What is a context initialization parameter : context-param ?

The ServletContext interface , defines methods , which can be used by a web application , in order to interact with the servlet container . A servlet container can pass initialization parameters , to a web application , by using what is called : context parameters.

context initialization parameter what is ?

Context parameters , can be defined either inside the web.xml file , located in : tomcat-version-number/conf/web.xml , in this case , it applies to every web application . Or it can be defined inside the web.xml file , located in : tomcat-version-number/webapps/web-app-name/WEB-INF/web.xml , in this case , it will apply only , to the specified web application .

Context parameters must be placed inside the web-app root tag , for example :

<!--
webapps/servletcontext-contex-param/WEB-INF/web.xml

The web application name is : servletcontext-contex-param
-->

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                        http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
    version="4.0"
    metadata-complete="false">
    
    <context-param>
        <param-name>web-application-initialization-param-1</param-name>
        <param-value>web-application-initialization-param-1-value</param-value>
    </context-param>
    
</web-app>

A context parameter , must have a name , which is defined by using the tag : param-name , and a value , which must be defined by using the tag : param-value .

To get the names , of initialization parameters , defined for a web application , the getInitParameterNames method , of ServletContext can be used . The getInitParameterNames method of ServletContext , has the following signature :

java.util.Enumeration<String> getInitParameterNames()

If no context initialization parameters , have been defined , the getInitParameterNames method , will return an empty Enumeration . An empty Enumeration does not contain any element .

To get the value of a context initialization parameter , the getInitParameter method of ServletContext , can be used . The getInitParameter method of ServletContext , has the following signature :

java.lang.String getInitParameter(java.lang.String name)

If the specified name does not exit , then getInitParameter , will return null .

For example , the following jsp servlet , named : index.jsp , retrieves the context initialization parameter , for a web application , named : servletcontext-contex-param , which web.xml , was defined earlier . No context initialization parameters , were defined inside the tomcat-version-number/conf/web.xml , so the only context initialization parameters , are those defined , inside the web.xml of the servletcontext-contex-param web application .

<html>

    <head>
        <title>ServletContext Context Parameters</title>
    </head>
    
    <body>
        <h1>getInitParameterNames</h1>
            <!--Get all the context parameters names , passed to the web application ,
                using 'application' implicit object  .
                The 'application' implicit object , is an instance of ServletContext .
                    The getInitParameterNames method , can be used to retrieve the names ,
                    of all the context initialization parameters , passed to a web application .
            -->
        <%
            java.util.Enumeration<String> context_Init_Parameter_Names_En  = application.getInitParameterNames();
            if(context_Init_Parameter_Names_En.hasMoreElements()){
                out.println("<ul>");
                while(context_Init_Parameter_Names_En.hasMoreElements()){
                    out.println("<li>" + context_Init_Parameter_Names_En.nextElement() + "</li>");
                }
                out.println("</ul>");
            }else{
                out.println("<p>No initialization parameters , passed to the web application </p>");
            }
            %>
        
        <h1>getInitParameter</h1>
            <!--The getInitParameter method of ServletContext ,
                    can be used to retrieve the value ,
                    of a context initialization parameter ,
                    by providing the name of the
                    context initialization parameter .
                    If found , the value is returned 
                    as a string , else the method will return null . 
            -->
            <ul>
            <%
                String webMasterEmail = application.getInitParameter("web-master-email");
                out.println("<li>Webmaster email is : "+ (webMasterEmail == null ? "undefined" : webMasterEmail) +"</li>");

                String debugMode = application.getInitParameter("debug-mode");
                out.println("<li>debugMode is : "+ (debugMode == null ? "undefined" : debugMode) +"</li>");

                String WebApplicationInitializationParam1 = application.getInitParameter("web-application-initialization-param-1");
                out.println("<li>WebApplicationInitializationParam1 is : "+ (WebApplicationInitializationParam1 == null ? "undefined" : WebApplicationInitializationParam1) +"</li>");
            %>
            </ul>
        
    </body>
</html>

When the web address http://localhost:8080/servletcontext-contex-param/ is visited , the result is :

A context parameter , cannot be defined by using web annotation , inside a servlet , so it cannot be defined in this way :

@WebServlet(
    urlPatterns = "/servlet-get-init-parameters",
    initParams = {
        @WebInitParam (name="an-init-param-name" , value="an-init-param-value")
    }
)
    
public class ServletGetInitParameter extends HttpServlet {

In this code fragment , what was defined , is an initialization parameter , for a servlet . A servlet initialization parameter , applies only to a specific servlet , whereas a context initialization parameter , applies to the whole web application .

In addition to defining context parameters using : web.xml , context parameters can also be defined , by using the setInitParameter method , of ServletContext . The setInitParameter method , can be used inside a class , which implements the : ServletContainerInitializer interface, or the ServletContextListener interface . It can also be used inside the init method of a servlet , which has the load-on-startup tag inside web.xml , set to a value larger or equal , to 0 . When a servlet has the load-on-startup tag , set to a value larger or equal to 0 , the servlet will be loaded when the web application is deployed .

For example , a web application has the following web.xml file :

<!--
webapps/servletcontext-contex-param/WEB-INF/web.xml

The web application name is : servletcontext-contex-param
-->

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                        http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
    version="4.0"
    metadata-complete="false">
    
    <context-param>
        <param-name>web-application-initialization-param-1</param-name>
        <param-value>web-application-initialization-param-1-value</param-value>
    </context-param>
    
    <servlet>
        <servlet-name>servlet-context-init-param-example</servlet-name>
        <servlet-class>com.difyel.test.ServletContextInitParamExample</servlet-class>
        <load-on-startup>0</load-on-startup>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>servlet-context-init-param-example</servlet-name>
        <url-pattern>/servlet-context-init-param-example</url-pattern>
    </servlet-mapping>

    </web-app>

The web.xml defines a servlet , named : servlet-context-init-param-example , the servlet class is defined to be com.difyel.test.ServletContextInitParamExample :

package com.difyel.test;

import java.util.Enumeration ; 

import java.io.IOException;
import java.io.PrintWriter; 

import javax.servlet.ServletException;
import javax.servlet.ServletContext ; 
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse; 
    
public class ServletContextInitParamExample extends HttpServlet{
    
    public void init(){
        ServletContext servletContext = this.getServletContext();
        servletContext.setInitParameter("context-init-param-name" , "context-init-param-value");
    }
    
    protected void doGet(HttpServletRequest req , HttpServletResponse res) throws ServletException , IOException{
        PrintWriter out = res.getWriter();
        out.println("<html>");
        out.println("<head><title>Servlet Context Init Param Example</title></head>");
        out.println("<body>");
        ServletContext servletContext = this.getServletContext();
        out.println("<ul>");
        Enumeration<String> contextInitParamNameEn = servletContext.getInitParameterNames();
        if(contextInitParamNameEn.hasMoreElements()){
        while(contextInitParamNameEn.hasMoreElements()){
            String contextInitParamName = contextInitParamNameEn.nextElement();
            String contextInitParamValue = servletContext.getInitParameter(contextInitParamName);
            out.println("<li>"+ contextInitParamName + " : " +  contextInitParamValue +  "</li>");
            }
        }else{
            out.println("<li>No context init parameters were defined for the web application </li>");
        }
        out.println("</ul>");
        out.println("<body>");
        out.println("<html>");
    }
}

When the web address http://localhost:8080/servletcontext-contex-param/servlet-context-init-param-example/ is visited , the result is :

servlet-context-init-param-example-localhost

The example web application , can be download from here