Advanced Java Applications

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Advanced JavaApplications

• Shon Vick
• CMSC 331

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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
• Client uses 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 a
  different 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 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

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Echo Echo Echo
import 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 //
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Establish the Socket connection

Output
Port
Host
try echoSocket new Socket(avatar ",
7777) out new PrintWriter(echoSocket.getOutp
utStream(), true) in new
BufferedReader(new
InputStreamReader(echoSocket.getInputStream()))
catch
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
Set up a mechanism to read from standard input
BufferedReader stdIn new
BufferedReader(
new InputStreamReader(System.in))
String userInput
while ((userInput stdIn.readLine()) ! null)
out.println(userInput)
System.out.println("echo "
in.readLine())
Read from standard input
Write to Server
Output whats read back from Server
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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 same basics 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

Ports
Client
Server
The things flowing through the Plumbing
The Socket Plumbing
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The Answer Is ..

• A Number of things can conceptually flow through
  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 program a very
  simple protocol give me back what I gave you
  (Strings)
• We also looked at simpleprotocol example
  (Protocol Objects)

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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 following mechanisms 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 data types
  and graphs of Java objects to an OutputStream.
• The objects can be read (reconstituted) using an
  ObjectInputStream.
• General Mechanism
• This works for the sockets as was just shown but
  is actually more general
• Persistent storage of objects can be accomplished
  by using 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(f
oo.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(istrea
m) 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 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

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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 was written.
• 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
  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
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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

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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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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()

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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 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

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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 Package To 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 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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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/