Design Patterns Category

Design Patterns Index

13. Chain of Responsibility

• Intent:
  • Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an objecthandles it.
• Applicability:
  • more than one object may handle a request, and the handler isn’t known a priori. The handler should be ascertained automatically
  • you want to issue a request to one of several objects without specifying the receiver explicitly.
  • the set of objects that can handle a request should be specified dynamically.
• Structure:

• Participants:
  • Handler
    • defines an interface for handling requests.
    • (optional) implements the successor link.
  • ConcreteHandler
    • handles requests it is responsible for.
    • can access its successor.
    • if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor.
  • Client
    • initiates the request to a ConcreteHandler object on the chain.
• Collaborations
  • When a client issues a request, the request propagates along the chain until a ConcreteHandler object takes responsibility for handling it.
• Consequences:
  • 1.Reduced coupling.
  • 2.Added flexibility in assigning responsibilities to objects.
  • 3.Receipt isn’t guaranteed.
• Related Patterns:
  • Chain of Responsibility is often applied in conjunction with Composite. There, a component’s parent can act as its successor.
• Code Sample: Chain of Responsibility

	public interface Handler {  
	    public void operator();  
	} 

	public abstract class AbstractHandler {  
	      
	    private Handler handler;  
	  
	    public Handler getHandler() {  
	        return handler;  
	    }  
	  
	    public void setHandler(Handler handler) {  
	        this.handler = handler;  
	    }  
	      
	}

	public class MyHandler extends AbstractHandler implements Handler {  
	  
	    private String name;  
	  
	    public MyHandler(String name) {  
	        this.name = name;  
	    }  
	  
	    @Override  
	    public void operator() {  
	        System.out.println(name+" test");  
	        if(getHandler()!=null){  
	            getHandler().operator();  
	        }  
	    }  
	}  

	public class Test {  
	  
	    public static void main(String[] args) {  
	        MyHandler h1 = new MyHandler("h1");  
	        MyHandler h2 = new MyHandler("h2");  
	        MyHandler h3 = new MyHandler("h3");  
	  
	        h1.setHandler(h2);  
	        h2.setHandler(h3);  
	  
	        h1.operator();  
	    }  
	} 	

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