A java servers

 To create a simple HTTP server in Java
 To use the implementation to illustrate a
number of advanced Java features:
› TCP/IP Sockets and Server Sockets
› Interfaces
› Software components (more from John later)
› Multithreading
 To show how to create executable
server objects (using Sun’s Servlets API)

 Java Network Programming, Elliotte
Rusty Harold, O’Reilly and Associates,
1997, ISBN 1-56592-227-1
 TCP/IP Network Administration, Second
Edition, Craig Hunt, O’Reilly and
Associates, 1997, ISBN 1-56592-322-7
 The Java Developer’s connection:
http://www.javasoft.com/jdc
 The Javadoc documentation

 Server must be able to process HTTP/1.0
file transfer requests and deliver files
 Connections are to be made via TCP/IP
 Must be efficient and prompt
 Must be simple to understand and
elegant in design

 Developed by Tim Berners-Lee at CERN
 Like most Internet protocols it is
described in an RFC (Request for
Comment document): RFC1945
 May be downloaded from the Internet
Engineering Task Force’s web site:
http://www.ietf.org

 Some of you may have covered this in
the introductory Java course
 Servers have a listener loop
› Loop until the server is shutdown
 Wait for a client to request a connection
 Read the details of the client’s request
 Provide the requested information to the client
 Here’s the listener loop from our
example:

ServerSocket socket = new ServerSocket(80, 5);
public void listen()
throws IllegalAccessException,
InstantiationException,
IOException
{
for (;;) {
System.err.println("HttpServer: waiting...");
Socket s = socket.accept();
FileServer f = createFileServer();
f.dispatch(s);
}
}

2037 80
2037 1583
2037 1583
Client (sid) Server (fred)
ServerSocket ss.
s = ss.accept()
s = new Socket
(“fred”, 80)
Socket s
s.getInputStream()
s.getOuputStream()
s.getInputStream()
s.getOuputStream()

 Good software is designed in a modular
fashion avoiding stovepipe designs!
 This is a form of software components
 Java has strong support for components
 Components hide their implementation
behind interfaces
 An interface defines a contract between
the supplier/server and the user/client.

 Simplifies client implementation
 Clients do not need to worry about the
implementation details
 Interfaces encapsulate state of different
subsystems ⇒ side effects reduced
 Define clear boundaries between different
teams of programmers
 Clients can substitute alternative
implementations: polymorphism
 Clients can purchase off the shelf solutions:
software components

Software Component
Client Program
Interface /ublic class HttpServer
{
/**
Listens indefinitely for transfer requests and creates a server
instance for each request.
*/
public void listen()
throws IllegalAccessException, InstantiationException, IOException
{
for (;;) {
/*
Block, waiting for a request to occur then spawns a new
(anonymous) socket with which to deal with the request.
*/
System.err.println("HttpServer: waiting...");
Socket s = socket.accept();
/*
Create a file server to deal with the new socket.
*/
FileServer f = createFileServer();
f.dispatch(s);
}
}
public static void main(String[] args)
{
try {
HttpServer htts = new HttpServer("sea.server.ThreadedFileServer");
htts.listen();
}
catch (Exception e) {
System.err.println("HttpServer: failed due to exception:n" + e);
}
}

public interface FileServer
{
/**
This method allows an incoming HTTP request to initiate a
file dispatch. The socket will provide an input stream (which
is at the beginning) from which an HTTP/1.0 header request may
be read.<p>
It also provides an output stream on which the request should be
delivered. The delivery should have an HTTP/1.0 header
prepended.
@param s The socket on which a request is being made.
Once this method has returned the socket will have
been closed by the dispatcher.
*/
public void dispatch(Socket s);
}

 Each interface is a contract between
two parties
 The contract should be made as strict
and precise as possible
 Avoid unnecessary ambiguity
 Document the contract within the
interface’s source file using Javadoc

 Two flavours of FileServer have been
provided using deferred instantiation
› A simple one but with low performance:
sea.server.SimpleFileServer
› A server that uses multiple threads to
increase performance:
sea.server.ThreadedFileServer
› A server which uses a pool of threads to
achieve the maximum possible
performance: sea.server.ThreadedServer2

 Must implement the FileServer interface
so that it can plug in to the HttpServer
 Reads the HTTP request from the Socket’s
input stream
 Decides which file is required
 Reads the file and spools to the Socket’s
output stream.

public class SimpleFileServer implements FileServer
{
protected Socket s = null;
public void dispatch(Socket s)
{
this.s = s;
respond();
}
. . . .
}

 Must get an input stream so that we
can analyse the request
 Socket provides the method
› InputStream getInputStream();
Socket s;
InputStream inStream = s.getInputStream();
InputStreamReader reader = new InputStreamReader(inStream);
BufferedReader input = new BufferedReader(reader);

 Request consists of a number of lines of
text separated by “rn”
 First line is all this server is interested in
 A typical request might be of the form:
GET /path/to/file.html HTTP/1.0
Accept: text/html
Accept: image/gif
User-Agent: Lynx/2.4

 Cuts out the file name
 Looks for the file relative to the current
working directory (not portable!!)
 If the file is a directory look for the file
“index.html” in the directory
 If the file does not exist then respond with
an error (code 404)

 Must construct a header for the response
 Code 200 means success
 Simple header takes the following form:
HTTP/1.0 200 OK
Server: SEA/1.0
MIME-version: 1.0
Content-type: text/html
Data starts after blank line. . .
More data, etc. . .

 Get the output stream from the Socket
› OutputStream getOutputStream()
 Spool (copy) the file contents into the
socket
 If the MIME type is textual then we must
make sure the lines are delimited by
“rn”.
 Otherwise we pass the file unmodified

 The SimpleFileServer is completely
sequential.
› It handles one request at a time.
 Reading a file from disk takes a long time
(around 10ms)
 The server will be sitting idle while it waits
for the file to load (wasting up to 106
instruction cycles)
 Other web browsers will be kept waiting

Start HTTP request loading
Block awaiting disk availability
Deliver web page across network
time

 Threaded servers can process several
requests at once. Each request is
handled by a separate thread.
 This doesn’t increase the overall amount
of work done (unless using SMP)
 . . . but it does reduce the wastage!
 Threaded operation is worthwhile when
threads are expected to block, awaiting
I/O operations

 Java provides very convenient
multithreading to programmers
 We can add threads using inheritance
› We can supplement the existing capabilities
of the SimpleFileServer class
› We create a class ThreadedFileServer which
extends the existing SimpleFileServer
 You may have covered threads in the
Introductory Java Course

public class ThreadedFileServer extends SimpleFileServer
implements FileServer, Runnable
{
private static int index = 0;
public void dispatch(Socket s) {
super.s = s;
Thread thread =
new Thread(this, ”Server-" + (index++));
thread.start();
}
public void run() {
super.respond();
}
}

 Creates new threads within the virtual
machine
 Classes which start threads must
implement interface java.lang.Runnable
interface Runnable
{
/**
This is the method that will be run when the
new thread is started.
*/
public void run();
}

 Must create a Thread object associated
with each new thread using the
constructor
› Thread(Runnable run, String threadName)
 Start a thread with the method
› void start()
 Other useful methods can be used to set
priorities and interrupt a running thread

 Our threads do not share any common
memory locations (except for index)
 When threads read/write a shared
memory area access must be
synchronized
 Otherwise it is impossible to predict how
the system will behave
 Java has mechanisms for achieving this

 Starting a thread can be relatively
expensive when performance is critical
 Our threaded server creates a new Thread
for each file to be transferred
 A better approach is to create a pool of
threads and recycle them
› Create a pool of threads which are ready to
work when needed
› Have threads wait until work is available
 Better, but more complex so look at the
class sea.server.ThreadedFileServer2

 Our example web server performs a very
simple task
› Accept a request from a client
› Retrieve the appropriate document from disk
› Return the document to the client
 This is too limiting
› How do we implement searches?
 We need to be able to run programs within
the server to process user requests
› Accept a client request including arguments
› Run a program on the arguments
› Return results in the form of a document

 When we run small Java programs within a
browser these are referred to as Applets. . .
 so we run small Java programs within a
server these are “Servlets”
 A servlet is a program designed to process
a client request (which requires
interactivity).
› It processes arguments and formats its results as
a short lived document.
 HTML servlets are becoming a popular
mechanism for creating interactive servers.

 Traditionally programs were run on web
servers using Common Gateway
Interface (CGI) scripts written in
languages such as Perl.
› Must create a new interpreter process for
each client request
› Comparatively slow to start
› Expensive of memory resources when serving
several clients at the same time
› Interpreted programs are CPU intensive

 Servlets use Java objects which persist
between requests to the server
› Low latency since requests run in threads
› Offer performance advantages since programs
are compiled and can take advantage of JITs
and/or Hotspot JVMs.
› Servlet groups can share a JVM leading to
smaller memory footprints.
› Servlets run in a Sandbox offering protection
from malicious (or accidental) damage
› Programs are future proofed since WORA offers
better scope for server upgrades.

 Servlets are written in a similar fashion to
applets
› Write a new servlet class which extends
javax.servlet.http.HttpServlet (or just
implements javax.servlet.Servlet)
› Override certain methods to deal with requests
› Get your methods to create an HTML document
to return information to the client’s browser
› Load the servlet byte codes onto your web
server (for example apache/jserv)

 When the servlet is first loaded it makes a
single call to the method
› public void init(ServletConfig config)
 This may optionally be overridden to initialise
the state of the servlet (for example loading
state information from a file).
 When a servlet is finally unloaded it
makes a single call to the method
› public void destroy()
 If you wish to save to servlet state to a file (or
using JDBC) this is the method to override

 To handle an HTTP GET request implement
› protected void doGet(HttpServletRequest request,
HttpServletResponse response)
 If a browser visits your servlet this is where you get
to create a document for it to display
 To handle an HTTP POST request provide
› protected void doPost(HttpServletRequest request,
 If your document contains an HTML form and the
user posts the results this is where you can extract
and process them
 Also methods for HTTP OPTIONS, TRACE and
DELETE (more exotic options)

 Two objects are passed as parameters to all
these handler methods:
 javax.servlet.http.HttpServletRequest
› Represents the formation that was passed to the
server when the user submitted the request by
visiting/posting to the servlets URL.
 javax.servlet.http.HttpServletResponse
› Used to construct a reponse document that is
returned to the user
 Each has a raft of methods so check the
Javadoc for details

 An web based chat room server
 A number of users can connect to the
servlet using browsers
 Read a list of the previous messages
 Optionally append new messages to the list
 Messages are attributed to a specific
author and are time stamped
 Messages do not persist after the chat
server is stopped (easy enough to rectify)

public class ChatServlet extends HttpServlet
{
Vector messages = new Vector();
public void init(ServletConfig config)
throws ServletException
{
super.init(config);
}
public void destroy()
{
// Currently does nothing
}
. . . .
}

protected void doGet(HttpServletRequest request,
throws ServletException, IOException {
createDocument(response);
}
protected void createDocument(HttpServletResponse response)
throws IOException {
response.setContentType("text/html");
response.setHeader("pragma", "no-cache");
PrintWriter writer = response.getWriter();
writer.println("<HTML>");
writer.println("<HEAD><TITLE>Chat Servlet</TITLE></HEAD>");
writer.println("<BODY>");
Date now = new Date();
writer.println("Current server time is " + now + "<P>");
. . . .
writer.println("</BODY></HTML>");
writer.close();
}

for (int i = 0; i < messages.size(); i++) {
writer.println("<HR>");
String messageString = (String) messages.elementAt(i);
writer.println(messageString);
}
writer.println("<HR><FORM METHOD=POST>");
writer.println("Enter your name: “ +
“<INPUT TYPE=TEXT SIZE=25 NAME=name><BR>");
writer.println("Enter your message:<BR>” +
“<TEXTAREA ROWS=5 COLS=40 NAME=message>” +
“Type your message here</TEXTAREA><BR>");
writer.println(
"<INPUT TYPE=SUBMIT NAME=action VALUE=Submit>");
writer.println("<HR></FORM>");

protected synchronized void doPost(
HttpServletRequest request,
throws ServletException, IOException
{
String name = request.getParameter("name");
String message = request.getParameter("message");
if (name != null && message != null) {
Date timeStamp = new Date();
String messageString = "<B>Message " + messages.size() +
" from " + name + " at " + timeStamp +
":</B><BR>" + message + "<P>";
messages.add(messageString);
}
createDocument(response);
}

 Servlets offer better performance than most
of the previous CGI like technologies
 But CGI/Servlets concentrate the load on
the server
 When designing high throughput servers
only use servlets where you really need
interactivity
› Searches/Shopping carts
› Data that is very short lived (stock quotes)
 This also applies to low throughput servers
that might need to scale later

 Consider using periodic programs to
generate static documents on disk
› The cost of serving fixed documents will always
be less than the cost of server side execution
› Disk space is cheap!
 Consider using applets when possible
› This places the load on the client machines
rather than the server
 Finally consider using SMP and/or server
farms
› Complex and very expensive

 How can a chat reader find out when a
new message has been posted by another
author?
› Only by repeatedly hitting the Reload button!
 HTTP (& TCP/IP services in general) transfer
documents on the user’s request
 To push updates automatically from the
server you will need to:
› Start a reverse server within each client
› Use a multicast group
› Use a remote procedure call system such as RMI
or CORBA

 Java Server Pages is an extension to the
servlets API.
 With conventional servlets you embed the
HTML that you need inside a Java program.
 With JSP you embed your Java program
within a HTML document (by using special
tags).
 Works rather like JavaScript but the JSP
script runs on the server before the page is
dispatched to the user’s browser.

 For information about HTML try
http://www.w3schools.com
 You can download Sun’s servlet
development kit from their web site at
the http://java.sun.com/products/servlet
 You can download apache’s Tomcat
server from http://jakarta.apache.org
 For other information about Servlet
development try
http://www.servlets.com

 Read through the sample code to
convince yourself you understand what’s
going on
 Sample code can be downloaded from
http://ciips.ee.uwa.edu.au/~gareth
 Read the code documentation
 If you can, run the examples to check
they work

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