Servlet

What is a Servlet?

Servlets, similar to Applets, are applications running at server side to replace the old CGI programs. Most functions of Servlets can be realized by another Java technology - JSP.

Servlet Lifecycle

It seems that (actually not) Servlet begins from calling init method in its lifecycle. It is not thread-safe because the service method will not be called until the call to init returns. After the Servlet is initialized, the key method service (ServletRequest req, ServletResponse res) is called by every request, even concurrently. The method should be implemented in a thread-safe manner. If the Servlet needs to be unloaded unconditionally, the thread-safe destroy() method will be called.

Servlet Functions

HTTP GET - getQueryString
HTTP POST, PUT, DELETE - BufferedReader getReader / ServletInputStream getReader
Text - getWriter
Stream - getOutputStream

A Template of A Servlet 

//import classes;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;


public class ATemplateServlet extends HttpServlet {
   private String filename;


   //initial some data coz this method is called only once

   public void init(ServletConfig config) throws ServletException {
      super.init(config);//may do it
      filename = config.getInitParameter("addressfile");
      if(filename == null)
         throw new UnavailableException(this);
   }
   //to display the form page
   protected void doGet(HttpServletRequest req, 
     HttpServletResponse res) throws ServletException, IOException {
      res.setContentType("text/html"); //must do first
      PrintWriter out = res.getWriter(); //get a object of writer
      out.print("<head></head>");
      out.print("...<form method=post action="myservlet">...");
      out.close(); 
   } 
   //deal with the user data
   protected void doPost(HttpServletRequest req, 
     HttpServletResponse res) throws ServletException, IOException {
      String email = req.getParameter("mydata"); //get the data
      String msg; 
      dealWith(email);
      //write the result
      res.setContentType("text/html"); 
      res.setHeader("pragma", "no-cache");
      PrintWriter out = res.getWriter(); 
      out.print("<head></head>"); 
      out.print("blar,blar,...");
      out.close(); 
   } 
   //since each request can call this method
   //be careful the share data - synchronized
   private synchronized boolean dealWith(String email) 
     throws IOException {
      if(xxx)) return false; 
   }
}

Web Sites

Paul Flavin's old Web page
Servlet.com - <JSP has a better future>
Sun's Servlet

Servlet Tutorial

Track session:
To overcome the stateless http, track session is important for e-commerce. HttpSession object called req.getSession (HttpSession session = req.getSession(true);)is used to solove the problem (identified by the cookie or URL). Certainly it should be called before response object. When the session is not needed, session.invalidate() is called to delete the session on the server side or timed out and to remove the session ID Cookie at client side.

Call another Servlet:
Use the following statement to call another Servlet to process a request.

getServletContext() 
.getRequestDispatcher("/servlet/AServlet?param="+data)
.include(req, res); //or forward() for header

Include an HTML file:

getServletContext() 
.getResource("/html/mydoc.htm");

Set Cookie:

Cookie aCookie = new Cookie("xxx", credentials); 
aCookie.setVersion(1); //not version 0
aCookie.setDomain(".mydomain.com");//indicate the server
res.addCookie(authCookie); //ask client write the cookie

Read Cookie:

Cookie[] cookies = req.getCookies(); 
for(int i=0; i < cookies.length; i++) { 
   String n = cookies[i].getName();
   String d = cookies[i].getDomain(); 
   if(n != null && n.equals("xxx") && 
      d != null && d.equals(".mydomain.com")) {
      String credentials = cookies[i].getValue(); 
      doSomething(credentials);
      break; 
   } 
}

Access a Database

public class Demo extends HttpServlet { 
  Connection con; 

  public void init(ServletConfig config) 
    throws ServletException { 
    super.init(config); 
    try { 
      // jdbc-odbc 
      Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver"); 
      con = DriverManager.getConnection 
      (“jdbc:odbc:DemoTable", “", “"); 
    }catch (ClassNotFoundException e){ 
      throw new UnavailableException(this, 
       “Couldnt load JdbcOdbcDriver"); 
    }catch (SQLException e) { 
       throw new UnavailableException(this, 
       “Couldnt get db connection"); 
    } 
  } 
  public void doGet(HttpServletRequest req, 
  HttpServletResponse res) throws ServletException, IOException{ 
    String sql = “SELECT * from DemoTable ORDER BY test_id"; 
    try { 
      res.setContentType(“text/plain"); 
      ServletOutputStream out = res.getOutputStream(); 
    try { 
      Statement stmt = con.createStatement(); 
      if (stmt.execute(sql)) { 
        ResultSet rs = stmt.getResultSet(); 
        out.println(“<TABLE>\n"); 
        ResultSetMetaData rsmd = rs.getMetaData(); 
        int numcols = rsmd.getColumnCount(); 
        out.println(“< TR >"); 
        for (int i = 1; i < = numcols; i++) 
          out.println(“< TH >" + rsmd.getColumnLabel(i)); 
        out.println(“< /TR >\n"); 
        while(rs.next()){ 
          out.println(“< TR >"); 
          for(int i = 1; i < = numcols; i++) { 
            out.println(“< TD >"); 
            Object obj = rs.getObject(i); 
            if (obj != null) 
              out.println(obj.toString()); 
            else 
              out.println(“&nbsp;"); 
            } 
          out.println(“< /TR >\n"); 
        } 
        out.println(“< /TABLE >\n"); 
      } else { 
        out.println(“< B >Records Affected:< /B > 
         " +stmt.getUpdateCount()); 
      } 
    }catch (SQLException e) { 
      out.println(“< /TABLE >< H1 >ERROR: 
      < /H1 > " + e.getMessage()); 
    } 
    con.close();
  }catch(SQLException e) { 
  } 
 } 
}