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#590 Add explanation for Chain of Responsibility

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Ilkka Seppälä 2017-09-19 23:20:40 +03:00
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126
chain/README.md
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@ -16,7 +16,131 @@ 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.
![alt text](./etc/chain_1.png "Chain of Responsibility")
## Explanation
Real world example
> The Orc King gives loud orders to his army. The closest one to react is the commander, then officer and then soldier. The commander, officer and soldier here form a chain of responsibility.
In plain words
> It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.
Wikipedia says
> 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.
**Programmatic Example**
Translating our example with orcs from above. First we have the request class
```
public class Request {
private final RequestType requestType;
private final String requestDescription;
private boolean handled;
public Request(final RequestType requestType, final String requestDescription) {
this.requestType = Objects.requireNonNull(requestType);
this.requestDescription = Objects.requireNonNull(requestDescription);
}
public String getRequestDescription() { return requestDescription; }
public RequestType getRequestType() { return requestType; }
public void markHandled() { this.handled = true; }
public boolean isHandled() { return this.handled; }
@Override
public String toString() { return getRequestDescription(); }
}
public enum RequestType {
DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX
}
```
Then the request handler hierarchy
```
public abstract class RequestHandler {
private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
private RequestHandler next;
public RequestHandler(RequestHandler next) {
this.next = next;
}
public void handleRequest(Request req) {
if (next != null) {
next.handleRequest(req);
}
}
protected void printHandling(Request req) {
LOGGER.info("{} handling request \"{}\"", this, req);
}
@Override
public abstract String toString();
}
public class OrcCommander extends RequestHandler {
public OrcCommander(RequestHandler handler) {
super(handler);
}
@Override
public void handleRequest(Request req) {
if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
printHandling(req);
req.markHandled();
} else {
super.handleRequest(req);
}
}
@Override
public String toString() {
return "Orc commander";
}
}
// OrcOfficer and OrcSoldier are defined similarly as OrcCommander
```
Then we have the Orc King who gives the orders and forms the chain
```
public class OrcKing {
RequestHandler chain;
public OrcKing() {
buildChain();
}
private void buildChain() {
chain = new OrcCommander(new OrcOfficer(new OrcSoldier(null)));
}
public void makeRequest(Request req) {
chain.handleRequest(req);
}
}
```
Then it is used as follows
```
OrcKing king = new OrcKing();
king.makeRequest(new Request(RequestType.DEFEND_CASTLE, "defend castle")); // Orc commander handling request "defend castle"
king.makeRequest(new Request(RequestType.TORTURE_PRISONER, "torture prisoner")); // Orc officer handling request "torture prisoner"
king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax")); // Orc soldier handling request "collect tax"
```
## Applicability
Use Chain of Responsibility when

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109
chain/etc/chain.ucls
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@ -1,109 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
realizations="true" associations="true" dependencies="false" nesting-relationships="true">
<class id="1" language="java" name="com.iluwatar.chain.Request" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/Request.java" binary="false" corner="BOTTOM_RIGHT">
<position height="196" width="228" x="168" y="182"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="2" language="java" name="com.iluwatar.chain.OrcOfficer" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/OrcOfficer.java" binary="false" corner="BOTTOM_RIGHT">
<position height="124" width="194" x="168" y="609"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="3" language="java" name="com.iluwatar.chain.OrcCommander" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/OrcCommander.java" binary="false" corner="BOTTOM_RIGHT">
<position height="124" width="217" x="402" y="609"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="4" language="java" name="com.iluwatar.chain.RequestHandler" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/RequestHandler.java" binary="false" corner="BOTTOM_RIGHT">
<position height="141" width="218" x="451" y="418"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="5" language="java" name="com.iluwatar.chain.OrcSoldier" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/OrcSoldier.java" binary="false" corner="BOTTOM_RIGHT">
<position height="124" width="194" x="659" y="609"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="6" language="java" name="com.iluwatar.chain.OrcKing" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/OrcKing.java" binary="false" corner="BOTTOM_RIGHT">
<position height="124" width="188" x="451" y="182"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<enumeration id="7" language="java" name="com.iluwatar.chain.RequestType" project="chain"
file="/chain/src/main/java/com/iluwatar/chain/RequestType.java" binary="false" corner="BOTTOM_RIGHT">
<position height="142" width="243" x="168" y="418"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</enumeration>
<association id="8">
<end type="SOURCE" refId="6" navigable="false">
<attribute id="9" name="chain"/>
<multiplicity id="10" minimum="0" maximum="1"/>
</end>
<end type="TARGET" refId="4" navigable="true"/>
<display labels="true" multiplicity="true"/>
</association>
<association id="11">
<end type="SOURCE" refId="4" navigable="false">
<attribute id="12" name="next"/>
<multiplicity id="13" minimum="0" maximum="1"/>
</end>
<end type="TARGET" refId="4" navigable="true"/>
<display labels="true" multiplicity="true"/>
</association>
<generalization id="14">
<end type="SOURCE" refId="5"/>
<end type="TARGET" refId="4"/>
</generalization>
<association id="15">
<end type="SOURCE" refId="1" navigable="false">
<attribute id="16" name="requestType"/>
<multiplicity id="17" minimum="0" maximum="1"/>
</end>
<end type="TARGET" refId="7" navigable="true"/>
<display labels="true" multiplicity="true"/>
</association>
<generalization id="18">
<end type="SOURCE" refId="2"/>
<end type="TARGET" refId="4"/>
</generalization>
<generalization id="19">
<end type="SOURCE" refId="3"/>
<end type="TARGET" refId="4"/>
</generalization>
<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</classifier-display>
<association-display labels="true" multiplicity="true"/>
</class-diagram>

61
chain/etc/chain.urm.puml
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@ -1,61 +0,0 @@
@startuml
package com.iluwatar.chain {
class App {
+ App()
+ main(args : String[]) {static}
}
class OrcCommander {
+ OrcCommander(handler : RequestHandler)
+ handleRequest(req : Request)
+ toString() : String
}
class OrcKing {
~ chain : RequestHandler
+ OrcKing()
- buildChain()
+ makeRequest(req : Request)
}
class OrcOfficer {
+ OrcOfficer(handler : RequestHandler)
+ handleRequest(req : Request)
+ toString() : String
}
class OrcSoldier {
+ OrcSoldier(handler : RequestHandler)
+ handleRequest(req : Request)
+ toString() : String
}
class Request {
- handled : boolean
- requestDescription : String
- requestType : RequestType
+ Request(requestType : RequestType, requestDescription : String)
+ getRequestDescription() : String
+ getRequestType() : RequestType
+ isHandled() : boolean
+ markHandled()
+ toString() : String
}
abstract class RequestHandler {
- LOGGER : Logger {static}
- next : RequestHandler
+ RequestHandler(next : RequestHandler)
+ handleRequest(req : Request)
# printHandling(req : Request)
+ toString() : String {abstract}
}
enum RequestType {
+ COLLECT_TAX {static}
+ DEFEND_CASTLE {static}
+ TORTURE_PRISONER {static}
+ valueOf(name : String) : RequestType {static}
+ values() : RequestType[] {static}
}
}
RequestHandler --> "-next" RequestHandler
Request --> "-requestType" RequestType
OrcKing --> "-chain" RequestHandler
OrcCommander --|> RequestHandler
OrcOfficer --|> RequestHandler
OrcSoldier --|> RequestHandler
@enduml

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2
pom.xml
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@ -473,6 +473,8 @@
<param>decorator</param>
<param>facade</param>
<param>flyweight</param>
<param>proxy</param>
<param>chain</param>
</skipForProjects>
</configuration>
</plugin>

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proxy/etc/proxy.ucls
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@ -1,72 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<class-diagram version="1.1.11" icons="true" automaticImage="PNG" always-add-relationships="false"
generalizations="true" realizations="true" associations="true" dependencies="false" nesting-relationships="true"
router="FAN">
<class id="1" language="java" name="com.iluwatar.proxy.WizardTowerProxy" project="proxy"
file="/proxy/src/main/java/com/iluwatar/proxy/WizardTowerProxy.java" binary="false" corner="BOTTOM_RIGHT">
<position height="149" width="191" x="388" y="271"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="2" language="java" name="com.iluwatar.proxy.Wizard" project="proxy"
file="/proxy/src/main/java/com/iluwatar/proxy/Wizard.java" binary="false" corner="BOTTOM_RIGHT">
<position height="113" width="102" x="619" y="271"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<interface id="3" language="java" name="com.iluwatar.proxy.WizardTower" project="proxy"
file="/proxy/src/main/java/com/iluwatar/proxy/WizardTower.java" binary="false" corner="BOTTOM_RIGHT">
<position height="77" width="116" x="388" y="460"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</interface>
<class id="4" language="java" name="com.iluwatar.proxy.IvoryTower" project="proxy"
file="/proxy/src/main/java/com/iluwatar/proxy/IvoryTower.java" binary="false" corner="BOTTOM_RIGHT">
<position height="113" width="116" x="761" y="271"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="5" language="java" name="com.iluwatar.proxy.App" project="proxy"
file="/proxy/src/main/java/com/iluwatar/proxy/App.java" binary="false" corner="BOTTOM_RIGHT">
<position height="95" width="114" x="917" y="271"/>
<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<association id="6">
<end type="SOURCE" refId="1" navigable="false">
<attribute id="7" name="tower"/>
<multiplicity id="8" minimum="0" maximum="1"/>
</end>
<end type="TARGET" refId="3" navigable="true"/>
<display labels="true" multiplicity="true"/>
</association>
<realization id="9">
<end type="SOURCE" refId="4"/>
<end type="TARGET" refId="3"/>
</realization>
<realization id="10">
<end type="SOURCE" refId="1"/>
<end type="TARGET" refId="3"/>
</realization>
<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</classifier-display>
<association-display labels="true" multiplicity="true"/>
</class-diagram>

32
proxy/etc/proxy.urm.puml
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@ -1,32 +0,0 @@
@startuml
package com.iluwatar.proxy {
class App {
+ App()
+ main(args : String[]) {static}
}
class IvoryTower {
- LOGGER : Logger {static}
+ IvoryTower()
+ enter(wizard : Wizard)
}
class Wizard {
- name : String
+ Wizard(name : String)
+ toString() : String
}
interface WizardTower {
+ enter(Wizard) {abstract}
}
class WizardTowerProxy {
- LOGGER : Logger {static}
- NUM_WIZARDS_ALLOWED : int {static}
- numWizards : int
- tower : WizardTower
+ WizardTowerProxy(tower : WizardTower)
+ enter(wizard : Wizard)
}
}
WizardTowerProxy --> "-tower" WizardTower
IvoryTower ..|> WizardTower
WizardTowerProxy ..|> WizardTower
@enduml