Chain of Responsibility Pattern Example

The following Java code illustrates the pattern with the example of a logging class. Each logging handler decides if any action is to be taken at this log level and then passes the message on to the next logging handler. The output is:

  Writing to stdout:   Entering function y.
  Writing to stdout:   Step1 completed.
  Sending via e-mail:  Step1 completed.
  Writing to stdout:   An error has occurred.
  Sending via e-mail:  An error has occurred.
  Writing to stderr:   An error has occurred.

Note that this example should not be seen as a recommendation on how to write logging classes.

Also, note that in a 'pure' implementation of the chain of responsibility pattern, a logger would not pass responsibility further down the chain after handling a message. In this example, a message will be passed down the chain whether it is handled or not.

import java.util.*;
 
abstract class Logger 
{
    public static int ERR = 3;
    public static int NOTICE = 5;
    public static int DEBUG = 7;
    protected int mask;
 
    // The next element in the chain of responsibility
    protected Logger next;
    public Logger setNext( Logger l )
    {
        next = l;
        return l;
    }
 
    public void message( String msg, int priority )
    {
        if ( priority <= mask ) 
        {
            writeMessage( msg );
        }
        if ( next != null )
        {
            next.message( msg, priority );
        }
    }
 
    abstract protected void writeMessage( String msg );
 
}
 
class StdoutLogger extends Logger 
{
 
    public StdoutLogger( int mask ) { this.mask = mask; }
 
    protected void writeMessage( String msg )
    {
        System.out.println( "Writing to stdout: " + msg );
    }
}
 
 
class EmailLogger extends Logger 
{
 
    public EmailLogger( int mask ) { this.mask = mask; }
 
    protected void writeMessage( String msg )
    {
        System.out.println( "Sending via email: " + msg );
    }
}
 
class StderrLogger extends Logger 
{
 
    public StderrLogger( int mask ) { this.mask = mask; }
 
    protected void writeMessage( String msg )
    {
        System.err.println( "Sending to stderr: " + msg );
    }
}
 
public class ChainOfResponsibilityExample
{
    public static void main( String[] args )
    {
        // Build the chain of responsibility
        Logger l,l1;
        l1 = l = new StdoutLogger( Logger.DEBUG );
        l1 = l1.setNext(new EmailLogger( Logger.NOTICE ));
        l1 = l1.setNext(new StderrLogger( Logger.ERR ));
 
        // Handled by StdoutLogger
        l.message( "Entering function y.", Logger.DEBUG );
 
        // Handled by StdoutLogger and EmailLogger
        l.message( "Step1 completed.", Logger.NOTICE );
 
        // Handled by all three loggers
        l.message( "An error has occurred.", Logger.ERR );
    }
}

Here another example of this pattern in java. In this example we have different roles, each having a fix purchase power limit and a successor. Everytime a user in a role receives a purchase request, when it's over his limit, he just passes that request to his successor.

The PurchasePower abstract class with the abstract method processRequest.

import java.io.*;
abstract class PurchasePower {
 
    protected final double base = 500;
    protected PurchasePower successor;
 
    public void setSuccessor(PurchasePower successor){
        this.successor = successor;
    }
 
    abstract public void processRequest(PurchaseRequest request);
}

Four implementations of the abstract class above: Manager, Director, Vice President, President

class ManagerPPower extends PurchasePower {
    private final double ALLOWABLE = 10 * base;
 
    public void processRequest(PurchaseRequest request ) {
        if( request.getAmount() < ALLOWABLE )
            System.out.println("Manager will approve $"+ request.getAmount());
        else
           if( successor != null)
               successor.processRequest(request);
  }
}
 
class DirectorPPower extends PurchasePower {
    private final double ALLOWABLE = 20 * base;
 
    public void processRequest(PurchaseRequest request ) {
        if( request.getAmount() < ALLOWABLE )
            System.out.println("Director will approve $"+ request.getAmount());
        else
           if( successor != null)
               successor.processRequest(request);
  }
}
 
class VicePresidentPPower extends PurchasePower {
    private final double ALLOWABLE = 40 * base;
 
    public void processRequest(PurchaseRequest request) {
        if( request.getAmount() < ALLOWABLE )
            System.out.println("Vice President will approve $" + request.getAmount());
        else
        if( successor != null )
            successor.processRequest(request);
  }
}
 
class PresidentPPower extends PurchasePower {
    private final double ALLOWABLE = 60 * base;
 
    public void processRequest(PurchaseRequest request){
        if( request.getAmount() < ALLOWABLE )
            System.out.println("President will approve $" + request.getAmount());
        else
            System.out.println( "Your request for $" + request.getAmount() + " needs a board meeting!");
    }
}

The PurchaseRequest class with its Getter methods which keeps the request data in this example.

class PurchaseRequest {
 
    private int number;
    private double amount;
    private String purpose;
 
    public PurchaseRequest(int number, double amount, String purpose){
        this.number = number;
        this.amount = amount;
        this.purpose = purpose;
    }
 
    public double getAmount() {
        return amount;
    }
    public void setAmount(double amt){
        amount = amt;
    }
 
    public String getPurpose() {
        return purpose;
    }
    public void setPurpose(String reason) {
        purpose = reason;
    }
 
    public int getNumber(){
        return number;
    }
    public void setNumber(int num) {
        number = num;
     }   
}


And here a usage example, the successors are set like this: Manager -> Director -> Vice President -> President

class CheckAuthority {
    public static void main(String[] args){
        ManagerPPower manager = new ManagerPPower();
        DirectorPPower director = new DirectorPPower();
        VicePresidentPPower vp = new VicePresidentPPower();
        PresidentPPower president = new PresidentPPower();
        manager.setSuccessor(director);
        director.setSuccessor(vp);
        vp.setSuccessor(president);
 
        //enter ctrl+c to kill.
        try{
            while (true) {
                System.out.println("Enter the amount to check who should approve your expenditure.");
                System.out.print(">");
                double d = Double.parseDouble(new BufferedReader(new InputStreamReader(System.in)).readLine());
                manager.processRequest(new PurchaseRequest(0, d, "General"));
           }
        }catch(Exception e){
            System.exit(1);
        }  
  }
}


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