Init-param 和 context-param

<init-param> will be used if you want to initialize some parameter for a particular servlet. When request come to servlet first its init method will be called then doGet/doPost whereas if you want to initialize some variable for whole application you will need to use <context-param> . Every servlet will have access to the context variable.

Consider the below definition in web.xml

<servlet>
<servlet-name>HelloWorld</servlet-name>
<servlet-class>TestServlet</servlet-class>
<init-param>
<param-name>myprop</param-name>
<param-value>value</param-value>
</init-param>
</servlet>

You can see that init-param is defined inside a servlet element. This means it is only available to the servlet under declaration and not to other parts of the web application. If you want this parameter to be available to other parts of the application say a JSP this needs to be explicitly passed to the JSP. For instance passed as request.setAttribute(). This is highly inefficient and difficult to code.

So if you want to get access to global values from anywhere within the application without explicitly passing those values, you need to use Context Init parameters.

Consider the following definition in web.xml

 <web-app>
<context-param>
<param-name>myprop</param-name>
<param-value>value</param-value>
</context-param>
</web-app>

This context param is available to all parts of the web application and it can be retrieved from the Context object. For instance, getServletContext().getInitParameter(“dbname”);

From a JSP you can access the context parameter using the application implicit object. For example, application.getAttribute(“dbname”);

<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
classpath*:/META-INF/PersistenceContext.xml
</param-value>
</context-param>

I have initialized my PersistenceContext.xml within <context-param> because all my servlets will be interacting with database in MVC framework.

Howerver,

<servlet>
<servlet-name>jersey-servlet</servlet-name>
<servlet-class>com.sun.jersey.spi.spring.container.servlet.SpringServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>
classpath:ApplicationContext.xml
</param-value>
</init-param>
<init-param>
<param-name>com.sun.jersey.config.property.packages</param-name>
<param-value>com.organisation.project.rest</param-value>
</init-param>
</servlet>

in the aforementioned code, I am configuring jersey and the ApplicationContext.xml only to rest layer. For the same I am using </init-param>

<init-param> and <context-param> are static parameters which are stored in web.xml file. If you have any data which doesn't change frequently you can store it in one of them.

If you want to store particular data which is confined to a particular servlet scope, then you can use <init-param> .Anything you declare inside <init-param> is only accessible only for that particular servlet.The init-param is declared inside the <servlet> tag.

<servlet>
<display-name>HelloWorldServlet</display-name>
<servlet-name>HelloWorldServlet</servlet-name>
<init-param>
<param-name>Greetings</param-name>
<param-value>Hello</param-value>
</init-param>
</servlet>

and you can access those parameters in the servlet as follows:

out.println(getInitParameter("Greetings"));

If you want to store data which is common for whole application and if it doesn't change frequently you can use <context-param> instead of servletContext.setAttribute() method of the application context. For more information regarding usage of <context-param> VS ServletContext.setAttribute() have a look at this question. context-param are declared under the tag web-app. You can declare and access the <context-param> as follows

<web-app>
<context-param>
<param-name>Country</param-name>
<param-value>India</param-value>
</context-param>
<context-param>
<param-name>Age</param-name>
<param-value>24</param-value>
</context-param>
</web-app>

Usage in the application either in a JSP or Servlet

getServletContext().getInitParameter("Country");
getServletContext().getInitParameter("Age");

What is the difference between <init-param> and <context-param> !?

Single servlet versus multiple servlets.

Other Answers give details, but here is the summary:

A web app, that is, a “context”, is made up of one or more servlets.

• <init-param> defines a value available to a single specific servlet within a context.
• <context-param> defines a value available to all the servlets within a context.