Adv Java Apps

7/28/2019 Adv Java Apps

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Advanced Java

Applications

Shon Vick

CMSC 331


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Agenda

We will look at some advanced techniquesand applications of Java

The first will take a quick look at some datastructures 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


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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


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Hashtable Example

Integer n = (Integer)numbers.get("two");

if (n != null) {

System.out.println("two = " + n);}

ObjectSpecific

Object


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Many Other Collections

Vector

Stack

LinkedList

Dictionary

ArrayList http://www.javasoft.com/products/jdk/1.2/d

ocs/api/index.html for a complete list


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Networking

Using the networking capabilities provided

in the Java environment is quite easy

We will see how to use Sockets


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Sockets

Lower-level network communication

Clientuses some service

Server - provides some service

TCP provides a reliable, point-to-point

communication channel for client-server

apps


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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.


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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


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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 adifferent port number) so that it can

continue to listen to the original socket


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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 programand another program on the network

It also includes the ServerSocket class,

which implements a socket that servers canuse to listen for and accept connections to

client


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Establish the Socket connection

try {

echoSocket = new Socket(avatar ", 7777);

out = new PrintWriter(echoSocket.getOutputStream(), true);

in = new BufferedReader(new

InputStreamReader(echoSocket.getInputStream()));}

catch

Host PortOutput

Input


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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);

}


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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 whats read back from Server

Write toServer

Read from standard input


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Close up Shop on Client side

out.close( );

in.close( );

stdIn.close( );

echoSocket.close( );


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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.


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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 samebasics as this simple example


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Server

A server must open a SeverSocket

ServerSocket server = new ServerSocket( 7777 );

Call accept on that socket creating a new

socket

Socket socket = server.accept();

Socket acts as socket from client


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If a socket is a pipe

We could conceptualize this like so:

Client

Server

Ports

The Socket Plumbing The things flowing

through the

Plumbing


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The Answer Is ..

A Number of things can conceptually flowthrough the pipe

We will focus on two: Objects Characters from a String

We looked at several examples last time

The first was a simple echo programa very simpleprotocolgive me back what I gave you (Strings)

We also looked at simpleprotocol example (ProtocolObjects)


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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 followingmechanisms for

Objects to be written by the server

Objects to be read by the client


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The newprotocol Client

public 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();


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The newprotocol Server

class 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 );


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Reading and Writing Objects

An ObjectOutputStream writes primitive datatypes and graphs of Java objects to anOutputStream.

The objects can be read (reconstituted) using anObjectInputStream.

General Mechanism

This works for the sockets as was just shown but isactually more general

Persistent storage of objects can be accomplished byusing a file for the stream.


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File example

For 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();


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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();


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The Needed Java Framework

Only objects that support the java.io.Serializable

interface can be written to streams.

The class of each serializable object is encodedincluding 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


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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 waswritten.

Marshalling of Objects (Serialize)

Un marshaling of Object (Serialize)


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Deserialization

& Object Reflection

Fields declared as transient or static are ignored by

the deserialization process.

References to other objects cause those objects tobe 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


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Reflection Allows

Determination of the class of an object.

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

Obtaining information about a class'smodifiers, fields, methods, constructors, and

superclasses. Determination of constants and method

declarations that belong to an interface


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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.


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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


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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();

Retrieving Class Objects

Foo

Bar


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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)


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Getting the Class Name

Every class in the Java programming language hasa name. When you declare a class, the nameimmediately follows the class keyword

At runtime, you can determine the name of a Classobject by invoking the getName method. TheString returned by getName is the fully-qualifiedname of the class.

A good home study question: Given an instanceprints the names of the classes its inheritancehierarchy from least specific to most specificexcluding Object


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An Example

import 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 Reflection

PackageTo Do this


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Selected References

Advanced Techniques for Java Developers,Berg&Fritzinger, Wiley, 1999 Chapter 4

http://java.sun.com/docs/books/tutorial/networking/index.html

The Java Programming Language , K.Arnold andJ. Gosling , Addison-Wesley , 1996

Java in a Nutshell , D. Flanagan, O'Reilly andAssociates

http://java.sun.com/products/jdk/1.2/docs/


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More References

The Java Programming Language ,

K.Arnold and J. Gosling , Addison-Wesley ,

1996 Java in a Nutshell , D. Flanagan, O'Reilly

and Associates

http://java.sun.com/products/jdk/1.2/docs/

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