Distributed Programming in Java

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Distributed Programming in Java

• Networking (1)

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Motivating Scenario
apps
apps
apps
apps
apps
apps
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nslookup
A program for looking up IP addresses
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InetAddress

• This class represents an IP address
• Each host on an IP network has a host name and a
  numeric address.

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NsLookup

• Simple Java version of nslookup

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Sockets

• The java.net package provides an object-oriented
  framework for using sockets
• Sockets offer a fast and flexible way for the
  communication between objects
• The Socket class represents an endpoint of a
  connection between two processes
• There are two flavors of sockets datagram (UDP)
  and stream (TCP) sockets

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Datagram Sockets

• Datagram sockets use the Unreliable Datagram
  Protocol (UDP)
• Low-level connectionless
• No guarantee about packet delivery
• Neither is their order guaranteed
• Packets transmitted by the sender either make it
  to the receiver or not
• Typically used in bandwidth-limited applications
  (e.g. real-time audio, webTV)

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Stream Sockets

• Stream sockets use the Transmission Control
  Protocol (TCP)
• Reliable connection-oriented
• Packets are guaranteed to be delivered
• Packets are delivered in order
• Packets not received within a set time are
  assumed lost and requested again
• Typically used when lost packets cannot be
  tolerated (e.g. message passing)

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Socket

• To create a client-side Socket to connect to a
  specific host and port number
• To create a server-side Socket to listen on a
  ServerSocket for a connection request

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Client Socket

• Key Socket methods

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ServerSocket

• Key ServerSocket methods

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NameServer

• Simple name server which replies to a name with a
  hostNameport or unknown0
• The server creates a ServerSocket, then waits in
  a loop to accept() connections, and processes the
  name lookup request

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NameServer
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NameServer
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NameServer
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Notes

• Following good practice we make the PORT on which
  the server listens a constant
• The classes DataInputStream and DataOutStream are
  filter streams that wrap around an input and
  output stream
• These streams read and write primitive Java
  types readInt(), writeInt(), etc.
• Strings are Unicode, which results in the
  methods readUTF(), and writeUTF()

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Name
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Streams

• The java.io package allows us to exchange
  formatted data over connections
• Streams support reading data serially from a
  source, or writing to a destination

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Byte Streams
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Character Streams
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Serialization

• Object serialization is a process through which
  objects are flattened into a form that can be
  written to and read from a stream
• Flattened objects can be written to a file or
  even sent across a network connection
• Supported by the java.io package
• Key component of the implementation of
  distributed object frameworks in Java

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Serializable

• Serialization is almost automatic in Java
• To use serialization, your objects need to
  implement the Serializable interface
• This is an example of a marking interface ...
• ... it does not have any methods
• ... but indicates (to the JVM) this class is
  designed to be serialized
• However, the internal machinery required makes
  serialization complex and costly

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Example Serializable

• Initial declaration of TestObject

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Object Streams

• Objects can be ...
• serialized with an ObjectOutputStream
• deserialized with an ObjectInputStream
• Similar to DataInput(Output)Stream
• Methods for writing and reading binary
  representations of primitive types
• But they add the ability to write and read
  non-primitive objects and array values

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Object Streams
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Not All Objects (and Data) Should be Serialized

• Inherently platform-specific objects
• e.g. FileDescriptors
• Redundant objects (eg that cache values)
• Only objects that implement Serializable can be
  serialized (must skip others)
• Can prevent fields from being serialized by
  declaring them as transient

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Example Serializable

• Revised declaration of TestObject
• both name and panel are now transient
• assumes that name is derived from value in this
  case (for sake of the example)

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References

• Online chapter on network streams
• Harold, Java I/O, Chapter 5
• http//www.oreilly.com/catalog/javaio/chapter/ch05
  .html
• More on using the serialization API
• Greanier, Flatten Your Objects
• http//www.javaworld.com/javaworld/jw-07-2000/jw-
  0714-flatten.html