task: Explanations and grammar fixes for all the GoF patterns (#1791)
* Grammatical fixes to command pattern * Update bridge pattern readme * Fixes to builder pattern grammar * Update chain of responsibility * Improvements to the composite example * Fixes to headings * Minor updates to decorator pattern * Update facade * Update factory example * Update factory method * Update flyweight * Interpreter explanation * Update iterator readme * Add explanation for mediator pattern * Grammatical fixes to memento * Grammar fixes for observer * Update explanation for the prototype pattern * Proxy pattern grammar fixes * Update singleton * Grammar fixes to state pattern * Grammar fixes for strategy * Grammar fixes, template method * Grammar fixes for visitor * Fix typo
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Ilkka Seppälä
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chain-of-responsibility/README.md
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chain-of-responsibility/README.md
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---
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layout: pattern
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title: Chain of responsibility
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folder: chain-of-responsibility
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permalink: /patterns/chain-of-responsibility/
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categories: Behavioral
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language: en
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tags:
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- Gang of Four
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---
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## Intent
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Avoid coupling the sender of a request to its receiver by giving more than one object a chance to
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handle the request. Chain the receiving objects and pass the request along the chain until an object
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handles it.
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## Explanation
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Real-world example
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> The Orc King gives loud orders to his army. The closest one to react is the commander, then
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> an officer, and then a soldier. The commander, officer, and soldier form a chain of responsibility.
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In plain words
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> It helps to build a chain of objects. A request enters from one end and keeps going from an object
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> to another until it finds a suitable handler.
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Wikipedia says
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> In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of
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> a source of command objects and a series of processing objects. Each processing object contains
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> logic that defines the types of command objects that it can handle; the rest are passed to the
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> next processing object in the chain.
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**Programmatic Example**
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Translating our example with the orcs from above. First, we have the `Request` class:
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```java
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public class Request {
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private final RequestType requestType;
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private final String requestDescription;
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private boolean handled;
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public Request(final RequestType requestType, final String requestDescription) {
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this.requestType = Objects.requireNonNull(requestType);
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this.requestDescription = Objects.requireNonNull(requestDescription);
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}
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public String getRequestDescription() { return requestDescription; }
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public RequestType getRequestType() { return requestType; }
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public void markHandled() { this.handled = true; }
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public boolean isHandled() { return this.handled; }
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@Override
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public String toString() { return getRequestDescription(); }
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}
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public enum RequestType {
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DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX
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}
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```
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Next, we show the request handler hierarchy.
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```java
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@Slf4j
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public abstract class RequestHandler {
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private final RequestHandler next;
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public RequestHandler(RequestHandler next) {
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this.next = next;
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}
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public void handleRequest(Request req) {
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if (next != null) {
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next.handleRequest(req);
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}
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}
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protected void printHandling(Request req) {
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LOGGER.info("{} handling request \"{}\"", this, req);
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}
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@Override
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public abstract String toString();
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}
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public class OrcCommander extends RequestHandler {
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public OrcCommander(RequestHandler handler) {
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super(handler);
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}
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@Override
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public void handleRequest(Request req) {
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if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
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printHandling(req);
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req.markHandled();
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} else {
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super.handleRequest(req);
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}
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}
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@Override
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public String toString() {
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return "Orc commander";
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}
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}
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// OrcOfficer and OrcSoldier are defined similarly as OrcCommander
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```
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The Orc King gives the orders and forms the chain.
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```java
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public class OrcKing {
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RequestHandler chain;
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public OrcKing() {
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buildChain();
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}
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private void buildChain() {
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chain = new OrcCommander(new OrcOfficer(new OrcSoldier(null)));
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}
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public void makeRequest(Request req) {
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chain.handleRequest(req);
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}
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}
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```
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The chain of responsibility in action.
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```java
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var king = new OrcKing();
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king.makeRequest(new Request(RequestType.DEFEND_CASTLE, "defend castle"));
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king.makeRequest(new Request(RequestType.TORTURE_PRISONER, "torture prisoner"));
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king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax"));
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```
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The console output.
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```
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Orc commander handling request "defend castle"
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Orc officer handling request "torture prisoner"
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Orc soldier handling request "collect tax"
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```
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## Class diagram
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## Applicability
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Use Chain of Responsibility when
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* More than one object may handle a request, and the handler isn't known a priori. The handler should be ascertained automatically.
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* You want to issue a request to one of several objects without specifying the receiver explicitly.
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* The set of objects that can handle a request should be specified dynamically.
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## Known uses
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* [java.util.logging.Logger#log()](http://docs.oracle.com/javase/8/docs/api/java/util/logging/Logger.html#log%28java.util.logging.Level,%20java.lang.String%29)
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* [Apache Commons Chain](https://commons.apache.org/proper/commons-chain/index.html)
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* [javax.servlet.Filter#doFilter()](http://docs.oracle.com/javaee/7/api/javax/servlet/Filter.html#doFilter-javax.servlet.ServletRequest-javax.servlet.ServletResponse-javax.servlet.FilterChain-)
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## Credits
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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