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Gareth Lee. John Morris. Advanced Java Programming Unit Two: Servers and Servlets. Gareth Lee Department of Electrical and Electronic Engineering, University of Western Australia. Overview. To create a simple HTTP server in Java

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Advanced Java Programming Unit Two: Servers and Servlets


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    1. GarethLee John Morris Advanced Java ProgrammingUnit Two: Servers and Servlets Gareth Lee Department of Electrical and Electronic Engineering, University of Western Australia

    2. Overview • 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)

    3. Recommended Reading • 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

    4. (Pseudo) requirements • 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

    5. HTTP protocol • 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

    6. Back to server example • 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:

    7. HttpServer 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); } }

    8. 2037 80 2037 1583 2037 1583 How it all fits together Client (sid) Server (fred) ServerSocket ss. s = ss.accept() s = new Socket (“fred”, 80) Socket s s.getInputStream() s.getOuputStream() s.getInputStream() s.getOuputStream()

    9. …but more of that later! • 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.

    10. How clients use interfaces 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); } }

    11. Interfaces benefit clients • 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

    12. Interfaces simplify clients Software Component 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); } } Client Program

    13. The FileServer interface 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); }

    14. Interfaces are contracts • 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

    15. Implementing FileServer • 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

    16. SimpleFileServer (1) • 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.

    17. SimpleFileServer (2) public class SimpleFileServer implements FileServer { protected Socket s = null; public void dispatch(Socket s) { this.s = s; respond(); } . . . . }

    18. SimpleFileServer (3) • 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);

    19. SimpleFileServer (4) • Request consists of a number of lines of text separated by “\r\n” • 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

    20. SimpleFileServer (5) • 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)

    21. SimpleFileServer (6) • 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. . .

    22. SimpleFileServer (7) • 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 “\r\n”. • Otherwise we pass the file unmodified

    23. Server performance (1) • 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

    24. time Start HTTP request loading Block awaiting disk availability Deliver web page across network Server performance (2)

    25. Threaded servers (1) • 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

    26. time Start HTTP request loading Block awaiting disk availability Deliver web page across network Threaded servers (2)

    27. Threaded FileServer (1) • 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

    28. Threaded FileServer (2) 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(); } }

    29. java.lang.Thread (1) • 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(); }

    30. java.lang.Thread (2) • 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

    31. java.lang.Thread (3) • 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

    32. But even threads cost • 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

    33. Making the server programmable • 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

    34. Servlets • 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.

    35. Servlets versus CGI (1) • 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

    36. Servlets versus CGI (2) • 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.

    37. Creating a simple servlet • 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)

    38. Import servlet methods (1) • 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

    39. Import servlet methods (2) • 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)

    40. Import servlet methods (3) • 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

    41. A simple chat server • 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)

    42. ChatServer (1) 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 } . . . . }

    43. ChatServer (2) 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(); }

    44. ChatServer (3) 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>");

    45. ChatServer (4) 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); }

    46. Performance (1) • 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

    47. Performance (2) • 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

    48. Pull versus Push transports • 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

    49. Servlets and JSP • 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.

    50. Useful sources of information • 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