1 Lecture 9 George Koutsogiannakis/ Summer 2011 CS441 CURRENT TOPICS IN PROGRAMMING LANGUAGES.

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Lecture 9

George Koutsogiannakis/ Summer 2011

CS441

CURRENT TOPICS IN PROGRAMMING LANGUAGES

Topics

• Continue with Servlets– Content Types– HTTP Tunneling.– URL connections.– Applet to Servlet communication.– Application to Servlet communication.– Session object.– Servlet to other server side resources

communication. 2

Servlets - Definition

• A servlet is a Java programming language class that is used to extend the capabilities of servers that host applications accessed by means of

a request-response programming model. • Although servlets can respond to any type of request, they are

commonly used to extend the applications hosted by web servers. For such applications, Java Servlet technology defines HTTP-specific

servlet classes.

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Servlets - Definition

• The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets.

• All servlets must implement the Servlet interface, which defines life-cycle methods.

• When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API.

• The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

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Servlet Engine and Servlets

• Communication from client is received by the web server in the form of HTTP packets.

• The web server reads the packets and passes information to the servlet engine (container).

• The container does the following:

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Servlet Engine (container) and Servlets

• Web Servers get the capability to communicate with servlet programs installed in their environment by having the servlet engine or container as part of their environment. The container:

– Activates the init and getServletContext methods if this is the first time that the servlet is being called after the web server was activated.

– Calls the service method.

– The service method creates the request and response objects.

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Servlet Engine (container) and Servlets

– The service method establishes the streams of communication between the container and the servlet.

– The service method calls either the doPost or the doGet method implementations in the servlet based on the type of request that was recognized coming from the client.

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Servlet Context• The servlet is responsible for setting the type of context

that would be include in the HTTP packets that will be sent back to the client. – The kind of content determines the type of streams needed on the

client side in order to receive the data sent as part of the response.

– If the client does not know the type of the context, it can be discovered from the headers of the incoming HTTP packets (received by the client)

• The various content types are defined in MIME – Multipart Internet Mail Protocol

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Servlet Context

• There are many different MIME content types i.e.– text/html : refers to HTML text data– text/plain : refers to plain text documents

– image/gif : refers to the fact that a gif file will be produced and sent– application/octet-stream : refers to the fact that the data is in the

form of an executable or binary data.– application/x-java : java program– application/x-csh : UNIX C-shell program

– Etc.• The complete MIME types can be found on the Internet by Googling

MIME

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Getting Info from Request Objects

• The Request object contains information extracted from the HTTP packets received from the client by the web server (web container).

• Such information includes:– Parameters that are passed from the client to the servlet.

– Objects and their attributes.

– Information about the protocol used to communicate between the client and the server.

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Getting Info from Request Objects

– Information about the client.

– Information about the server.

– The type of request (GET or POST).

– The type of content (MIME)

– Other

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Information placed on Response Objects

• Content type returned by the response to the client (i.e. content generated by a servlet , sent to the web container, web container extracts the data from the response object and inserts it in the HTTP packets to be sent to the client)

• The data sent by the server to the client.

• Status codes that indicate why a request could not be honored.

• Session id.

• Cookies (files to be sent to the Browser and reside in local client environment).

• Information about the client and the server.

• Other.

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Filters

• Programs that are used to:– Authenticate a request

– Modify a request or a response

– Block the request and response form going through.

• A web resource can be filtered by one or more filter programs.

• Filters are used by using the Filter interface and overriding its doFilter method (javax.servlet package).

• The web container decides how to use filter programs based on filter mapping tags entered in the deployment descriptor (web.xml).

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HTTP Tunneling

• Refers to sending data back in a different protocol than HTTP nevertheless enveloped by HTTP.

i.e. the server is sending data back to an applet client program in a binary form or bytes.

• Or the server contacting another server, such as a RMI server, to send data using the Remote Method Invocation Protocol as the transport protocol (or the RMI over IIOP transport protocol).

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HTTP Tunneling-Applets to Servlets communication-GET

• Example of tunneling is an applet communicating with a servlet via either GET or POST

• i.e Applet generates data for Web Component using GET / Reading

Binary data by an applet:– Create a url object that identifies the host url.

URL url= new URL(“http://localhost:8080/Bank/ShowBalance?id=custid”);

Notice that it is assumed that the HTTP method is GET since the data is part of the URL.

Notice that the parameter is id but the value of it is held by variable custid.

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HTTP Tunneling-Applets to Servlets communication-GET

Parameters are separated from the address part of URL by ?

Additional parameters are separated by &URL url= new URL(“http://localhost:8080/Bank/ShowBalance?

id=custid&firstn=fn”);

The parameters are id and fn and their values are held by variables custid and fn

(notice the name/value pair for each parameter)

– Create a URLConnection object.• Calling OpenConnection method returns a URLConnection object that can be

used to obtain streams .

URLConnection uc=url.openConnection();

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HTTP Tunneling-GET

– Instruct the Browser not to cache the data:

uc.setCaches(false);

– Set other information in HTTP headers

uc.setRequestProperty(“header_name”, “value”);

– Use one of the input streams to read the response back from the servlet.

• If the data from the servlet , for instance, was text/html (IN OTHER WORDS THE CONTEXT WAS SET FOR text/html BY THE SERVLET)

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HTTP Tunneling-GET

BufferedReader in = new BufferedReader ( new InputStreamReader (uc.getInputStream()));

String line;

while ( (line=in.readLine()) ! = null )

{

read the data and do something

}

in.close();

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HTTP Tunneling

• Other types of context require different type of streams (for instance object streams if the content is set for serializable objects)

• Note: HTTP packets have a Header part and a Body part. The header contains standard named attributes whose values we can set.

• The String “header_name” in method setRequestProperty (see previous slide) represents the name of the HTTP header attribute whose value we are setting .

The http Header has pre defined fields whose value scan be set programmatically by the applet (or an java application program acting as clients. Or by a Browser).

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HTTP Tunneling-Serializable Data Received by the Client

– What if the data is binary in the form of object’ s data? In other words serializable objects have been created by the servlet and sent over the network to the applet client.

– Serializable interface creates serializable objects using Java’ s serialization protocol. Serialized objects can be sent to another resource in binary form and deserialized at the destination.

– To receive (by the Applet client as an example) the serialized data sent by the servlet the code is formed as follows by the client:

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– Assume that we have a separate class on the server side, that has a method which gets the data from the data base puts it into a vector and that this class implements the interface Serializable.. This class returns a vector object which is serializable. The serilizable object is sent by the servlet to the client.

– Create an ObjectInputStream ObjectInputStream in = new ObjectInputStream

( uc.getInputStream()));

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– Now read the object. As an example: The servlet got data from a database and place it into a vector. The servlet then serialized the vector and sent it as part of the response. The code:

Vector myvector = (Vector)in. readObject();

deserializes the Object received into a Vector data type.

The object myvector now has the data sent by the servlet.

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HTTP Tunneling-Serializable Data Sent by the Servlet

• What does the servlet have to do to send the serialized data:– Specify that the content is of binary format data.

res.setContentType(“application/x-java-serialized-object”);

– Create an object output stream

ObjectOutputStream out = new ObjectOutputStream (res.getOutputStream() );

– Write the object into the output stream:

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HTTP Tunneling-Serializable Data

MyVectorClass mvc= new MyVectorClass();

Vector myvector = mvc.getVector();

out.write(myvector);

out.flush();

Where MyVectorClass is the class, on the server side, that serializes the vector that contains the data.

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Applet sends binary data to Servlet as POST

• Create URL object and URLConnection as in slides 5 and 6. The data , however, will not be part of the url this time.

• Tell system to permit applet to sent data (this is NOT in lieu of the required certificate).

uc.setDoOutput(true);

• Create a ByteArrayOutputStream object

ByteArrayOutputStream bytestream= new ByteArrayOutputStream (512);

512 bytes of data to be sent.

PrintWriter out =new PrintWriter(bytestream, true);

• Place data into a buffer:

Use either print if data is Strings or writeObject

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• Suppose it is a String that represents the data:

Need to url encode any parameters:

String val1= URLEncode(someval1);

String val2= URLEncode (someval2);

String data= “param1=“+val1+”param2=“+val2;

• Write the data into the stream:

out.print(data);

out.flush();

out.close(); // the data is written into the bytestream object

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• Set the Content Length (required for POST)

uc.setRequestProperty(“Content-length”, String.valueOf(bytestream.size());

• Set the content type:

uc.setProperty(“Conetent-type”, “application/x-www-form-urlencoded”);

• Send the data out:

bytestream.writeTo(uc. getOutputStream());

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Applications to Servlet

• If the client is an application program instead of an applet the same procedure is followed to have the application communicate either via GET or via POST with the servlet.

• The only difference is that no Browser is needed to execute the applet and the code for the application did not come from another server but it resided in the client’ s local system.

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Sessions

• Collaborating web resources in a server share information via objects. These objects are maintained as attributes of 4 scope objects.

• The attributes are accessible via methods like getAttribute or setAttribute.

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Scope Object Class Accessible From

Web context javax.servlet.ServletContext Web components within a web context

Session javax.servlet.http.HttpSession Web components handling a request that belongs to a session

Request javax.servlet.SerletRequest Web components that handle the request

Page javax.servlet.jsp.JspResponse The JSP page that creates the object

Session Management

• Sessions have time out periods if no request is received from a client that has a session established

• The time out period can be set in the web.xml file that is in the conf directory of the server (not the web.xml for your web application)

<session-config>

<session-timeout>30</session-timeout>

</session-config>

Timeout is set to 30 minutes in the above example.

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Session Management

• Keeping track of a session via a session object is called server side session tracking.

• This is different than using cookies which are files that are stored on the client side by the server.

• The web container associates a session with a client. The web container is responsible for generating a session id.

– A session identifier is created for each client.

– The identifier is sent to the client as a cookie and it is included or as an alternative the web component (servlet) can include the session id in every response to the client.

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Session Management

• Create a session object in your web component:

HttpSession session = request.getSession(true);

Either a new object is created or an existing one is returned to the servlet by the web container each time a request is made.

• You can store attribute_name/value pairs in the session object.• There are pieces of information that you can extract using the session object:

– session.getID(); returns the session id for the client as a String.

– session.isNew(); returns true if the client has never seen a session.

– session,getCreationTime(); returns the time in milliseconds since the session was created.

– session.getLastAccessedTime(); returns the time in milliseconds since the last request generated a session.

– session.getMaxInactiveInterval(); returns the time in milliseconds that the session can go without access before it gets invalidated.

– session.getAttributeName(String name); returns the object bound by the name in the session.

– session.setAttribute(String name, Object obj); bounds an object obj by the String name. In the session 32

Session Management

• Example of session management:

HttpSession session= request.getSession(true);

session.setAttribute("userName",request.getParameter("myname"));

-----We store in the session object, as an object, the name retrieved from parameter myname (assuming that the user has entered a name captured by myname).

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Session Management

-------Later on we check for the session:

session = request.getSession(true);

if (session.getAttribute("userName") == null)

{ session.setAttribute("userName", "Stranger"); }

else

PrintWriter out = response.getWriter();

out.println("Welcome " + (String) session.getAttribute("userName") + "!");

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Session Management

• Another example:• Suppose class ShoppingCart stores selected items in a

shopping cart object.• HttpSession session = request.getSession(true);

ShoppingCart cart = (ShoppingCart)session.getAttribute(session.getId()); // If the user has no cart, create a new one

if (cart == null) { cart = new ShoppingCart(); session.setAttribute(session.getId(), cart); }

The card object is associated with the particular session id associated with the client.

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Session Management

• Because an object can be associated with a session, the Duke's Bookstore example keeps track of the books that a user has ordered within an object. The object is type ShoppingCart and each book that a user orders is stored in the shopping cart as a ShoppingCartItem object.

• Retrieving the shopping cart from the session in order to add new item in it:

HttpSession session = request.getSession(true); ShoppingCart cart = (ShoppingCart)session.getAttribute(session.getId());

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Servlet calls other resources

• RequestDispatcher Object allows another resource to be called:

Example of calling an html file from a servlet:

getServletContext().getRequestedDispatcher(“examples/applications/bookstore/bookstore.html”);

Returns the html page .

• The resources called can be– Another servlet

– A Java Server Page

– Html

– Other37


• Two types of requests for resources:– Forward request

– Include request.

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1. ……………….2 …………………..3 …………………..4 …………………..5…………………..6…………………

….

Servlet 1

Request object from container

1. ……………….2 …………………..3 …………………..4 …………………..5…………………..6…………………

….

Servlet 2

Request/Response objects sent to servllet 2

Response object to container

Forward Request


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1. ……………….2 …………………..3 …………………..4 …………………..5…………………..6…………………

….

Servlet 1

Request object from container

1. ……………….2 …………………..3 …………………..4 …………………..5…………………..6…………………

….

Servlet 2

Request/Response objects sent to servllet 2.Servlet 2 return s objects to calling servlet.

Response object to container

Include Request


• Forwarding using Dispatcher object: Servlet1:

1. HttpSession session=request.getSession(true);

………..do something with request and response…………. 2. RequestDispatcher dispatcher=

getServletContext().getRequestDispatcher(“/AnotherServlet”);

3. dispatcher.forward(request, response);

Servlet 2:

After execution of line 3 servler AnotherServlet starts execution of its doGet or doPost method. It receives the request and response obejcts with whatever information has been encapsulated, does its tasks and proceeds to write into the proper streams using the response object.

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Servlet calls other resources• Including using the Dispatcher object:

Servlet1:

1. HttpSession session=request.getSession(true);

………..do something with request and response…………. 2. Dispatcher dispatcher=

getSevletContext().getRequestDispatcher(“/AnotherServlet”);

3. dispatcher.include(request, response);

4. Other code

Servlet 2:Servlet AnotherServlet receives the request and response objects after execution of line 3. Adds information to the response object and finishes its tasks. Servlet 1 captures the request and response objects returned from servlet 2at the eend of line 3 execution.

It proceeds with line 4.41


• Another approach besides the usage of the Dispatcher object is to use the redirect command:

response.sendRedirect(String name_of_resource);

• Sends a temporary redirect response to a client using the resource specified. The String passed in the argument can be a URL.– A slash / in the URL means that the resource is relative to the

servlet folder root (WEB_INF/classes).

– The response will be sent to the client by the redirected resource.

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• A chain of servlets can be formed:

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Servlet 1 Servlet 2 Servlet 3 Servlet 4 request

Request/response

Request/response

Request/response response


• Using Scope Objects• Collaborating web components share information by means of objects

that are maintained as attributes of four scope objects. You access these attributes using the [get|set]Attribute methods of the class representing the scope.

• Scope Objects

• Session

• Request

• Page44

POST type of communication between an applet (or an application) and a

servlet• Remember that:

– If POST is to be used then the client (an applet for example) has to place the data in the body of the HTTP packet.

– That means that the url approach where the parameters and their values are part of the url CAN NOT be used.

– It also means that the servlet must have implemented the doPost method.

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Session Tracking with Cookies

• Session tracking can be established with cookies.– Cookies are files sent by the web server to the client

and stored in the client’s system.

– The server reads the files every time contact is made and extracts information about the client.

– The Servlets Api can generate cookies via

Cookie mycookie=new Cookie(cookiename, value);

response.addCookie( mycookie );

Request.getCookies() receives an array of cookie sfrom client. 46

Information on Servlets

• Further Information about Java Servlet Technology

• For more information on Java Servlet technology, see:

• Java Servlet 3.0 specification:

http://jcp.org/en/jsr/detail?id=315

• The Java Servlet web site:

http://java.sun.com/products/servlet

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Study Guide

• WBAD TEXT: Chapter 8.

• Java EE6 Tutorial- Part II Chapter 10.

• Read examples from course’ s web site and implement them on Tomcat.

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1 Lecture 9 George Koutsogiannakis/ Summer 2011 CS441 CURRENT TOPICS IN PROGRAMMING LANGUAGES.

Documents

servlet communication

servlet context

servlet interface

servlet programs

servlet engine container

java servlet technology

java servlet api

httpspecific servlet