Chain-of-responsibility pattern
In Object Oriented Design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain. A mechanism also exists for adding new processing objects to the end of this chain.
In a variation of the standard chain-of-responsibility model, some handlers may act as dispatchers, capable of sending commands out in a variety of directions, forming a tree of responsibility. In some cases, this can occur recursively, with processing objects calling higher-up processing objects with commands that attempt to solve some smaller part of the problem; in this case recursion continues until the command is processed, or the entire tree has been explored. An XML interpreter might work in this manner.
This pattern promotes the idea of loose coupling, which is considered a programming best practice.
Example
The following 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. 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.
Java
package chainofresp;
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 log) {
next = log;
return log;
}
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 logger, logger1,logger2;
logger = new StdoutLogger(Logger.DEBUG);
logger1 = logger.setNext(new EmailLogger(Logger.NOTICE));
logger2 = logger1.setNext(new StderrLogger(Logger.ERR));
// Handled by StdoutLogger
logger.message("Entering function y.", Logger.DEBUG);
// Handled by StdoutLogger and EmailLogger
logger.message("Step1 completed.", Logger.NOTICE);
// Handled by all three loggers
logger.message("An error has occurred.", Logger.ERR);
}
}
/*
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.
*/
C#
using System;
using System.IO;
namespace ChainOfResponsibility
{
public enum LogLevel
{
Info=1,
Debug=2,
Warning=4,
Error=8,
FunctionalMessage=16,
FunctionalError=32,
All = 63
}
/// <summary>
/// Abstract Handler in chain of responsibility Pattern
/// </summary>
public abstract class Logger
{
protected LogLevel logMask;
// The next Handler in the chain
protected Logger next;
public Logger(LogLevel mask)
{
this.logMask = mask;
}
/// <summary>
/// Sets the Next logger to make a list/chain of Handlers
/// </summary>
public Logger SetNext(Logger nextlogger)
{
next = nextlogger;
return nextlogger;
}
public void Message(string msg, LogLevel severity)
{
if ((severity & logMask) != 0)
{
WriteMessage(msg);
}
if (next != null)
{
next.Message(msg, severity);
}
}
abstract protected void WriteMessage(string msg);
}
public class ConsoleLogger : Logger
{
public ConsoleLogger(LogLevel mask)
: base(mask)
{ }
protected override void WriteMessage(string msg)
{
Console.WriteLine("Writing to console: " + msg);
}
}
public class EmailLogger : Logger
{
public EmailLogger(LogLevel mask)
: base(mask)
{ }
protected override void WriteMessage(string msg)
{
//Placeholder for mail send logic, usually the email configurations are saved in config file.
Console.WriteLine("Sending via email: " + msg);
}
}
class FileLogger : Logger
{
public FileLogger(LogLevel mask)
: base(mask)
{ }
protected override void WriteMessage(string msg)
{
//Placeholder for File writing logic
Console.WriteLine("Writing to Log File: " + msg);
}
}
public class Program
{
public static void Main(string[] args)
{
// Build the chain of responsibility
Logger logger, logger1, logger2;
logger1 = logger = new ConsoleLogger(LogLevel.All);
logger1 = logger1.SetNext(new EmailLogger(LogLevel.FunctionalMessage | LogLevel.FunctionalError));
logger2 = logger1.SetNext(new FileLogger(LogLevel.Warning | LogLevel.Error));
// Handled by ConsoleLogger
logger.Message("Entering function ProcessOrder().", LogLevel.Debug);
logger.Message("Order record retrieved.", LogLevel.Info);
// Handled by ConsoleLogger and FileLogger
logger.Message("Customer Address details missing in Branch DataBase.", LogLevel.Warning);
logger.Message("Customer Address details missing in Organization DataBase.", LogLevel.Error);
// Handled by ConsoleLogger and EmailLogger
logger.Message("Unable to Process Order ORD1 Dated D1 For Customer C1.", LogLevel.FunctionalError);
// Handled by ConsoleLogger and EmailLogger
logger.Message("Order Dispatched.", LogLevel.FunctionalMessage);
}
}
}
/* Output
Writing to console: Entering function ProcessOrder().
Writing to console: Order record retrieved.
Writing to console: Customer Address details missing in Branch DataBase.
Writing to Log File: Customer Address details missing in Branch DataBase.
Writing to console: Customer Address details missing in Organization DataBase.
Writing to Log File: Customer Address details missing in Organization DataBase.
Writing to console: Unable to Process Order ORD1 Dated D1 For Customer C1.
Sending via email: Unable to Process Order ORD1 Dated D1 For Customer C1.
Writing to console: Order Dispatched.
Sending via email: Order Dispatched.
*/
Another Example
Below is another example of this pattern in Java. In this example we have different roles, each having a fixed purchasing limit and a successor. Every time a user in a role receives a purchase request that exceeds his or her limit, the request is passed to his or her successor.
The PurchasePower abstract class with the abstract method processRequest.
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 following code defines the PurchaseRequest class that 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;
}
}
In the following usage example, the successors are set as follows: 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);
// Press Ctrl+C to end.
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);
}
}
}
See also
External links
- Article "The Chain of Responsibility pattern's pitfalls and improvements" by Michael Xinsheng Huang
- Article "Follow the Chain of Responsibility" by David Geary
- Article "Pattern Summaries: Chain of Responsibility" by Mark Grand
- Behavioral Patterns - Chain of Responsibility Pattern
- Descriptions from Portland Pattern Repository
- Apache Jakarta Commons Chain
- PerfectJPattern Open Source Project, Provides a context-free and type-safe implementation of the Chain of Responsibility Pattern in Java
- Chain.NET(NChain) - Ready-to-use, generic and lightweight implementation of the Chain of Responsibility pattern for .NET and Mono
- Chain of Responsibility Design Pattern - An Example
- Sourcemaking Tutorial