CS 580 Client-Server Spring Semester, 2004 States
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Doc 22, States Slide # 2

States

• If (case) statements
• Table of function pointers
• State Objects (State pattern)

Doc 22, States Slide # 3

Finite Automata - State Machines

Doc 22, States Slide # 4

Implementing a State Machine: switch

int state = 0;
while (true) {
   command = input.read();
   switch (state) {
      case 0:
         if (command.isUser()) {
            username = command.argument();
            state = 1;
         }
         else if (command.isQuit())
            state = 4;
         else
            error("Illegal command: " + command);
         break;
      case 1:
         if (command.isPassword()) {
            if (valid(username, command.argument()))
               state = 2;
            else {
               error("Unauthorized User");
               state = 3;
            }
         }
         else
            error("Unknown: " + command);
         break;
...

Doc 22, States Slide # 5

int state = 0;
while (true) {
   command = input.read();
   switch (state) {
      case NO_AUTH:
         noAuthorizationStateHandle( command );
         break;
      case HAVE_USER:
         haveUserStateHandle( command );
         break;
      case PROCESS:
         processStateHandle( command );
         break;
      case INVALID:
         invalidStateHandle( command );
         break;
      case QUIT:
         quitStateHandle( command );
         break;
   }

Doc 22, States Slide # 6

void noAuthorizationStateHandle(PopCommand a Command) {
   if (command.isUser()) {
      username = command.argument();
      state = 1;
   }
   else if (command.isQuit())
      state = 4;
   else
      error("Illegal command: " + command);
}

Doc 22, States Slide # 7

• Hard to read.

Need the state machine picture to understand what is going on.

• Hard to modify.

If the protocol (and therefore the state machine) changes, the code will most likely have to be rewritten.

• Hard to debug.

• The code will get very long very quickly.

• The code within the while (true) can be put into a function and only called when there is new input.
• Everyone understands if statements
• Easy to start out with

Doc 22, States Slide # 8

Implementing a State Machine with a Table

• A function to process request
• Next state on success
• Next state on failure

Doc 22, States Slide # 9

Doc 22, States Slide # 10

Doc 22, States Slide # 11

• Use function pointers

• Use symbols and reflection
• Use blocks

• Use reflection
• Use Inner classes

Doc 22, States Slide # 12

Function Pointers in C/C++

void quickSort( int* array, int LowBound, int HighBound)
{
   // source code to sort array from LowBound to HighBound
   // using quicksort has been removed to save room on page
}
   
void mergeSort(int* array, int LowBound, int HighBound)
{   // same here}
   
void insertionSort(int* array, int LowBound, int HighBound)
{   // ditto }
   
void main()
{
   void (*sort) (int*, int, int);
   int size;
   int data[100];
   
   // pretend data and Size are initialized
   
   if (size < 25)
      sort = insertionSort;
   
   else if (size > 100) 
      sort = quickSort;
   
   else
      sort = mergeSort;
   
   sort(data, 0, 99);
}
Doc 22, States Slide # 13

struct
{
   int      currentState;
   char      *command;
   int      stateIfSucceed;
   int      stateIfFailed;
   int      (*action)(char **);
} actionTable[] =
{
   {0, "USER", 1, 3, actionUser},
   {0, "QUIT", 4, 4, actionQuit},
   {1, "PASS", 2, 3, actionPass},
   {1, "QUIT", 4, 4, actionQuit},
   {2, "LIST", 2, 2, actionList},
   {2, "RETR", 2, 2, actionList},
   {2, "QUIT", 4, 4, actionList},
   {0, 0, 0, 0, 0}
};

• Easy to see what is going on.
  • Even easier if the states are given names.

• Easy to add new commands.

Doc 22, States Slide # 14

Smalltalk - Symbols & Reflection

   3 squared
   2 + 3 
   'A cat in the hat' copyFrom: 3 to: 5

   3 perform: #squared 
   2 perform: #+ with: 3
   'A cat in the hat' perform: #copyFrom:to: with: 3 with: 5

Doc 22, States Slide # 15

Function Pointers in Java

public Method getMethod(String name,Class parameterTypes[])
      throws NoSuchMethodException, SecurityException

Throws: NoSuchMethodException 
   if a matching method is not found. 
Throws: SecurityException 
   if access to the information is denied. 

Doc 22, States Slide # 16

class Example
   {
   public void getLunch()
      {
      System.out.println( "Lunch Time!");
      }
   public void getLunch( String day)
      {
      System.out.println( "Lunch Time for " + day);
      }
      
   public void eatOut( String where)
      {
      System.out.println( "MacDonalds? ");
      }
   
   public void eatOut( int where)
      {
      System.out.println( "PizzaHut? " + where );
      }
   }

Doc 22, States Slide # 17

import java.lang.reflect.Method;
   
class Test
   {
   public  static  void  main( String  args[] ) throws Exception
      {
      Example a = new Example();
   
      Class[] stringType = { Class.forName( "java.lang.String" ) };
   
      Object[] stringParameter = { "Monday" };
   
      Method tryMe;
   
      tryMe = a.getClass().getMethod( "getLunch", stringType );
   
      tryMe.invoke( a, stringParameter );
   
      }
   }

Doc 22, States Slide # 18

• Compact view of states and transitions

• Easy to add remove states

• Easy to modify transitions

• Language support varies

• Compile time checks are replaced by runtime check

Doc 22, States Slide # 19

Implementing a State Machine: Objects

Doc 22, States Slide # 20

Strawman Driver Program

class SPopServer 
   {
   public void processRequest(InputStream in, OutputStream out, 
      InetAddress clientAddress) throws IOException 
      {
   
      SPopState currentState = new NoAuth();
      do
         {
         ProtocolParser requestData = new ProtocolParser( in );
         String request = requestData.getCommand();
         if ( request.isPassword() )
            currentState = currentState.pass( request, this);
   
         else if ( request.isUser())
            currentState = currentState.user(this);
         etc.
            
            send response to client
         }
      while ( ! currentState instanceof Quit  );
      }
   }
Doc 22, States Slide # 21

public class SPopState {
   public SPopState quit( SPopServer parent) {
      return new Quit();
      }
   
   public SPopState pass( PopCommand clientRequest, SPopServer parent) 
      throws IllegalCommand {
      throw new IllegalCommand();
      }
   
   public SPopState user( PopCommand clientRequest, SPopServer parent) 
      throws IllegalCommand {
      throw new IllegalCommand();
      }
   
   public SPopState list( PopCommand clientRequest, SPopServer parent) 
      throws IllegalCommand {
      throw new IllegalCommand();
      }

Doc 22, States Slide # 22

public class NoAuth extends SPopState {
   public SPopState user( PopCommand clientRequest, SPopServer parent) {
      parent.setUser( clientRequest.getArgument() );
      parent.sendOKResponse();
      return new HaveUser();
   }
}
   
public class HaveUser extends SPopState {
   public SPopState pass( PopCommand clientRequest, SPopServer parent) {
      parent.setPassword( clientRequest.getArgument() );
      if ( parent.user&PasswordValid() ) {
         parent.sendOKResponse();
         return new Process();
      }
      else {
         parent.sendErrorResponse();
         return new NoAuth();
      }
   }
}
Doc 22, States Slide # 23

Smalltalk Example From VW 7 POP3 Client

• Pop3AuthorizationState
• Pop3TransactionState

Doc 22, States Slide # 24

Doc 22, States Slide # 25

Smalltalk.Net defineClass: #Pop3State
   superclass: #{Core.Object}
   private: false
   instanceVariableNames: ''
   classInstanceVariableNames: ''
   category: 'Net-Pop Rocks'
   
Net.Pop3State methodsFor: 'commands'
   
delete: message for: connection
   Pop3StateError raiseSignal
   
list: number for: connection 
   Pop3StateError raiseSignal
   
pass: aConnection 
   Pop3StateError raiseSignal
   
quit: aConnection 
   aConnection sendQuit
   
retrieveMessage: number for: connection 
   Pop3StateError raiseSignal
   
stat: aConnection 
   Pop3StateError raiseSignal
   
user: aConnection 
   Pop3StateError raiseSignal 

Doc 22, States Slide # 26

public class Pop3AuthorizationState extends Pop3State  {
   
   public void pass( Pop3Client aConnection) {
      aConnection.sendPassword();
   }
   
   public void user( Pop3Client aConnection) {
      aConnection.sendUser();
   }
}

Doc 22, States Slide # 27

Smalltalk.Net defineClass: #Pop3AuthorizationState
   superclass: #{Net.Pop3State}
   indexedType: #none
   private: false
   instanceVariableNames: ''
   classInstanceVariableNames: ''
   imports: ''
   category: 'Net-Pop Rocks'
   
Net.Pop3AuthorizationState methodsFor: 'commands' 
   
pass: aConnection 
   aConnection sendPassword
   
user: aConnection 
   aConnection sendUser 

Doc 22, States Slide # 28

public class Pop3TransactionState extends Pop3State  {
   
   public Object deleteFor( message, Pop3Client connection) {
      return connection.sendDeleteMessage(message);
   }
   
   public Object list( Pop3Client aConnection) {
      return aConnection.sendList();
   }
   
   public Object listFor( int number, Pop3Client connection) {
      return connection.sendList( number);
   }
   
   public Object retrieveMessageFor( int number, Pop3Client connection) {
      return connection.sendRetrieveMessage( number);
   }
   
   public Object stat( Pop3Client aConnection) {
      return aConnection.sendStat();
   }
   
   public void user( Pop3Client aConnection) {
      throw new Pop3StateError();
   }
}
Doc 22, States Slide # 29

Smalltalk.Net defineClass: #Pop3TransactionState
   superclass: #{Net.Pop3State}
   indexedType: #none
   private: false
   instanceVariableNames: ''
   classInstanceVariableNames: ''
   imports: ''
   category: 'Net-Pop Rocks'
   
Net.Pop3TransactionState methodsFor: 'commands'
   
delete: message for: connection
   ^connection sendDeleteMessage: message
   
list: aConnection
   ^aConnection sendList
   
list: number for: connection 
   ^connection sendList: number
   
retrieveMessage: number for: connection 
   ^connection sendRetrieveMessage: number
   
stat: aConnection
   ^aConnection sendStat

Doc 22, States Slide # 30

public class Pop3Client extends NetClient  {
   
   private NetUser user;      //Contains user name and password
   private String hostName;
   private int portNumber;
   private int retries;
   private Socket connection;
   private ReadAppendStream stream;
   private Pop3State state;
   private String serverResponse;

   public Pop3Client() {
      this.state( new Pop3State());
   }
   public Pop3State state() {
      return state;
   }
   public void state(Pop3State aState) {
      state = aState;
   }
many methods not shown

Doc 22, States Slide # 31

(Pop3Client)
   public boolean login() {
      this.state().user( this);
      if (this.hasPositiveResponse())
         return false;
      this.state().pass( this);
      if (this.hasPositiveResponse() )
         return false;
      this.state( new Pop3TransactionState());
      return true;
   }

(Pop3AuthorizationState)
   public void user( Pop3Client aConnection) {
      aConnection.sendUser();
   }

(Pop3Client)
   public void sendUser() {
      this.sendCommand("USER " , this.user().username());
      this.waitForResponse();
      if (!this.hasPositiveResponse() ) 
         throw new NetClientError();
   }

Doc 22, States Slide # 32

   public void sendCommand( aString) {
      this.log("C: ” , hostname() , “S: “ , aString);
      stream.write( aString , CRLF);
      stream.flush();
   }

   public void waitForResponse() 
      String result = stream.throughAll( CRLF);
      this.serverResponse( result);
   }

   public boolean hasPositiveResponse() {
      return (this.serverResponse().startsWith( "+OK" );
   }

   public void sendPassword() {
      this.sendCommand("PASS " , this.user().password());
      this.waitForResponse();
      if (!this.hasPositiveResponse() ) 
         throw new NetClientError();
   }

Doc 22, States Slide # 33

• Lots of little parts

• Algorithm distributed among different classes

• Easy to add new states

• Easy to change state transitions

• Each State class deals with one state

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