Advanced JavaApplications

Overview

We will see some advanced techniques and applications of Java

We will take a quick look at some data structures built into the language

Next will show be how Java and Object Orientation can be applied to networking

Finally we will be to show Reflection in Java

Hashtable Example

Hashtable numbers = new Hashtable();

numbers.put("one", new Integer(1));numbers.put("two", new Integer(2));numbers.put("three", new Integer(3));

key value

Hashtable Example

Integer n = (Integer)numbers.get("two"); if (n != null) { System.out.println("two = " + n); }

ObjectSpecificObject

Many Other Collections

VectorStackLinkedListDictionary ArrayListhttp://www.javasoft.com/products/jdk/1.2/docs/api/

index.html for a complete list

Networking

Using the networking capabilities provided in the Java environment is quite easy

We will see how to use Sockets

Sockets

Lower-level network communication- Client – uses some service- Server - provides some service

TCP provides a reliable, point-to-point communication channel for client-server apps

What Is a Socket?

A socket is one endpoint of a two-way communication link between two programs running on the network.

A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent.

How do Sockets work?

A server runs on a specific computer and has a socket that is bound to a specific port number.

Client knows the hostname and port of server and tries to make a connection request

Connection established

If the server accepts the connection it gets a new socket bound to a different port.

It needs a new socket (and consequently a different port number) so that it can continue to listen to the original socket

How does Java support Sockets

The java.net package provides a class, Socket, that implements one side of a two-way connection between your Java program and another program on the network

It also includes the ServerSocket class, which implements a socket that servers can use to listen for and accept connections to client

Echo Echo Echoimport java.io.*;import java.net.*;

public class EchoClient { public static void main(String[] args) throws IOException {

Socket echoSocket = null; PrintWriter out = null; BufferedReader in = null;// …

Establish the Socket connection

try { echoSocket = new Socket(“image ", 7777); out = new PrintWriter(echoSocket.getOutputStream(), true); in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream())); } catch …

Host Port Output

Input

Need to Catch Exceptions

} catch (UnknownHostException e) { System.err.println("Don't know about host: avatar."); System.exit(1); } catch (IOException e) { System.err.println("Couldn't get I/O for " + "the connection to: avatar."); System.exit(1);}

Simple Socket Example

BufferedReader stdIn = new BufferedReader( new InputStreamReader(System.in)); String userInput;

while ((userInput = stdIn.readLine()) != null) {

out.println(userInput);

System.out.println("echo: " + in.readLine()); }

Set up a mechanism to read from standard input

Output what’s read back from Server

Write to Server

Read from standard input

Close up Shop on Client side

out.close( );

in.close( );

stdIn.close( );

echoSocket.close( );

Basic Steps

Open a socket. Open an input stream and output stream to the

socket. Read from and write to the stream according to the

server's protocol. Close the streams. Close the socket.

Same Basic Steps

This client program is straightforward and simple because the Echo server implements a simple protocol

Even with more complicated protocols such as HTTP server, your client program while more complicated will follow the same basics as this simple example

Server

A server must open a SeverSocket ServerSocket server = new ServerSocket( 7777 );

Call accept on that socket creating a new socketSocket socket = server.accept();

Socket acts as socket from client

If a socket is a pipe …

We could conceptualize this like so:

ClientServer

Ports

The Socket Plumbing The things flowing throughthe Plumbing

The Answer Is ..A Number of things can conceptually flow through the

pipeWe will focus on two:

Objects Characters from a String

We looked at several examples last timeThe first was a simple echo program – a very simple

protocol – give me back what I gave you (Strings)We also looked at simpleprotocol example (Protocol

Objects)

Objects flow through the Pipe

Let first address the case where we want to have objects flowing over the pipe

Must have at least the following mechanisms for Objects to be written by the serverObjects to be read by the client

The newprotocol Clientpublic class Client { Socket socket = new Socket( "127.0.0.1", 9999 ); // ObjectInputStream input = new ObjectInputStream(socket.getInputStream() );

// read using serialization NewProtocol protocol = (NewProtocol)(input.readObject() ); System.out.println(“Protocol: “ + protocol); socket.close();

The newprotocol Serverclass ThreadedSocket extends Thread {// here is where all the real work is done. private Socket socket; ThreadedSocket( Socket socket ) { this.socket = socket; //… ObjectOutputStream output = new ObjectOutputStream(socket.getOutputStream() ); output.writeObject( protocol );

Reading and Writing Objects

An ObjectOutputStream writes primitive data types and graphs of Java objects to an OutputStream.

The objects can be read (reconstituted) using an ObjectInputStream.

General MechanismThis works for the sockets as was just shown but is

actually more generalPersistent storage of objects can be accomplished by

using a file for the stream.

File exampleFor example to write an object that can be read by

the example in ObjectInputStream

FileOutputStream ostream = new FileOutputStream(“foo.bar");ObjectOutputStream p = new ObjectOutputStream(ostream);p.writeInt(12345);p.writeObject("Today");p.writeObject(new Date());p.flush();ostream.close();

The read counterpart

FileInputStream istream = new FileInputStream(" foo.bar ");

ObjectInputStream p = new ObjectInputStream(istream);

int i = p.readInt();

String today = (String)p.readObject();

Date date = (Date)p.readObject();

istream.close();

The Needed Java FrameworkOnly objects that support the java.io.Serializable

interface can be written to streams.

The class of each serializable object is encoded including the class name and signature of the class, the values of the object's fields and arrays, and the closure of any other objects referenced from the initial objects

This relates to introspection/reflection which we will discuss shortly

More about the Framework

The default deserialization mechanism for objects restores the contents of each field to the value and type it had when it was written.

Marshalling of Objects (Serialize)Un marshaling of Object (Serialize)

Deserialization& Object Reflection

Fields declared as transient or static are ignored by the deserialization process.

References to other objects cause those objects to be read from the stream as necessary. Graphs of objects are restored correctly using a

reference sharing mechanism. New objects are always allocated when deserializing,

which prevents existing objects from being overwritten

Reflection

Reflection Allows

Determination of the class of an object.

Creation of an instance of a class whose name is not known until runtime.

Obtaining information about a class's modifiers, fields, methods, constructors, and superclasses.

Determination of constants and method declarations that belong to an interface

Reflection Also Allows

• Allows one to get and set the value of an object's field, even if the field name is unknown to your program until runtime.

• Allows one to invoke a method on an object, even if the method is not known until runtime.

• Create a new array, whose size and component type are not known until runtime, and then modify the array's components.

Examining Classes

A way to get information about classes at runtime

For each class, the Java Runtime Environment (JRE) maintains an immutable Class object that contains information about the class. A Class object represents, or reflects, the class

To get this information you need to get the Class object that reflects the class

Retrieving Class Objects

You can retrieve a Class object in several ways: Class c = foo.getClass() // for some object named foo

Bar b = new Bar(); Class c = b.getClass(); Class s = c.getSuperclass();

Foo

Bar

Other Ways of Retrieving Class Objects

If you know the name of the class at compile time, you can retrieve its Class object by appending .class to its name:

Class c = java.awt.Button.class;

You can also use the Class.forName static method:

Class c = Class.forName(commandNameToken)

Getting the Class Name Every class in the Java programming language has a

name. When you declare a class, the name immediately follows the class keyword

At runtime, you can determine the name of a Class object by invoking the getName method. The String returned by getName is the fully-qualified name of the class.

A good home study question: Given an instance prints the names of the classes its inheritance hierarchy from least specific to most specific excluding Object

An Exampleimport java.lang.reflect.*;

import java.awt.*;

class SampleName {

public static void main(String[] args) {

Button b = new Button();

printName(b);

}

static void printName(Object o) {

Class c = o.getClass();

String s = c.getName();

System.out.println(s);

}}

Need ReflectionPackage

To Do this