Servlet Scopes and attributes by R4R Team

Servlet Scopes and attributes:

For the fundamental servlet attribute scopes (request, session, and context): write servlet code to add, retrieve, and remove attributes; given a usage scenario, identify the proper scope for an attribute; and identify multi-threading issues associated with each scope.

What is an attribute

Attributes allow data to be stored and used in inter servlet communication. For example you might set some attributes in a login servlet that hands off to other servlets for the actual processing. Attributes can also be used with the session mechanism to preserve data between requests. The main methods associated with attribute manipulation are

setAttribute
getAttribute
Attributes are stored using setAttribute as name/value pairs with the signature

setAttribute(String name, Object value);

As you can see from this attributes are stored with a type of Object Take the example of storing an Integer number as an attribute called count

setAttribute(“count”,new Integer(99));
To retrieve it you will need to cast back to the original type thus.

Integer count = (Integer)session.getAttribute("count");

Attribute in Servlet:

An attribute in servlet is an object that can be set, get or removed from one of the following scopes:

request scope
session scope
application scope

The servlet programmer can pass informations from one servlet to another using attributes. It is just like passing object from one class to another so that we can reuse the same object again and again.

Attribute specific methods of ServletRequest, HttpSession and ServletContext interface

There are following 4 attribute specific methods. They are as follows:

public void setAttribute(String name,Object object):sets the given object in the application scope.
public Object getAttribute(String name):Returns the attribute for the specified name.
public Enumeration getInitParameterNames():Returns the names of the context's initialization parameters as an Enumeration of String objects.
public void removeAttribute(String name):Removes the attribute with the given name from the servlet context.

The three attribute scopes (request,session,and context) represent different visibility and availability. The following table sums up their visibility.

In the context of a JSP page this means that the implicit objects can be used as follows to set attributes in each scope.

request.setAttribute("username","marcus");
session.setAttribute("username","marcus");
application.setAttribute("username","marcus");

setAttribute and getAttribute are available to in request, session and application scopes.

ServletRequest attributes:

The scope of ServletRequest is based on the fundamental nature of the HttpRequest. The Servlet is executing and data is available to the executing code. Once the request is complete there is no connection between the client and the servlet and thus all data is effectively discarded. This is essential to the stateless nature of the HTTP Protocol.

Session attributes:

The scope of HttpSession is based on extending the connection between the client and the server code by implementing some way of allowing a state to exist between requests. This depends on using a unique identifier based on a cookie, which is called JSESSIONID. Data in the HttpSession scope is only available to a single client and is not shared. Note that there is no guarantee of how session attribute data is stored. Thus it might be stored in a file and not be available if the server is restarted, or it could be stored in a database and thus be available if the server is restarted.

ServletContext attributes:

Data in the ServletContext can be shared by any servlet of the same web application. You have seen examples of data in this context previously with information such as the database type or default language setting.

getAttributeNames:

Each of the objects has a getAttributeNames method that returns an enumeration of the names of the attributes (no surprises there). If you didn't know what attributes were available you could get the names and then loop through them getting the values they contain.

Attributes and threading:

Servlet containers such as Jakarta Tomcat optimise performance by providing execution through multiple threads. This optimises performance but introduces the problem that the action of one thread might cause an inconsistent data state, i.e. for example, if thread T1 sets an attribute A, and thread T2 sets the same attribute A, there might be unpredictable results if thread T1 reads attribute A back again.

The javax.servlet.SingleThreadModel interface:

This is a marker interface, i.e. it has no methods or fields but acts as a marker to other code. The servlet specification guarantees that if a Servlet implements this interface it will not execute the service method in more than one thread simultaneously. This ensures that the doXXX methods are not executed in more than one thread at a time.

Example of ServletContext to set and get attribute:

In this example, we are setting the attribute in the application scope and getting that value from another servlet.

r4r.java

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class r4r extends HttpServlet{

public void doGet(HttpServletRequest req,HttpServletResponse res)

{

try{

res.setContentType("text/html");

PrintWriter out=res.getWriter();

ServletContext context=getServletContext();

context.setAttribute("company","IBM");

out.println("Welcome to first servlet");

out.println("<a href='servlet2'>visit</a>");

out.close();

}catch(Exception e){out.println(e);}

r4r2.java

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class r4r2 extends HttpServlet{

public void doGet(HttpServletRequest req,HttpServletResponse res)

{

try{

res.setContentType("text/html");

PrintWriter out=res.getWriter();

ServletContext context=getServletContext();

String n=(String)context.getAttribute("company");

out.println("Welcome to "+n);

out.close();

}catch(Exception e){out.println(e);}

}}

web.xml

<web-app>

<servlet>

<servlet-name>s1</servlet-name>

<servlet-class>r4r</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>s1</servlet-name>

<url-pattern>/servlet1</url-pattern>

</servlet-mapping>

<servlet>

<servlet-name>s2</servlet-name>

<servlet-class>r4r2</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>s2</servlet-name>

<url-pattern>/servlet2</url-pattern>

</servlet-mapping>
</web-app>