Servlets

03:18
Java-Examples

Introduction to Java Servlet

A Servlet is a Java programming language class used to extend the capabilities of a server. Although servlets can respond to any types of requests, they are commonly used to extend the applications hosted by web servers, so they can be thought of as Java Applets that run on servers instead of in web browsers.[1] These kinds of servlets are the Java counterpart to non-Java dynamic Web content technologies such as PHP and ASP.NET.

A Servlet is a Java-based server-side web technology. Technically speaking, a Servlet is a Java class in Java EE that conforms to the Java Servlet API, a protocol by which a Java class may respond to requests. Servlets could in principle communicate over any client-server protocol, but they are most often used with the HTTP protocol. Therefore, the word "Servlet" is often used in the meaning of "HTTP Servlet".[2] Thus, a software developer may use a servlet to add dynamic content to a web server using the Java platform. The generated content is commonly HTML, but may be other data such as XML. Servlets can maintain state in session variables across many server transactions by using HTTP cookies, or URL rewriting.

To deploy and run a Servlet, a web container must be used. A web container (also known as a Servlet container) is essentially the component of a web server that interacts with the Servlets. The web container is responsible for managing the lifecycle of Servlets, mapping a URL to a particular Servlet and ensuring that the URL requester has the correct access rights. The Servlet API, contained in the Java package hierarchy javax.servlet, defines the expected interactions of the web container and a servlet.[2] A Servlet is an object that receives a request and generates a response based on that request. The basic Servlet package defines Java objects to represent Servlet requests and responses, as well as objects to reflect the Servlet's configuration parameters and execution environment. The package javax.servlet.http defines HTTP-specific subclasses of the generic servlet elements, including session management objects that track multiple requests and responses between the web server and a client. Servlets may be packaged in a WAR file as a web application. Servlets can be generated automatically from JavaServer Pages (JSP) by the JavaServer Pages compiler. The difference between Servlets and JSP is that Servlets typically embed HTML inside Java code, while JSPs embed Java code in HTML. While the direct usage of Servlets to generate HTML (as shown in the example below) has become rare, the higher level MVC web framework in Java EE (JSF) still explicitly uses the Servlet technology for the low level request/response handling via the FacesServlet. A somewhat older usage is to use Servlets in conjunction with JSPs in a pattern called "Model 2", which is a flavor of the model-view-controller pattern.         



History


The complete Servlet specification was created by Sun Microsystems, with version 1.0 finalized in June 1997. Starting with version 2.3, the Servlet specification was developed under the Java Community Process. JSR 53 defined both the Servlet 2.3 and JavaServer Page 1.2 specifications. JSR 154 specifies the Servlet 2.4 and 2.5 specifications. As of March 26, 2010, the current version of the Servlet specification is 3.0. In his blog on java.net, Sun veteran and GlassFish lead Jim Driscoll details the history of Servlet technology. James Gosling first thought of Servlets in the early days of Java, but the concept did not become a product until Sun shipped the Java Web Server[clarify] product. This was before what is now the Java Platform, Enterprise Edition was made into a specification.
  • Servlet API history
    Servlet API versionReleasedPlatformImportant Changes
    Servlet 3.0December 2009JavaEE 6, JavaSE 6Pluggability, Ease of development, Async Servlet, Security, File Uploading
    Servlet 2.5September 2005JavaEE 5, JavaSE 5Requires JavaSE 5, supports annotation
    Servlet 2.4November 2003J2EE 1.4, J2SE 1.3web.xml uses XML Schema
    Servlet 2.3August 2001J2EE 1.3, J2SE 1.2Addition of Filter
    Servlet 2.2August 1999J2EE 1.2, J2SE 1.2Becomes part of J2EE, introduced independent web applications in .war files
    Servlet 2.1November 1998UnspecifiedFirst official specification, added RequestDispatcher, ServletContext
    Servlet 2.0JDK 1.1Part of Java Servlet Development Kit 2.0
    Servlet 1.0June 1997

Usage

Servlets are most often used to :
    process or store data that was submitted from an HTML form
    provide dynamic content such as the results of a database query
      manage state information that does not exist in the stateless HTTP protocol, such as filling the articles into the shopping cart of the appropriate customer.

Life cycle of a Servlet

A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet
  • The servlet is initialized by calling the init () method.
  • The servlet calls service() method to process a client's request.
  • The servlet is terminated by calling the destroy() method.
  • Finally, servlet is garbage collected by the garbage collector of the JVM.
Now let us discuss the life cycle methods in details.

The init() method :

The init method is designed to be called only once. It is called when the servlet is first created, and not called again for each user request. So, it is used for one-time initializations, just as with the init method of applets.
The servlet is normally created when a user first invokes a URL corresponding to the servlet, but you can also specify that the servlet be loaded when the server is first started.
When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost as appropriate. The init() method simply creates or loads some data that will be used throughout the life of the servlet.
The init method definition looks like this:
public void init() throws ServletException {
  // Initialization code...
}

The service() method :

The service() method is the main method to perform the actual task. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client( browsers) and to write the formatted response back to the client.
Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.
Here is the signature of this method:
public void service(ServletRequest request, 
                    ServletResponse response) 
      throws ServletException, IOException{
}
The service () method is called by the container and service method invokes doGe, doPost, doPut, doDelete, etc. methods as appropriate. So you have nothing to do with service() method but you override either doGet() or doPost() depending on what type of request you receive from the client.
The doGet() and doPost() are most frequently used methods with in each service request. Here are the signature of these two methods.

The doGet() Method

A GET request results from a normal request for a URL or from an HTML form that has no METHOD specified and it should be handled by doGet() method.
public void doGet(HttpServletRequest request,
                  HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet code
}

The doPost() Method

A POST request results from an HTML form that specifically lists POST as the METHOD and it should be handled by doPost() method.
public void doPost(HttpServletRequest request,
                   HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet code
}

The destroy() method :

The destroy() method is called only once at the end of the life cycle of a servlet. This method gives your servlet a chance to close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities.
After the destroy() method is called, the servlet object is marked for garbage collection. The destroy method definition looks like this:
  public void destroy() {
    // Finalization code...
  }

Architecture Digram:

The following figure depicts a typical servlet life-cycle scenario.
  • First the HTTP requests coming to the server are delegated to the servlet container.
  • The servlet container loads the servlet before invoking the service() method.
  • Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet.
Servlet Life Cycle

Creating and Initializing a Servlet

Servlets are Java classes which service HTTP requests and implement the javax.servlet.Servlet interface. Web application developers typically write servlets that extend javax.servlet.http.HttpServlet, an abstract class that implements the Servlet interface and is specially designed to handle HTTP requests.

Sample Code for Hello World:

Following is the sample source code structure of a servlet example to write Hello World:
// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// Extend HttpServlet class
public class HelloWorld extends HttpServlet {
 
  private String message;

  public void init() throws ServletException
  {
      // Do required initialization
      message = "Hello World";
  }

  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set response content type
      response.setContentType("text/html");

      // Actual logic goes here.
      PrintWriter out = response.getWriter();
      out.println("<h1>" + message + "</h1>");
  }
  
  public void destroy()
  {
      // do nothing.
  }
}

Compiling a Servlet:

Let us put above code if HelloWorld.java file and put this file in C:\ServletDevel (Windows) or /usr/ServletDevel (Unix) then you would need to add these directories as well in CLASSPATH.
Assuming your environment is setup properly, go in ServletDevel directory and compile HelloWorld.java as follows:
$ javac HelloWorld.java
If the servlet depends on any other libraries, you have to include those JAR files on your CLASSPATH as well. I have included only servlet-api.jar JAR file because I'm not using any other library in Hello World program.
This command line uses the built-in javac compiler that comes with the Sun Microsystems Java Software Development Kit (JDK). For this command to work properly, you have to include the location of the Java SDK that you are using in the PATH environment variable.
If everything goes fine, above compilation would produce HelloWorld.class file in the same directory. Next section would explain how a compiled servlet would be deployed in production.

Servlet Deployment:

By default, a servlet application is located at the path <Tomcat-installation-directory>/webapps/ROOT and the class file would reside in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/classes.
If you have a fully qualified class name of com.myorg.MyServlet, then this servlet class must be located in WEB-INF/classes/com/myorg/MyServlet.class.
For now, let us copy HelloWorld.class into <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/classes and create following entries in web.xml file located in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/
    <servlet>
        <servlet-name>HelloWorld</servlet-name>
        <servlet-class>HelloWorld</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloWorld</servlet-name>
        <url-pattern>/HelloWorld</url-pattern>
    </servlet-mapping>
Above entries to be created inside <web-app>...</web-app> tags available in web.xml file. There could be various entries in this table already available, but never mind.
You are almost done, now let us start tomcat server using <Tomcat-installation-directory>\bin\startup.bat (on windows) or <Tomcat-installation-directory>/bin/startup.sh (on Linux/Solaris etc.) and finally type http://localhost:8080/HelloWorld in browser's address box. If everything goes fine, you would get following result:
Servlet Example

  • Servlet - Client Request

    When a browser requests for a web page, it sends lot of information to the web server which can not be read directly because this information travel as a part of header of HTTP request. You can check HTTP Protocol for more information on this.
    Following is the important header information which comes from browser side and you would use very frequently in web programming:
    HeaderDescription
    AcceptThis header specifies the MIME types that the browser or other clients can handle. Values of image/png or image/jpeg are the two most common possibilities.
    Accept-CharsetThis header specifies the character sets the browser can use to display the information. For example ISO-8859-1.
    Accept-EncodingThis header specifies the types of encodings that the browser knows how to handle. Values of gzip or compress are the two most common possibilities.
    Accept-LanguageThis header specifies the client's preferred languages in case the servlet can produce results in more than one language. For example en, en-us, ru, etc.
    AuthorizationThis header is used by clients to identify themselves when accessing password-protected Web pages.
    ConnectionThis header indicates whether the client can handle persistent HTTP connections. Persistent connections permit the client or other browser to retrieve multiple files with a single request. A value of Keep-Alive means that persistent connections should be used
    Content-LengthThis header is applicable only to POST requests and gives the size of the POST data in bytes.
    CookieThis header returns cookies to servers that previously sent them to the browser.
    HostThis header specifies the host and port as given in the original URL.
    If-Modified-SinceThis header indicates that the client wants the page only if it has been changed after the specified date. The server sends a code, 304 which means Not Modified header if no newer result is available.
    If-Unmodified-SinceThis header is the reverse of If-Modified-Since; it specifies that the operation should succeed only if the document is older than the specified date.
    RefererThis header indicates the URL of the referring Web page. For example, if you are at Web page 1 and click on a link to Web page 2, the URL of Web page 1 is included in the Referer header when the browser requests Web page 2.
    User-AgentThis header identifies the browser or other client making the request and can be used to return different content to different types of browsers.

    Methods to read HTTP Header:

    There are following methods which can be used to read HTTP header in your servlet program. These method are available with HttpServletRequest object.
    S.N.Method & Description
    1Cookie[] getCookies()
    Returns an array containing all of the Cookie objects the client sent with this request.
    2Enumeration getAttributeNames()
    Returns an Enumeration containing the names of the attributes available to this request.
    3Enumeration getHeaderNames()
    Returns an enumeration of all the header names this request contains.
    4Enumeration getParameterNames()
    Returns an Enumeration of String objects containing the names of the parameters contained in this request.
    5HttpSession getSession()
    Returns the current session associated with this request, or if the request does not have a session, creates one.
    6HttpSession getSession(boolean create)
    Returns the current HttpSession associated with this request or, if if there is no current session and create is true, returns a new session.
    7Locale getLocale()
    Returns the preferred Locale that the client will accept content in, based on the Accept-Language header
    8Object getAttribute(String name)
    Returns the value of the named attribute as an Object, or null if no attribute of the given name exists.
    9ServletInputStream getInputStream()
    Retrieves the body of the request as binary data using a ServletInputStream.
    10String getAuthType()
    Returns the name of the authentication scheme used to protect the servlet, for example, "BASIC" or "SSL," or null if the JSP was not protected
    11String getCharacterEncoding()
    Returns the name of the character encoding used in the body of this request.
    12String getContentType()
    Returns the MIME type of the body of the request, or null if the type is not known.
    13String getContextPath()
    Returns the portion of the request URI that indicates the context of the request.
    14String getHeader(String name)
    Returns the value of the specified request header as a String.
    15String getMethod()
    Returns the name of the HTTP method with which this request was made, for example, GET, POST, or PUT.
    16String getParameter(String name)
    Returns the value of a request parameter as a String, or null if the parameter does not exist.
    17String getPathInfo()
    Returns any extra path information associated with the URL the client sent when it made this request.
    18String getProtocol()
    Returns the name and version of the protocol the request.
    19String getQueryString()
    Returns the query string that is contained in the request URL after the path.
    20String getRemoteAddr()
    Returns the Internet Protocol (IP) address of the client that sent the request.
    21String getRemoteHost()
    Returns the fully qualified name of the client that sent the request.
    22String getRemoteUser()
    Returns the login of the user making this request, if the user has been authenticated, or null if the user has not been authenticated.
    23String getRequestURI()
    Returns the part of this request's URL from the protocol name up to the query string in the first line of the HTTP request.
    24String getRequestedSessionId()
    Returns the session ID specified by the client.
    25String getServletPath()
    Returns the part of this request's URL that calls the JSP.
    26String[] getParameterValues(String name)
    Returns an array of String objects containing all of the values the given request parameter has, or null if the parameter does not exist.
    27boolean isSecure()
    Returns a boolean indicating whether this request was made using a secure channel, such as HTTPS.
    28int getContentLength()
    Returns the length, in bytes, of the request body and made available by the input stream, or -1 if the length is not known.
    29int getIntHeader(String name)
    Returns the value of the specified request header as an int.
    30int getServerPort()
    Returns the port number on which this request was received.

    HTTP Header Request Example:

    Following is the example which uses getHeaderNames() method of HttpServletRequest to read the HTTP header infromation. This method returns an Enumeration that contains the header information associated with the current HTTP request.
    Once we have an Enumeration, we can loop down the Enumeration in the standard manner, using hasMoreElements() method to determine when to stop and using nextElement() method to get each parameter name.
    // Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
 
// Extend HttpServlet class
public class DisplayHeader extends HttpServlet {
 
  // Method to handle GET method request.
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set response content type
      response.setContentType("text/html");
 
      PrintWriter out = response.getWriter();
   String title = "HTTP Header Request Example";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +
      "transitional//en\">\n";
      out.println(docType +
        "<html>\n" +
        "<head><title>" + title + "</title></head>\n"+
        "<body bgcolor=\"#f0f0f0\">\n" +
        "<h1 align=\"center\">" + title + "</h1>\n" +
        "<table width=\"100%\" border=\"1\" align=\"center\">\n" +
        "<tr bgcolor=\"#949494\">\n" +
        "<th>Header Name</th><th>Header Value(s)</th>\n"+
        "</tr>\n");
 
      Enumeration headerNames = request.getHeaderNames();
      
      while(headerNames.hasMoreElements()) {
         String paramName = (String)headerNames.nextElement();
         out.print("<tr><td>" + paramName + "</td>\n");
         String paramValue = request.getHeader(paramName);
         out.println("<td> " + paramValue + "</td></tr>\n");
      }
      out.println("</table>\n</body></html>");
  }
  // Method to handle POST method request.
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
     doGet(request, response);
  }
}
    Now calling the above servlet would generate following result:

    HTTP Header Request Example

    Header NameHeader Value(s)
    accept */*
    accept-language en-us
    user-agent Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; InfoPath.2; MS-RTC LM 8)
    accept-encoding gzip, deflate
    host localhost:8080
    connection Keep-Alive
    cache-control no-cache

  • Servlet - Server Response

    As discussed in previous chapter, when a Web server responds to a HTTP request to the browser, the response typically consists of a status line, some response headers, a blank line, and the document. A typical response looks like this:
    HTTP/1.1 200 OK
Content-Type: text/html
Header2: ...
...
HeaderN: ...
  (Blank Line)
<!doctype ...>
<html>
<head>...</head>
<body>
...
</body>
</html>
    The status line consists of the HTTP version (HTTP/1.1 in the example), a status code (200 in the example), and a very short message corresponding to the status code (OK in the example).
    Following is a summary of the most useful HTTP 1.1 response headers which go back to the browser from web server side and you would use them very frequently in web programming:
    HeaderDescription
    AllowThis header specifies the request methods (GET, POST, etc.) that the server supports.
    Cache-ControlThis header specifies the circumstances in which the response document can safely be cached. It can have values public, private or no-cache etc. Public means document is cacheable, Private means document is for a single user and can only be stored in private (nonshared) caches and no-cache means document should never be cached.
    ConnectionThis header instructs the browser whether to use persistent in HTTP connections or not. A value of close instructs the browser not to use persistent HTTP connections and keep-alive means using persistent connections.
    Content-DispositionThis header lets you request that the browser ask the user to save the response to disk in a file of the given name.
    Content-EncodingThis header specifies the way in which the page was encoded during transmission.
    Content-LanguageThis header signifies the language in which the document is written. For example en, en-us, ru, etc.
    Content-LengthThis header indicates the number of bytes in the response. This information is needed only if the browser is using a persistent (keep-alive) HTTP connection.
    Content-TypeThis header gives the MIME (Multipurpose Internet Mail Extension) type of the response document.
    ExpiresThis header specifies the time at which the content should be considered out-of-date and thus no longer be cached.
    Last-ModifiedThis header indicates when the document was last changed. The client can then cache the document and supply a date by an If-Modified-Since request header in later requests.
    LocationThis header should be included with all responses that have a status code in the 300s. This notifies the browser of the document address. The browser automatically reconnects to this location and retrieves the new document.
    RefreshThis header specifies how soon the browser should ask for an updated page. You can specify time in number of seconds after which a page would be refreshed.
    Retry-AfterThis header can be used in conjunction with a 503 (Service Unavailable) response to tell the client how soon it can repeat its request.
    Set-CookieThis header specifies a cookie associated with the page.

    Methods to Set HTTP Response Header:

    There are following methods which can be used to set HTTP response header in your servlet program. These method are available with HttpServletResponse object.
    S.N.Method & Description
    1String encodeRedirectURL(String url)
    Encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged.
    2String encodeURL(String url)
    Encodes the specified URL by including the session ID in it, or, if encoding is not needed, returns the URL unchanged.
    3boolean containsHeader(String name)
    Returns a boolean indicating whether the named response header has already been set.
    4boolean isCommitted()
    Returns a boolean indicating if the response has been committed.
    5void addCookie(Cookie cookie)
    Adds the specified cookie to the response.
    6void addDateHeader(String name, long date)
    Adds a response header with the given name and date-value.
    7void addHeader(String name, String value)
    Adds a response header with the given name and value.
    8void addIntHeader(String name, int value)
    Adds a response header with the given name and integer value.
    9void flushBuffer()
    Forces any content in the buffer to be written to the client.
    10void reset()
    Clears any data that exists in the buffer as well as the status code and headers.
    11void resetBuffer()
    Clears the content of the underlying buffer in the response without clearing headers or status code.
    12void sendError(int sc)
    Sends an error response to the client using the specified status code and clearing the buffer.
    13void sendError(int sc, String msg)
    Sends an error response to the client using the specified status.
    14void sendRedirect(String location)
    Sends a temporary redirect response to the client using the specified redirect location URL.
    15void setBufferSize(int size)
    Sets the preferred buffer size for the body of the response.
    16void setCharacterEncoding(String charset)
    Sets the character encoding (MIME charset) of the response being sent to the client, for example, to UTF-8.
    17void setContentLength(int len)
    Sets the length of the content body in the response In HTTP servlets, this method sets the HTTP Content-Length header.
    18void setContentType(String type)
    Sets the content type of the response being sent to the client, if the response has not been committed yet.
    19void setDateHeader(String name, long date)
    Sets a response header with the given name and date-value.
    20void setHeader(String name, String value)
    Sets a response header with the given name and value.
    21void setIntHeader(String name, int value)
    Sets a response header with the given name and integer value.
    22void setLocale(Locale loc)
    Sets the locale of the response, if the response has not been committed yet.
    23void setStatus(int sc)
    Sets the status code for this response.

    HTTP Header Response Example:

    You already have seen setContentType() method working in previous examples and following example would also use same method, additionally we would use setIntHeader() method to set Refresh header.
    // Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
 
// Extend HttpServlet class
public class Refresh extends HttpServlet {
 
  // Method to handle GET method request.
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set refresh, autoload time as 5 seconds
      response.setIntHeader("Refresh", 5);
 
      // Set response content type
      response.setContentType("text/html");
 
      // Get current time
      Calendar calendar = new GregorianCalendar();
      String am_pm;
      int hour = calendar.get(Calendar.HOUR);
      int minute = calendar.get(Calendar.MINUTE);
      int second = calendar.get(Calendar.SECOND);
      if(calendar.get(Calendar.AM_PM) == 0)
        am_pm = "AM";
      else
        am_pm = "PM";
 
      String CT = hour+":"+ minute +":"+ second +" "+ am_pm;
    
      PrintWriter out = response.getWriter();
      String title = "Auto Refresh Header Setting";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +
      "transitional//en\">\n";
      out.println(docType +
        "<html>\n" +
        "<head><title>" + title + "</title></head>\n"+
        "<body bgcolor=\"#f0f0f0\">\n" +
        "<h1 align=\"center\">" + title + "</h1>\n" +
        "<p>Current Time is: " + CT + "</p>\n");
  }
  // Method to handle POST method request.
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
     doGet(request, response);
  }
}
    Now calling the above servlet would display current system time after every 5 seconds as follows. Just run the servlet and wait to see the result:

    Auto Refresh Header Setting

    Current Time is: 9:44:50 PM

  • Servlets - Cookies Handling

    Cookies are text files stored on the client computer and they are kept for various information tracking purpose. Java Servlets transparently supports HTTP cookies.
    There are three steps involved in identifying returning users:
    • Server script sends a set of cookies to the browser. For example name, age, or identification number etc.
    • Browser stores this information on local machine for future use.
    • When next time browser sends any request to web server then it sends those cookies information to the server and server uses that information to identify the user.
    This chapter will teach you how to set or reset cookies, how to access them and how to delete them.

    The Anatomy of a Cookie:

    Cookies are usually set in an HTTP header (although JavaScript can also set a cookie directly on a browser). A servlet that sets a cookie might send headers that look something like this:
    HTTP/1.1 200 OK
Date: Fri, 04 Feb 2000 21:03:38 GMT
Server: Apache/1.3.9 (UNIX) PHP/4.0b3
Set-Cookie: name=xyz; expires=Friday, 04-Feb-07 22:03:38 GMT; 
                 path=/; domain=tutorialspoint.com
Connection: close
Content-Type: text/html
    As you can see, the Set-Cookie header contains a name value pair, a GMT date, a path and a domain. The name and value will be URL encoded. The expires field is an instruction to the browser to "forget" the cookie after the given time and date.
    If the browser is configured to store cookies, it will then keep this information until the expiry date. If the user points the browser at any page that matches the path and domain of the cookie, it will resend the cookie to the server. The browser's headers might look something like this:
    GET / HTTP/1.0
Connection: Keep-Alive
User-Agent: Mozilla/4.6 (X11; I; Linux 2.2.6-15apmac ppc)
Host: zink.demon.co.uk:1126
Accept: image/gif, */*
Accept-Encoding: gzip
Accept-Language: en
Accept-Charset: iso-8859-1,*,utf-8
Cookie: name=xyz
    A servlet will then have access to the cookie through the request method request.getCookies() which returns an array of Cookie objects.

    Servlet Cookies Methods:

    Following is the list of useful methods which you can use while manipulating cookies in servlet.
    S.N.Method & Description
    1public void setDomain(String pattern)
    This method sets the domain to which cookie applies, for example tutorialspoint.com.
    2public String getDomain()
    This method gets the domain to which cookie applies, for example tutorialspoint.com.
    3public void setMaxAge(int expiry)
    This method sets how much time (in seconds) should elapse before the cookie expires. If you don't set this, the cookie will last only for the current session.
    4public int getMaxAge()
    This method returns the maximum age of the cookie, specified in seconds, By default, -1 indicating the cookie will persist until browser shutdown.
    5public String getName()
    This method returns the name of the cookie. The name cannot be changed after creation.
    6public void setValue(String newValue)
    This method sets the value associated with the cookie.
    7public String getValue()
    This method gets the value associated with the cookie.
    8public void setPath(String uri)
    This method sets the path to which this cookie applies. If you don't specify a path, the cookie is returned for all URLs in the same directory as the current page as well as all subdirectories.
    9public String getPath()
    This method gets the path to which this cookie applies.
    10public void setSecure(boolean flag)
    This method sets the boolean value indicating whether the cookie should only be sent over encrypted (i.e. SSL) connections.
    11public void setComment(String purpose)
    This method specifies a comment that describes a cookie's purpose. The comment is useful if the browser presents the cookie to the user.
    12public String getComment()
    This method returns the comment describing the purpose of this cookie, or null if the cookie has no comment.

    Setting Cookies with Servlet:

    Setting cookies with servlet involves three steps:
    (1) Creating a Cookie object: You call the Cookie constructor with a cookie name and a cookie value, both of which are strings.
    Cookie cookie = new Cookie("key","value");
    Keep in mind, neither the name nor the value should contain white space or any of the following characters:
    [ ] ( ) = , " / ? @ : ;
    (2) Setting the maximum age: You use setMaxAge to specify how long (in seconds) the cookie should be valid. Following would set up a cookie for 24 hours.
    cookie.setMaxAge(60*60*24);
    (3) Sending the Cookie into the HTTP response headers: You use response.addCookie to add cookies in the HTTP response header as follows:
    response.addCookie(cookie);

    Example:

    Let us modify our Form Example to set the cookies for first and last name.
    // Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
// Extend HttpServlet class
public class HelloForm extends HttpServlet {
 
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Create cookies for first and last names.      
      Cookie firstName = new Cookie("first_name",
                      request.getParameter("first_name"));
      Cookie lastName = new Cookie("last_name",
                      request.getParameter("last_name"));

      // Set expiry date after 24 Hrs for both the cookies.
      firstName.setMaxAge(60*60*24); 
      lastName.setMaxAge(60*60*24); 

      // Add both the cookies in the response header.
      response.addCookie( firstName );
      response.addCookie( lastName );

      // Set response content type
      response.setContentType("text/html");
 
      PrintWriter out = response.getWriter();
      String title = "Setting Cookies Example";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +
      "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" +
                "<h1 align=\"center\">" + title + "</h1>\n" +
                "<ul>\n" +
                "  <li><b>First Name</b>: "
                + request.getParameter("first_name") + "\n" +
                "  <li><b>Last Name</b>: "
                + request.getParameter("last_name") + "\n" +
                "</ul>\n" +
                "</body></html>");
  }
}
    Compile above servlet HelloForm and create appropriate entry in web.xml file and finally try following HTML page to call servlet.
     
<html>
<body>
<form action="HelloForm" method="GET">
First Name: <input type="text" name="first_name">
<br />
Last Name: <input type="text" name="last_name" />
<input type="submit" value="Submit" />
</form>
</body>
</html>
    Keep above HTML content in a file Hello.htm and put it in <Tomcat-installation-directory>/webapps/ROOT directory. When you would access http://localhost:8080/Hello.htm, here is the actual output of the above form.
    First Name:
    Last Name:
    Try to enter First Name and Last Name and then click submit button. This would display first name and last name on your screen and same time it would set two cookies firstName and lastName which would be passed back to the server when next time you would press Submit button.
    Next section would explain you how you would access these cookies back in your web application.

    Reading Cookies with Servlet:

    To read cookies, you need to create an array of javax.servlet.http.Cookie objects by calling the getCookies( ) method of HttpServletRequest. Then cycle through the array, and use getName() and getValue() methods to access each cookie and associated value.

    Example:

    Let us read cookies which we have set in previous example:
    // Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
// Extend HttpServlet class
public class ReadCookies extends HttpServlet {
 
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      Cookie cookie = null;
   Cookie[] cookies = null;
      // Get an array of Cookies associated with this domain
      cookies = request.getCookies();
      
   // Set response content type
      response.setContentType("text/html");
 
      PrintWriter out = response.getWriter();
      String title = "Reading Cookies Example";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +
      "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" );
      if( cookies != null ){
         out.println("<h2> Found Cookies Name and Value</h2>");
         for (int i = 0; i < cookies.length; i++){
            cookie = cookies[i];
            out.print("Name : " + cookie.getName( ) + ",  ");
            out.print("Value: " + cookie.getValue( )+" <br/>");
         }
      }else{
          out.println(
            "<h2>No cookies founds</h2>");
      }
      out.println("</body>");
      out.println("</html>");
   }
}
    Compile above servlet ReadCookies and create appropriate entry in web.xml file. If you would have set first_name cookie as "John" and last_name cookie as "Player" then running http://localhost:8080/ReadCookies would display the following result:

    Found Cookies Name and Value

    Name : first_name, Value: John
    Name : last_name, Value: Player

    Delete Cookies with Servlet:

    To delete cookies is very simple. If you want to delete a cookie then you simply need to follow up following three steps:
    1. Read an already exsiting cookie and store it in Cookie object.
    2. Set cookie age as zero using setMaxAge() method to delete an existing cookie.
    3. Add this cookie back into response header.

    Example:

    Following example would delete and existing cookie named "first_name" and when you would run ReadCookies servlet next time it would return null value for first_name.
    // Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
// Extend HttpServlet class
public class DeleteCookies extends HttpServlet {
 
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      Cookie cookie = null;
   Cookie[] cookies = null;
      // Get an array of Cookies associated with this domain
      cookies = request.getCookies();
      
   // Set response content type
      response.setContentType("text/html");
 
      PrintWriter out = response.getWriter();
      String title = "Delete Cookies Example";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +
      "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" );
       if( cookies != null ){
         out.println("<h2> Cookies Name and Value</h2>");
         for (int i = 0; i < cookies.length; i++){
            cookie = cookies[i];
            if((cookie.getName( )).compareTo("first_name") == 0 ){
                 cookie.setMaxAge(0);
                 response.addCookie(cookie);
                 out.print("Deleted cookie : " + 
                              cookie.getName( ) + "<br/>");
            }
            out.print("Name : " + cookie.getName( ) + ",  ");
            out.print("Value: " + cookie.getValue( )+" <br/>");
         }
      }else{
          out.println(
            "<h2>No cookies founds</h2>");
      }
      out.println("</body>");
      out.println("</html>");
   }
}
    Compile above servlet DeleteCookies and create appropriate entry in web.xml file. Now running http://localhost:8080/DeleteCookies would display the following result:

    Cookies Name and Value

    Deleted cookie : first_name
    Name : first_name, Value: John
    Name : last_name, Value: Player
    Now try to run http://localhost:8080/ReadCookies and it would display only one cookie as follows:

    Found Cookies Name and Value

    Name : last_name, Value: Player
    You can delete your cookies in Internet Explorer manually. Start at the Tools menu and select Internet Options. To delete all cookies, press Delete Cookies.

  • Finalizing a Servlet

    Finalizing a Servlet


    When a servlet container determines that a servlet should be removed from service (for example, when a container wants to reclaim memory resources, or when it is being shut down), it calls thedestroy method of the Servlet interface. In this method, you release any resources the servlet is using and save any persistent state. The following destroy method releases the database object created in the init method described in Initializing a Servlet:
    public void destroy() {
   bookDB = null;
}
    All of a servlet's service methods should be complete when a servlet is removed. The server tries to ensure this completion by calling the destroy method only after all service requests have returned or after a server-specific grace period, whichever comes first.
    If your servlet has potentially long-running service requests, use the techniques described below to do the following:
    • Keep track of how many threads are currently running the service method.
    • Provide a clean shutdown by having the destroy method notify long-running threads of the shutdown and wait for them to complete.

  • Servlet- Page redirection

    Page redirection is generally used when a document moves to a new location and we need to send the client to this new location or may be because of load balancing, or for simple randomization.
    The simplest way of redirecting a request to another page is using method sendRedirect() of response object. Following is the signature of this method:
    public void HttpServletResponse.sendRedirect(String location)
throws IOException
    This method sends back the response to the browser along with the status code and new page location. You can also use setStatus() and setHeader() methods together to achieve the same:
    ....
String site = "http://www.newpage.com" ;
response.setStatus(response.SC_MOVED_TEMPORARILY);
response.setHeader("Location", site); 
....

    Example:

    This example shows how a servlet performs page redirection to an another location:
    import java.io.*;
import java.sql.Date;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class PageRedirect extends HttpServlet{
    
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set response content type
      response.setContentType("text/html");

      // New location to be redirected
      String site = new String("http://www.photofuntoos.com");

      response.setStatus(response.SC_MOVED_TEMPORARILY);
      response.setHeader("Location", site);    
    }
}
    Now let us compile above servlet and create following entries in web.xml
    ....
 <servlet>
     <servlet-name>PageRedirect</servlet-name>
     <servlet-class>PageRedirect</servlet-class>
 </servlet>

 <servlet-mapping>
     <servlet-name>PageRedirect</servlet-name>
     <url-pattern>/PageRedirect</url-pattern>
 </servlet-mapping>
....
    Now call this servlet using URL http://localhost:8080/PageRedirect. This would take you given URL http://www.photofuntoos.com.

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