While developing a web application with Java, as per your
business logic, situations will come when you need to do some
pre-decided time-consuming initialization jobs like Database
Initialization or some external file download, etc. at the very
first when your application gets loaded into the servlet container.
In these scenarios, to make your life easy, JEE framework
provides a nice little concept of servlet Load On Startup.
Assuming that you are using java 5.0 or higher, you can use
webServlet
annotation along with its attribute
loadOnStartup = 1
to explicitly direct your container to load your servlet at the
very first before any other servlets of your application get
loaded.Let us suppose that your application has got three servlets
namely OneServ, TwoServ, and ThreeServ. Following are the code
snippets for these three servlets.
Code snippet for OneServ
@webServlet(urlPattern= "/first")
public class OneServ extends HttpServlet{
// Your Servlet code
}
Code snippet for TwoServ
@webServlet(urlPattern= "/second", loadOnStartup = 1)
public class TwoServ extends HttpServlet{
// Your Servlet code
}
Code snippet for ThreeServ
@webServlet(urlPattern= "/third")
public class ThreeServ extends HttpServlet{
// Your Servlet code
}
The above code snippets will allow container to load servlet
TwoServ before OneServ and ThreeServ servlets. This has to happen
as the webServlet annotation for TwoServ has attribute
loadOnStartup with value equals to 1, while the other two servlets
have no specified loadOnStartup value with
webServlet
annotation.So whatever code you write in TwoServ will run before the codes
written in OneServ or ThreeServ.
For your holistic learning, let us discuss some fundamental
concepts related to loadOnStartup attribute of webServlet
annotation
- loadOnStartup attribute will always accept an integer. So it can have any values like 0 or 1 or -10 or 50 etc.
- If a servlet has loadOnStartup value 0 or negative or if the loadOnStartup attribute is not present at all (as in case of servlets OneServ or ThreeServ), then the container will load the corresponding servlets as and when required, typically at the very first time, it is accessed.
- If any two servlets have loadOnStartup values declared as x and y (x > y), the container will ensure that the servlet with loadOnStartup value x gets loaded only after the servlet with loadOnStartup value y has been loaded. So any servlet with loadOnStartup value 1 will always load at the very first.
- A servlet that has explicitly declared
loadOnStartup value in
webServletannotation will always call itsinit()method once it is loaded into the memory.
All the concepts presented above can thoroughly be demonstrated
with the following working codes of a sample application. But
before you start walking through it; please note the application
structure as below
This sample application (
LoadOnStartUpExample as
you can see in the figure above) has three servlets OneServ,
TwoServ, and ThreeServ. OneServ has no specified loadOnStartup value
whereas TwoServ and ThreeServ have loadOnStartup values 1 and 2
respectively.
OneServ (which reverses a name taken as input) is accessed from a webpage index.html through a form submission, but before it is accessed from the web page, as the application is already loaded into the container, first TwoServ will be called, then ThreeServ, as the loadOnStartup values are 1 and 2 respectively. Before you submit the form in index.html to call OneServ, please check your console to check that indeed TwoServ and ThreeServ are called in proper order.
Coding index.html
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>BunksAllowed-LoadOnStartUp Example</title>
</head>
<body>
<h2>Load On Start Up Example</h2>
<p>
When you submit the following form, a servlet named <b>OneServ</b> will be called and this servlet will show
the input name in reverse order.<br>
Please note this <b>OneServ</b> has no specified loadOnStartup value.
</p>
<p>
This little sample application has two more servlets <b>TwoServ</b> and <b>ThreeServ</b>.
<b>TwoServ</b> has a specified loadOnStartup value <b>1</b> and <b>ThreeServ</b> has a
specified loadOnStartup value<b>2</b>.
</p>
<p>
Hence at this instance, when you are watching this web page, as the application has
already been loaded into the container, first <b>TwoServ</b> has been called followed by
<b>ThreeServ</b>. The proof of their calling can be verified into your console. Hence please
look into the console before submitting the below form.
</p>
<p>
Now if you submit the following form, <b>OneServ</b> will be called as usual. Hence the order of
accessing the servlets will be <b>TwoSrv</b>, <b>ThreeServ</b> then <b>OneServ</b>.
</p>
<br>
<form action="OneServ" method="get">
<label>Enter your name to get it revered</label>
<br>
<input type="text" name="nameInput" />
<input type="submit" value="Submit to OneServ" />
</form>
<div align="center">
<p>© <a href="http://www.bunksallowed.com">BunksAllowed</a> 2019</p>
</div>
</body>
</html>
Coding OneServ.java
package com.bunksallowed.example.lose;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class OneServ
*/
@WebServlet("/OneServ")
public class OneServ extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* Default constructor.
*/
public OneServ() {
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String name = request.getParameter("nameInput");
String reversedName = "";
for(int i = name.length() - 1; i>= 0; i-- )
reversedName = reversedName + name.charAt(i);
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>
<head>
<title>Name In Reverse Order</title>
</head>
<body>" + reversedName + "</body>
</html>");
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
}
CodingTwoServ.java
package com.bunksallowed.example.lose;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class TwoServ
*/
@WebServlet(urlPatterns="/TwoServ",loadOnStartup=1)
public class TwoServ extends HttpServlet {
private static final long serialVersionUID = 1L;
public void init()
{
System.out.println("This is init() of TwoServ, it has loadOnStartup value 1");
}
/**
* @see HttpServlet#HttpServlet()
*/
public TwoServ() {
super();
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.getWriter().append("Served at: ").append(request.getContextPath());
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
}
Coding ThreeServ.java
package com.bunksallowed.example.lose;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class ThreeServ
*/
@WebServlet(urlPatterns ="/ThreeServ", loadOnStartup=2)
public class ThreeServ extends HttpServlet {
private static final long serialVersionUID = 1L;
public void init()
{
System.out.println("This is init() of ThreeServ, it has loadOnStartup value 2");
}
/**
* @see HttpServlet#HttpServlet()
*/
public ThreeServ() {
super();
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.getWriter().append("Served at: ").append(request.getContextPath());
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
}
If everything goes fine, once you run this application, you will find this web page as following
Now, before submitting the form on the web page, you can check your console, it will look something like the following figure. Here you will see that the servlet with loadOnStartup value 1 has been called first, followed by the servlet with loadOnStartUp value 2.
Now, if you submit this form the sample application will run as usual and give you a web page showing your name in reversed order.
Hope, you have enjoyed learning with this tutorial. More on queue.
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