Flight Simulation ClientServer

1
Flight Simulation Client/Server

• Malcolm Mumme

2
Introduction

• Using Java features to program Client/Server
  applications
• Threads, Sockets, inheritance, c.
• (all page number references are to the textbook)

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Topics of Discussion

• Java Threads and Strand Threads
• Socket input buffering with ReEnumerator
• Simulation state updates with Entity and
  SimMonitorMessage
• Output Synchronization
• Managing clients in SimEngine
• Display updates using SimulationClient and
  Radar/WinDisplay

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Java Threads and Strand Threads

• Text implies that Threads may outlive the objects
  that contain them. (p. 842)
• This is not consistent with the usual approach to
  objects
• Define Strand class as a more OO Thread
• Strand interface similar to Thread
• new Strand( (Runnable)this )
• Strand will automatically start the thread at
  construction and stop the thread upon
  finalization (p. 472)

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Java Threads and Strand Threads

• A classs finalizers can be overriden by
  subclasses, parent finalizers are not
  automaticaly called as with constructors(p. 516)
• Avoid this problem by using a helper final class
  StrandThreadContainer whos finalize() method
  stop()s the Thread.

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Socket input buffering with ReEnumerator

• Why Socket input buffering?
• A multi-user server must not be forced to wait on
  an individual Socket
• An Enumeration(p. 769) is a convenient way to
  package a sequence of messages
• A ReEnumerator packages a sequence of Enumerations

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Socket input buffering with ReEnumerator
A Server
Client1
in
in Stream
Socket
Socket
out Stream
out
Client2
in
Socket
in
Socket
out
out

• A multi-user server must not be forced to wait on
  an individual Socket

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Socket input buffering with ReEnumerator
A Server
Client1
in
in Stream
Socket
Socket
out Stream
out
Client2
in
Socket
in
Socket
out
out

• A multi-user server must not be forced to wait on
  an individual Socket

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Socket input buffering with ReEnumerator
An Enumeration
an Object
nextElement()
an Object
an Object
hasMoreElements()
an boolean

• An Enumeration(p. 769) is a convenient way to
  package a sequence of messages

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Socket input buffering with ReEnumerator

• A ReEnumerator contains a Strand Thread which
  keeps the main thread from having to wait on a
  Stream
• nextElement() returns an Enumeration
• That Enumeration supplies those messages that
  have already been buffered.
• MessageEnumeration is an extended Enumeration
  interface for this program that has a method
  nextSimMonitorMessage()

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Socket input buffering with ReEnumerator
A ReEnumerator buffer
an Enumeration
nextElement()
an Enumeration
an Enumeration
buffering Strand
a boolean
hasMoreElements()

• A ReEnumerator packages a sequence of Enumerations

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Socket input buffering with ReEnumerator

• A MessageEnumerator wraps(p. 725) an input Stream
  with a MessageEnumeration
• MessageReEnumerator is a special ReEnumerator
  that works with a MessageEnumeration
• new MessageReEnumerator( new
  MessageEnumerator( in ) ) wraps and buffers the
  stream

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Simulation state updates with Entity and
SimMonitorMessage

• Each simulated thing has a corresponding Entity
  Object in the SimEngine
• Each Entity contains several simulation
  parameters relating to it( position, velocity,
  orientation, shape, color, name,spin rate c. )
• An Entity can step() itself forward in time to do
  its part of the simulation

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Simulation state updates with Entity and
SimMonitorMessage

• A SimMonitorMessage communicates partial update
  information about an Entity and can
  update(Entity)
• There is a subclass of SimMonitorMessage for each
  parameter in an Entity
• A SimMonitorMessage can be read from a
  DataInputStream and can update an Entity
• A SimMonitorMessage can be constructed from an
  Entity and can write itself to a DataOutputStream
  

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Output Synchronization

• A SimMonitorMessage writes itself to a
  DataOutputStream in many little pieces.
• There is more that one Strand(Thread) in the
  SimulationServer in which this may occurr.
• This must always be done with synchronized
  methods(p. 851)
• public synchronized static void sendAllMessages(
  )
• public synchronized static void
  sendUserControlMessages()

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Output Synchronization
In Thread A, SimMonitorMessage X writes itself to
out
OutputDataStream out
In Thread B, SimMonitorMessage Y writes itself to
out
X0 X1 X2 Y0 X3 Y1 X4 X5

• in many little pieces. There is more that one
  Strand in which this may occurr.

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Managing clients in SimEngine

• The SimEngine starts running a simulation Strand
  at construction
• The SimulationServer Object adds clients to the
  SimEngine by invoking the addMember(Socket)
  method
• Clients may leave at any time on request or by
  just aborting and causing an IOException on a
  Socket
• The SimulationServer may destroy the SimEngine at
  any time in response to the Quit button

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Managing clients in SimEngine

• When the SimulationServer Object adds clients to
  the SimEngine by invoking the addMember(Socket)
  method,
• A new simulation Entity corresponding to the
  client must be created and initialized
• All existing clients must be updated adding the
  new Entity
• The new client must be informed of its
  corresponding Entity
• The new client must be downloaded with the entire
  simulation state

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Managing clients in SimEngine

• A client may quit at any time.
• The SimEngine may be informed of this by
• 1 a request from the client
• 2 an IOException on that clients Socket
• The corresponding Entity must be removed, and all
  remaining clients must be then informed of that.
• When the client requests to quit, the SimEngine
  also informs the same client (as well as all the
  remaining clients) of its Entity being removed

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Managing clients in SimEngine

• The SimulationServer may destroy the SimEngine at
  any time in response to the Quit button A
  client may quit at any time.
• The SimEngine has a finalize() method which does
  the following
• Each Entity must be removed and each
  corresponding client must be then informed of
  that.

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Display updates using SimulationClient and
Radar/WinDisplay

• A SimulationClient connects to the
  SimulationServer with a Socket in the usual
  way(p. 862)
• The simulation state is duplicated on each client
  using the same class Entity used on the server
• The SimMonitorMessage class is used for all
  messages both from and to the SimulationClient

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Display updates using SimulationClient and
Radar/WinDisplay

• A SimulationClient must receive all messages from
  the SimulationServer
• It must create an Entity Object when informed of
  its existence
• It must update the corresponding Entity when it
  receives a SimMonitorMessage from the
  SimulationServer
• It must remove an Entity for which it receives a
  Quit message

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Display updates using SimulationClient and
Radar/WinDisplay

• A SimulationClient must remember to which Entity
  it corresponds
• It must render visual images from the point of
  view of its corresponding Entity
• It must update its Entity and the
  SimulationServer whenever the user changes a
  control on the Applet Window
• It must quit when its Entity is removed by the
  SimulationServer

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Display updates using SimulationClient and
Radar/WinDisplay

• A SimulationClient display images are rendered in
  the WinDisplay Applet that maintains the primary
  display area(by means of the renderAll() method).
• Within renderAll ,a polygonal model is extracted
  from each Entity
• The light on each polygon is adjusted according
  to its orientation.
• Each polygonal model is geometricaly manipulated
  to its relative orientation and location as it
  should appear on the display.

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Display updates using SimulationClient and
Radar/WinDisplay

• A shadow polygon at altitude 0 is created in
  some cases.
• Each polygonal model is clipped to remove parts
  that would not be visible within the display
  window
• After all polygonal models are geometricaly
  processed, they are reordered to approximate a
  visibility algorithm.
• All polygons are then rendered to a clean Image
  using the fillPolygon method, which is then
  copied to the window by drawImage to minimize
  flicker.

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Potential improvements

• Add a dialog to the client for obtaining the
  Server host name
• Get some better scenery
• Correct the visibility algorithm
• Add realistic flight dynamics
• Improve the appearance of planes

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What This Means

• With Sockets and Threads, you can do fun stuff