More on Network Programming in Java

1
More on Network Programming in Java

• Chapter 22

2
Transmitting Objects over the Network

• Objects to be transmitted over the network via
  object serialization
• The object to be transmitted implements the
  Serializable interface
• Classes that do not implement this interface will
  not have any of their state serialized or
  deserialized.
• All subtypes of a serializable class are
  themselves serializable.
• The serialization interface has no methods or
  fields and serves only to identify the semantics
  of being serializable.
• To allow subtypes of non-serializable classes to
  be serialized, the subtype may assume
  responsibility for saving and restoring the state
  of the supertype's public, protected, and (if
  accessible) package fields. The subtype may
  assume this responsibility only if the class it
  extends has an accessible no-arg constructor to
  initialize the class's state. It is an error to
  declare a class Serializable if this is not the
  case. The error will be detected at runtime.

3
ChangeMaker Server

• import java.io.import java.net.public
  class ChangeMaker static final public int
  portNumber 3339 static final public
  double exchangeBPenniestoAPennies 1.615
  public ChangeMaker() throws IOException
  ServerSocket server new ServerSocket(portNumb
  er) while (true) Socket sock
  server.accept() // got client, make the
  connection System.out.println("After socket
  creation")
• ObjectInputStream in new
  ObjectInputStream(sock.getInputStream())
  System.out.println("After ObjectInputStream
  creation") ObjectOutputStream out
  new ObjectOutputStream(sock.getOutputStream())
  System.out.println("After ObjectOutputStream
  creation")
• // read the value PocketChange
  coins try coins
  (PocketChange) in.readObject() catch
  (ClassNotFoundException e)
  continue // end try-catch

4
ChangeClient

• Socket sock new Socket(InetAddress.getLocalHost(
  ), portNumber)
• System.out.println("After socket creation")
• ObjectInputStream in new ObjectInputStream(sock.
  getInputStream())
• System.out.println("After ObjectInputStream
  creation")
• ObjectOutputStream out new ObjectOutputStream(so
  ck.getOutputStream())
• System.out.println("After ObjectOutputStream
  creation")
• // write out british object
• out.writeObject(bc)
• out.flush()

5
Lab 12 Observed Problem

• Both Client and Server hang when trying to
  perform new ObjectInputStream()
• Server
• ObjectInputStream in new ObjectInputStream(sock.
  getInputStream())
• System.out.println("After ObjectInputStream
  creation") / never printed /
• ObjectOutputStream out new ObjectOutputStream(so
  ck.getOutputStream())
• Client
• ObjectInputStream in new ObjectInputStream(sock.
  getInputStream())
• System.out.println("After ObjectInputStream
  creation") / never printed /
• ObjectOutputStream out new ObjectOutputStream(so
  ck.getOutputStream())

6
Lab 12 Problem

• public ObjectInputStream(InputStream in) throws
  IOException
• Creates an ObjectInputStream that reads from the
  specified InputStream.
• A serialization stream header is read from the
  stream and verified.
• This constructor will block until the
  corresponding ObjectOutputStream has written and
  flushed the header.
• So whats the problem???
• So whats the solution???

7
Lab 12 Problem

• public ObjectOutputStream(OutputStream out)
  throws IOException
• Creates an ObjectOutputStream that writes to the
  specified OutputStream.
• This constructor writes the serialization stream
  header to the underlying stream
• callers may wish to flush the stream immediately
  to ensure that constructors for receiving
  ObjectInputStreams will not block when reading
  the header.

8
This is a common problem in CS

• Deadlock two (or more) programs are waiting
  indefinitely for an event that can only be caused
  by another waiting program.

9
Corrected ChangeMaker Server

• import java.io.import java.net.public
  class ChangeMaker static final public int
  portNumber 3339 static final public
  double exchangeBPenniestoAPennies 1.615
  public ChangeMaker() throws IOException
  ServerSocket server new ServerSocket(portNumb
  er) while (true) Socket sock
  server.accept() // got client, make the
  connection System.out.println("After socket
  creation")
• ObjectOutputStream out new
  ObjectOutputStream(sock.getOutputStream())
  System.out.println("After ObjectOutputStream
  creation")
• out.flush() // did not seem to be
  needed, but a good idea
• ObjectInputStream in new
  ObjectInputStream(sock.getInputStream())
  System.out.println("After ObjectInputStream
  creation") // read the value
  PocketChange coins try coins
  (PocketChange) in.readObject() catch
  (ClassNotFoundException e)
  continue // end try-catch

10
To the lab

• Work on completing Part B of Lab 12.