What are the Attributes in Servlet?

An object, which can be set, get, or removed, is referred to as an Attribute. Attributes in Servlet can get or set using any of the below scopes:

1. request: This is used while processing the results of a submitted form.
2. Session: This is used to create the session by the Web Container when a user visits the web application.
3. Application: This scope persists until the application is deployed.

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Syntax to set an Attribute:

public void setAttribute(String name, Object obj)

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Methods used in Attributes:

• public void setAttribute(String name, Object object): This method will set Attribute provided object in the application scope.
• public Object getAttribute(String name): This method will return the attribute for the specified name.
• public Enumeration getInitParameterNames(): This will return the names of the context’s initialization parameters as an Enumeration of String objects.
• public void removeAttribute(String name): This will remove the attribute with the given name from the servlet context.

Request Scope:

• Request Scope starts when an HTTP request hits the Servlet and ends when the Servlet sends an HTTP response.
• Request Scope is denoted as javax.servlet.http.HttpServletRequest interface.
• Web Container uses the request object as an argument of type HttpServletRequest.

Session Scope:

• Session scope starts when a connection is established between the user and the web application and remains until the browser is closed.
• Session Scope is denoted as javax.servlet.http.HttpSession interface.
• Session object can be retrieved using request.getSession().

Application Scope:

• Application scope starts when the browser is open and stops when the browser is closed.
• Application Scope is denoted as javax.servlet.ServletContext interface.
• Application object can be retrieved using getServletContext() directly or by making an explicit call to getServletConfig().getServletContext().

Example to set an Attribute:

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class First extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        ServletContext sc = getServletContext();
        sc.setAttribute("user","ABC"); //setting attribute on context scope
    }
}

Example to get an Attribute: 

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class Second extends  HttpServlet {
  protected void  doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        ServletContext sc = getServletContext();
        String str = sc.getAttribute("user");  //getting attribute from context scope
        out.println("Welcome"+str);  // Prints : Welcome ABC   
    }
}

Example showing all the three scopes that is session, request and application in a single code:

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;
@WebServlet("/get-attributes")
public class GetAttributesServlet extends HttpServlet{
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // get application scoped attribute
        String applicationScope = (String)req.getServletContext().getAttribute("a");
        // get session scoped attribute
        HttpSession session = req.getSession();
        String sessionScope = (String)session.getAttribute("b");
        // get request scoped attribute
        String requestScope = (String)req.getAttribute("c");
        // print response
        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        out.write("<html><body>");
        out.write("<h2>Servlet attributes example: applicationScope, sessionScope and requestScope</h2>");
        out.write("<p>applicationScope: " + applicationScope + "</p>");
        out.write("<p>sessionScope: " + sessionScope + "</p>");
        out.write("<p>requestScope: " + requestScope + "</p>");
    }
}