlet lifecycle, it will be created only once during webapp startup (publish, create of context), the init() method will be called and the instance will be kept in the server's memory in a sort of a Map<Url-Pattern, Servlet>. If you have overriden the init() method in your Servlet, then it will be called. The servlet container will do the same for all of the servlets declared in web.xml (or as per Java EE 6, annotated with #WebServlet).
Everytime a request whose URL matches the url-pattern of the Servlet, the (inherited) service() method will be invoked. The normal HttpServlet implementation will then determine the method to be executed based on HttpServletRequest#getMethod(). If you did override any of those methods (doGet(), doPost(), etc) in your Servlet, then it will be invoked accordingly.
Finally, when the webapp is about to shutdown (unpublish, destroy of context), then the destroy() will be invoked for any of the Servlet instances kept in server's memory. If you have overriden the destroy() method in your Servlet, then it will be called.

A servlet "runs" only when it's invoked. The server will wait for a connection to come from the client, read the headers, find the proper servlet based on the mappings in web.xml, and then call the service() method of that servlet. The servlet object, will remain in-memory until the container decides to dispose it (which it may do at any time that it's not servicing requests). If the server decides to dispose of a particular servlet instance, it will create a new one the next time a request comes in for the servlet.
Which means you should not be getting an error that says the server can't find your servlet. Assuming that the application has been deployed, and there is a correct servlet mapping, the container will be able to process the request. If you edit your request and paste the exact error message, someone may be able to tell you why this isn't happening.

you need to research the servlet lifecycle - that's what the init() and destroy methods are there for
normally init() is called once, when the serlvet is first called (unless you did something like set it to autorun in tomcat)
and destroy() is called when the container shuts down
dopost() or doGet() (if it's an HTTP servlet) are called for each request