1. Flight Simulation Client/Server
Malcolm Mumme
4/25/2019

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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3. 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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4. 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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5. Java Threads and Strand Threads
A class’s 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 who’s finalize() method stop()s the Thread.
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6. 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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7. Socket input buffering with ReEnumerator
A Server
Client1 in
in Stream
Socket
Socket
out
out Stream
Client2 in in
Socket
Socket
out
out
A multi-user server must not be forced to wait on an individual Socket
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8. Socket input buffering with ReEnumerator
A Server
Client1 in
in Stream
Socket
Socket
out
out Stream
Client2 in in
Socket
Socket
out
out
A multi-user server must not be forced to wait on an individual Socket
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9. Socket input buffering with ReEnumerator
an Object
An Enumeration
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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10. 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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11. Socket input buffering with ReEnumerator
an Enumeration
A ReEnumerator
buffer
nextElement()
an Enumeration
an Enumeration
buffering Strand
a boolean
hasMoreElements()
A ReEnumerator packages a sequence of Enumerations
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12. 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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13. 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 it’s part of the simulation
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14. 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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15. 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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16. 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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17. 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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18. 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 it’s corresponding Entity
The new client must be downloaded with the entire simulation state
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19. 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 it’s Entity being removed
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20. 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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21. 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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22. 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 it’s 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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23. 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 it’s corresponding Entity
It must update it’s Entity and the SimulationServer whenever the user changes a control on the Applet Window
It must quit when it’s Entity is removed by the SimulationServer
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24. 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 it’s orientation.
Each polygonal model is geometricaly manipulated to it’s relative orientation and location as it should appear on the display.
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25. 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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26. 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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27. What This Means With Sockets and Threads, you can do fun stuff
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