Download the working example demo code in java from my GIT repository –https://github.com/premaseem/designPatterns/tree/4bb9beca7bab5b5e71d02b76e4f1ad48fce4aca6/ZipDownloadableProjects
Example Code Description :
1. Number Handler : Client passes a request (signed number ) to first node of change. Positive number handler will see if number is positive it will print it else pass it to next handler which is Zero number handlers, which in return do the same and pass it to negative number handler.
2. ATM Money dispatcher : Client would enter the amount ex $576/- to first node which might have different handlers for each denomination like 500 dollar bill handler, 100, 50, 10, 5, 2 and 1 dollar bill handers and finally dispatch the money.
3. Log Handler : Client would enter a message and set the logging level based on which all the consoles would get notifications. In this example the chain is set, however the first node / handler is decided dynamically based on the log level (to skip unused nodes in the chain)
Download example code in zipped format : https://drive.google.com/file/d/0B-9WtmpPhI4ZYVNIc2hOcE5PSkU/edit?usp=sharing
- 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 object handles it.
- Launch-and-leave requests with a single processing pipeline that contains many possible handlers.
- An object-oriented linked list with recursive traversal.
Highlights of Chain of Responsibility
- Sender will not know which object in the chain will serve its request.
- Every node in chain will have the responsibility to decide, if they can serve the request.
- If node decides to forward the request, it should be capable of choosing the next node and forward it.
- There is a possibility where none of the node may serve the request.
There is a potentially variable number of “handler” or “processing element” or “node” objects, and a stream of requests that must be handled. Need to efficiently process the requests without hard-wiring handler relationships and precedence, or request-to-handler mappings.
Encapsulate the processing elements inside a “pipeline” abstraction; and have clients “launch and leave” their requests at the entrance to the pipeline.
The pattern chains the receiving objects together, and then passes any request messages from object to object until it reaches an object capable of handling the message. The number and type of handler objects isn’t known a priori, they can be configured dynamically. The chaining mechanism uses recursive composition to allow an unlimited number of handlers to be linked.
Chain of Responsibility simplifies object interconnections. Instead of senders and receivers maintaining references to all candidate receivers, each sender keeps a single reference to the head of the chain, and each receiver keeps a single reference to its immediate successor in the chain.
Make sure there exists a “safety net” to “catch” any requests which go unhandled.
Do not use Chain of Responsibility when each request is only handled by one handler, or, when the client object knows which service object should handle the request.
The derived classes know how to satisfy Client requests. If the “current” object is not available or sufficient, then it delegates to the base class, which delegates to the “next” object, and the circle of life continues.
Multiple handlers could contribute to the handling of each request. The request can be passed down the entire length of the chain, with the last link being careful not to delegate to a “null next”.
The Chain of Responsibility pattern avoids coupling the sender of a request to the receiver by giving more than one object a chance to handle the request. ATM use the Chain of Responsibility in money giving mechanism.
- The base class maintains a “next” pointer.
- Each derived class implements its contribution for handling the request.
- If the request needs to be “passed on”, then the derived class “calls back” to the base class, which delegates to the “next” pointer.
- The client (or some third party) creates and links the chain (which may include a link from the last node to the root node).
- The client “launches and leaves” each request with the root of the chain.
- Recursive delegation produces the illusion of magic.
Rules of thumb
- Chain of Responsibility, Command, Mediator, and Observer, address how you can decouple senders and receivers, but with different trade-offs. Chain of Responsibility passes a sender request along a chain of potential receivers.
- Chain of Responsibility can use Command to represent requests as objects.
- Chain of Responsibility is often applied in conjunction with Composite. There, a component’s parent can act as its successor.