universidad de chile - tupper 2007, santiago - fono: 678 4888 - fax: 698 8427 - email:...

1Universidad de Chile - Tupper 2007, Santiago - Fono: 678 4888 - Fax: 698 8427 - Email: victoria.gaete @die.uchile.cl 2000/1

Módulo 9: Desarrollo de Aplicaciones en Redes de Computadores

Java Spaces



Definition according to sun

JavaSpacesTM technology is a simple unified mechanism for dynamic communication, coordination, and sharing of objects between

JavaTM technology-based network resources like clients and servers. In a distributed application, JavaSpaces technology acts as a virtual space between providers and requesters of network resources or objects. This allows participants in a distributed solution to exchange tasks, requests and information in the form of Java technology-based objects. JavaSpaces technology provides developers with the ability to create and store objects with persistence, which allows for process integrity.



JavaSpaces is part of the Jini spec.

Jini is a collection of service specifications They help services to find each other on the

network The most common notion people have from Jini

is that enables jini powered machines to be attached to the network and automatically find out which services are available, as well as post its own services to the rest

Jini based machines are servers and client at the same time



Example

? ?



Services provided by Jini

Lookup Services (reggie)

rmid

HTTP-Server (tools.jar)

Transaction Services (mahalo.jar)

JavaSpace (outriggger)Fiddler

MercuryNorm



What does JavaSpaces provides

A space in which objects can be stored in, retrieved from, copied from.

A set of methods: write, read, take, (readIfExists, takeIfExists)

A mechanism to ensure completeness of transactions

An event notify mechanism



The write and read

write

Space

read

A copy



The take

X

write

Space

take

Exists only here



How can I create a JavaSpace in a host

A JavaSpace can be created by starting the outrigger service of Jini with a certain name as parameter. This will be the name of the space.

Before starting the outrigger it is necessary to start:The http server tools.jar

An rmid

The lookup server rigger

The transaction server mahalo



How can I write objects into a Space

User defined object classes must declare they implement the interface Entry.

This does not mean it has to implement any methods. It is just a tag to tell the system an object of this class can be written in a JavaSpace

Fields (variables) in this class should be objects. If primitives are needed (such as integers) use wrappers Integer i = new Integer(4)



Example of an Entry classimport net.jini.core.entry.Entry;public class Message implements Entry { public String content; public Integer counter;

public Message() { //this is mandatory !!!!! } public Message(String content, int initVal) { this.content = content; counter = new Integer(initVal); } public String toString() { return content + counter + " times."; } public void increment() { counter = new Integer(counter.intValue() + 1); }}



Storing objects in a spaceimport net.jini.core.lease.Lease;import net.jini.space.JavaSpace;import java.io.*;

public class WriteMessages { public static void main(String[] args) { try {

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

JavaSpace space = SpaceAccessor.getSpace(); System.out.print("Message ? "); String m = in.readLine();

Message2 msg = new Message2(m,0); space.write(msg, null, Lease.FOREVER);

} catch (Exception e) {e.printStackTrace(); } }}



The write sentence

space.write(msg, null, Lease.FOREVER);

The Parameters: an Entry object (the Message), a transaction object (null for now) and a long (leasing time in milliseconds, Lease.FOREVER is the value to make it permanent).



Retrieving objects in a spaceimport java.io.*;import net.jini.core.lease.Lease;import net.jini.space.JavaSpace;

public class ReadMessage { public static void main(String[] args) { try { Message2 template; JavaSpace space = SpaceAccessor.getSpace();

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

System.out.print("Message (**=null)? "); String m = in.readLine();

template = new Message2(m); Message2 result = (Message2)space.read(template,null,Long.MAX_VALUE); System.out.println("Got "+result.toString()); } catch (Exception e) {e.printStackTrace(); }}



Rules for retrieving objects The object must match the template according to

the class and the content Null references in the template act as wildcards There is no rule for deciding which of all the

matching objects in the space matching the template will be retrieved

The parameters: a template object, a transaction (null) and a waiting time before the read sentence gives up if it does not find an object which matches the template. Long.MAX_VALUE is for waiting forever



Taking objects It works just like read but the object is deleted

from the space

In order to avoid deadlocks or waiting, it is possible to use readIfExists and takeIfExists

Their structure is the same as read and take, but they will return immediately if they do not find any matching object in the space

The SpaceAccessor class with the getSpace method is not standard, see SpaceAccessor.java



Synchronizing clients

Ticket Dispenser

number

Customer1- takes a number, 2- increments Ticket Disp.3- takes the service object when it has this number4- waits for being served5-Increments service number

A client will be served Only if it has the servicenumber

Service Number

number

See: TicketInit.java Customer.java



And of course…Chatting

position

A Tail object indicates which is the number of the last message

Every message has a content And a number

The channel identifies a „chat.-thred“

Tail

MessageChatnumber

Channel

Channel

MessageChatnumber

Channel

MessageChatnumber

Channel

See: Tail.java, MessageChannel.java, CreateChannel.java, ChatSpace.java



Distributed Events

1- Create a Listener object

2- Notify space about interestWith a template

3- an object matching templateEnters the space

4- the listener is nofied



How to write a listenerimport java.rmi.server.*;import java.rmi.RemoteException;import net.jini.core.event.*;import net.jini.space.JavaSpace;

public class Listener implements RemoteEventListener { private JavaSpace space; public Listener(JavaSpace space) throws RemoteException { this.space = space; UnicastRemoteObject.exportObject(this); } public void notify(RemoteEvent ev) { Message template = new Message(); try { Message result = (Message)space.read(template, null, Long.MAX_VALUE); System.out.println(result.content); } catch (Exception e) { e.printStackTrace(); } }}



A program that listensimport net.jini.core.lease.Lease;import net.jini.space.JavaSpace;

public class HelloWorldNotify { public static void main(String[] args) { JavaSpace space = SpaceAccessor.getSpace(); try { Listener listener = new Listener(space); Message template = new Message(); space.notify(template, null, listener, Lease.FOREVER, null); } catch (Exception e) { e.printStackTrace(); } }}



Calling the notify method

After this, if any program writes a message (no matter the content) the Listener object of the HelloWorldNotify program will be „notified“, this means, the notify method will be called

Message msg = new Message(); msg.content = "Hello World"; space.write(msg, null, Lease.FOREVER);



Transactions A transaction is a set of instructions which should be

all executed atomically or none at all. In JavaSpaces this refers to write, read, and take

instructions that can be scheduled to be performed in this way.

For this, a transaction object should be created and passed as an argument in every instruction which belongs to the transaction

After all write, read, take, etc.instructions with (the same) transaction are “executed”, a commit statement will either perform all the operations or no one.

In the latter case, an exception is thrown



An exampleJavaSpace space = SpaceAccessor.getSpace();TransactionManager mgr = TransactionManagerAccessor.getManager();//get a reference to the transaction manager serviceTry { Transaction.Created trc =

TransactionFactory.create(mgr, 3000);Transaction txn = trc.transaction;SharedVar template = new SharedVar(url);SharedVar counter =

(SharedVar) space.take(template, txn, Long.MAX_VALUE);counter.increment();space.write(counter, txn, Lease.FOREVER);txn.commit();

} catch (Exception e) { System.err.println("Transaction failed"); return;}


Módulo 9: Desarrollo de Aplicaciones en Redes de ComputadoresJava Applets

<applet code=Animator.class ><parameters></applet>

Java programrunning on the client

Html

Animator.class

Applets are java programs which are downloaded with the HTML page.

Animator.class


Módulo 9: Desarrollo de Aplicaciones en Redes de ComputadoresJava Script

Java programrunning on the client

Html & Script

The code of the java program is written directly in the HTML page

<script language = “JavaScript”>the code</script>


Módulo 9: Desarrollo de Aplicaciones en Redes de ComputadoresJava ServletsThe code of the java program which runs on the serverand can dynamically produce HTML content accordingto the particular situation (the client of a bank)

MyServlet.class

HTML-page with a reference to a servlet

HTML from page

HTML from servlet



Java Server Pages (and ...)

Like Java Script for applets, JSP is a script language programming for servlets

The code is written inside the HTML page but it is executed on the server

The server will dynamically generate HTML code, which will be “written” into the client’s browser



How can I explain what is an Applet ?

For people like you, who already master Java programming it is very easy: an applet is a Panel in an HTML page in which I can program (almost) everything I can program in a normal Panel.

The html viewer

The applet

Bla Bla Bla Bla

Bla Bla Bla Bla



How do I link an Applet in my HTML Page ?

<HTML><HEAD><TITLE> My first Applet </TITLE></HEAD><BODY>.......Here is how you attach an applet to your html page!!!<APPLET CODE = “MyApplet.class” WIDTH=150 HEIGTH=25></APPLET>.....</BODY>

You must provide an Applet which is programmed in a filenamed MyApplet.java and compiled just as any other class



How do I Program an Applet ?

import java.applet.*;import java.awt.*;

public class MyApplet1 extends Applet {

public void paint(Graphics g) {g.drawString(“Hello World”,50,25);

}}

Let´s start with a very simple one

This tells us the applet is a kind of frame !!


Módulo 9: Desarrollo de Aplicaciones en Redes de ComputadoresRemember the Jalisco Program ?import java.awt.*;import java.applet.*;import java.awt.event.*;public class Jalisco1 extends Applet { Label question = new Label("Enter a number :"); Label answer = new Label(" "); TextField number = new TextField(9);

public void init() {add(question);add(number);add(answer);setBackground(Color.green);number.addActionListener( new ActionListener() { //this will make the thexfield react when an enter is typed

public void actionPerformed(ActionEvent x) { String s = number.getText(); int n = Integer.parseInt(s); answer.setText("I win with the " + (n+1));

} }); } }


Módulo 9: Desarrollo de Aplicaciones en Redes de ComputadoresThe applet has a life cycle When an applet is loaded

The applet initializes itself, running the init() method if provided

The applet starts running, executing the start() method if provided

When you leave and return to the pagewhen leaving the page (going to another page) the applet stops

itself, executing the stop() method before if providedwhen returning to the page it srarts again executing the start()

method When quitting the browser:

the applet can run the destroy() method if provided



Secutiry with applets The applet is a running program in my computer which

I just downloaded from the network: how can I be sure it will not harm my computer ?Applets cannot read or write the disks of the host computer !Because java has not pointer arithmetic a class generated by

the compiler cannot read or write other memory than just the memory of the program (in C we have this problem !!)

In order to avoid Trojan Horses, every applet is “signed” by the compiler. This means, if a .class file was not generated by the compiler, it can be easily detected by the applet loader of the viewer and it doesn't allow it to run



What Applets Can't Do

It cannot ordinarily read or write files on the host that is executing it.

It cannot make network connections except to the host that it came from.

It cannot start any program on the host that's executing it.

It cannot read certain system properties. Windows that an applet brings up look

different than windows that an application brings up.



What Applets Can Do Applets can usually make network connections to the host

they came from. Applets running within a Web browser can easily cause

HTML documents to be displayed. Applets can invoke public methods of other applets on the

same page. Applets that are loaded from the local file system (from a

directory in the user's CLASSPATH) have none of the restrictions that applets loaded over the network do.

Although most applets stop running once you leave their page, they don't have to.



Servlets Servlets are modules that extend request/response-oriented

servers, such as Java-enabled web servers. A servlet might be responsible for taking data in an HTML

order-entry form and applying the business logic used to update a company's order database.

Servlets are an effective replacement for CGI scripts. They provide a way to generate dynamic documents that is both easier to write and faster to run.

Servlets also address the problem of doing server-side programming with platform-specific APIs: they are developed with the Java Servlet API, a standard Java extension.



Preliminary Work The javax.servlet package provides interfaces and

classes for writing servlets This is not part of the standard language, it is

necessary to download JSDK(2.1) package Not all web servers can serve servlets, you should

either have one with this capability(e.g. Tomcat), download a plug-in, or use the servletrunner

Each servlet server has it rules how to make servers accessible. In most cases, you should specify a certain path where servlets are located

In order to be able to compile servlets you will need to put the jsdk.jar file in the propper directory (c:\jdk1.3\jre\lib\ext)



The Anatomy of a Servlet A new servlet class is defined by extending HttpServlet

class (in most cases) The most important pre-defined methods of a servlet are:

init() it is called when the servlet is called for the first time (uploaded)

doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException

Called when servlet invoked by a GET Http request doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException

Called when servlet invoked by a POST Http request



Anatomy of a servlet A GET request is always generated when an http

request is typed into a browser (http://www.yahoo.com/ means GET index.html Httpx.x )

When the server is called for the first time, it is initialized and 4-6 threads for attending clients are started, this means they may be served in parallel

In most cases, servlets are contacted by forms in html pages. In this cases, it is possible to specify some parameters and contact the servlet by the POST method.



A First example

The servlet will be contacted directly by an http expression in the browser:http://collide.

informatik.uni-duisburg.de/servlets/SimpleServlet

This will cause the doGet method to be called

The servlet will answer by writing in the client‘s browser a page with a simple message



A Second example

The servlet will be contacted by an action triggered from the web browser:http://collide.informatik.uni-duisburg.de/

servlets/Servlet1.htm

The different buttons will call the doPost and the doGet methods



A Third Example

We will now develop a web counter It will keep track about how many

instances of the servlet have been created and how many times these instances have reacted to client‘s requests

See Count.java



Passing parameters

It is possible to pass parameters to the doGet method in the command line http://host:port/servlet?param1=value1&param2=value2..

The servlet can ask for a value parameter value:String value = req.getParametervalues(param1)

Parameters are only strings See ServletParameter1.java



We can use the same with Forms

A form will be an html page in which we can write a string and send the request to the servlet:

We can use the same servlet !!!!see ServletParameter1.html

Another example with more parametersSurveyServlet with JdcSurvey



Obtaining more information about the client

We can get a lot of information about the request made by the client such like:URL requestData about the client’s hostParameters names and valuesHeader of the requestetc

Try SnoopServlet?par1=val1&par2=val2



Session Tracking

Session tracking is a mechanism that servlets use to maintain state about a series of requests from the same user (that is, requests originating from the same browser) across some period of time.

See SessionServlet



Using Cookies Cookies are a way for a servlet to send some information to

a client to store, and for the server to later retrieve its data from that client.

Servlets send cookies to clients by adding fields to HTTP response headers.

Clients automatically return cookies by adding fields to HTTP request headers.

Cookies have a name and a value (additionally they can have some comments).

In addition to a name and a value, you can also provide optional attributes such as comments.

A server can provide one or more cookies to a client.



Using Cookies To send a cookie 1.Instantiate a Cookie object

2.Set any attributes

3.Send the cookie

To get information from a cookie, 1.Retrieve all the cookies from the user's request

2.Find the cookie by its name

3.Get the values of the cookies that you found

universidad de chile - tupper 2007, santiago - fono: 678 4888 - fax: 698 8427 - email:...

Documents

chile tupper

santiago fono

cl20001 mdulo

aplicaciones en redes

mechanism slide

time slide

copy slide

transaction services