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

Introduction Using Java features to program Client/Server applications Threads, Sockets, inheritance, &c. (all page number references are to the textbook) 4/25/2019

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 4/25/2019

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) 4/25/2019

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. 4/25/2019

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 4/25/2019

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 4/25/2019

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 4/25/2019

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 4/25/2019

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() 4/25/2019

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 4/25/2019

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 4/25/2019

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 4/25/2019

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 … 4/25/2019

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(…) … 4/25/2019

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. 4/25/2019

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 4/25/2019

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 4/25/2019

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 4/25/2019

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. 4/25/2019

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 4/25/2019

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 4/25/2019

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 4/25/2019

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. 4/25/2019

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. 4/25/2019

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 … 4/25/2019

What This Means With Sockets and Threads, you can do fun stuff 4/25/2019