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A java servers
1.
2.
3. 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)
4. 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
5. 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
6. 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
7. 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:
8. 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);
}
}
9. 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()
10. 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.
11. 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);
}
}
12. 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
13. 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);
}
}
14. 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);
}
15. 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
16. 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
17. 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.
18. public class SimpleFileServer implements FileServer
{
protected Socket s = null;
public void dispatch(Socket s)
{
this.s = s;
respond();
}
. . . .
}
19. 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);
20. 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
21. 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)
22. 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. . .
23. 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
24. 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
25. Start HTTP request loading
Block awaiting disk availability
Deliver web page across network
time
26. 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
27. Start HTTP request loading
Block awaiting disk availability
Deliver web page across network
time
28. 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
29. 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();
}
}
30. 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();
}
31. 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
32. 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
33. 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
34. 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
35. 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.
36. 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
37. 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.
38. 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)
39. 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
40. 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,
HttpServletResponse response)
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)
41. 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
42. 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)
43. 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
}
. . . .
}
44. protected void doGet(HttpServletRequest request,
HttpServletResponse response)
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();
}
45. 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>");
46. protected synchronized void doPost(
HttpServletRequest request,
HttpServletResponse response)
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);
}
47. 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
48. 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
49. 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
50. 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.
51. 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
52. 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