Merge pull request iluwatar#258 from amit2103/master

MonoState Pattern

Author: iluwatar

monostate/etc/MonoState.ucls

@@ -0,0 +1,53 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.monostate.LoadBalancer" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="187" width="158" x="100" y="66"/>    
+    <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.monostate.Server" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/Server.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="176" y="372"/>    
+    <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.monostate.Request" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/Request.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="329" y="352"/>    
+    <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>  
+  <dependency id="4">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="3"/>  
+  </dependency>  
+  <dependency id="5">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="3"/>  
+  </dependency>  
+  <association id="6">    
+    <end type="SOURCE" refId="1" navigable="false">      
+      <attribute id="7" name="servers"/>      
+      <multiplicity id="8" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="2" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <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>

monostate/etc/monostate.png

@@ -0,0 +1,28 @@
+---
+layout: pattern
+title: MonoState
+folder: monostate
+permalink: /patterns/monostate/
+categories: Creational
+tags: Java
+---
+
+**Intent:** Enforces a behaviour like sharing the same state amongst all instances.
+
+![alt text](./etc/monostate.png "MonoState")
+
+**Applicability:** Use the Monostate pattern when
+
+* The same state must be shared across all instances of a class.
+* Typically this pattern might be used everywhere a Singleton might be used. Singleton usage however is not transparent, Monostate usage is.
+* Monostate has one major advantage over singleton. The subclasses might decorate the shared state as they wish and hence can provide dynamically different behaviour than the base class.
+
+**Typical Use Case:**
+
+* the logging class
+* managing a connection to a database
+* file manager
+
+**Real world examples:**
+
+Yet to see this.

monostate/index.md

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.7.0</version>
+  </parent>
+  <artifactId>monostate</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

monostate/pom.xml

@@ -0,0 +1,35 @@
+package com.iluwatar.monostate;
+
+
+
+/**
+ * 
+ * The MonoState pattern ensures that all instances of the class will have the same state. This can
+ * be used a direct replacement of the Singleton pattern.
+ * 
+ * <p>
+ * In the following example, The {@link LoadBalancer} class represents the app's logic. It contains
+ * a series of Servers, which can handle requests of type {@link Request}. Two instances of
+ * LoadBalacer are created. When a request is made to a server via the first LoadBalancer the state
+ * change in the first load balancer affects the second. So if the first LoadBalancer selects the
+ * Server 1, the second LoadBalancer on a new request will select the Second server. If a third
+ * LoadBalancer is created and a new request is made to it, then it will select the third server as
+ * the second load balancer has already selected the second server.
+ * <p>
+ * .
+ * 
+ */
+public class App {
+  /**
+   * Program entry point
+   * 
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+    LoadBalancer loadBalancer1 = new LoadBalancer();
+    LoadBalancer loadBalancer2 = new LoadBalancer();
+    loadBalancer1.serverequest(new Request("Hello"));
+    loadBalancer2.serverequest(new Request("Hello World"));
+  }
+
+}

monostate/src/main/java/com/iluwatar/monostate/App.java

@@ -0,0 +1,52 @@
+package com.iluwatar.monostate;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * The LoadBalancer class. This implements the MonoState pattern. It holds a series of servers. Upon
+ * receiving a new Request, it delegates the call to the servers in a Round Robin Fashion. Since all
+ * instances of the class share the same state, all instances will delegate to the same server on
+ * receiving a new Request.
+ * 
+ */
+
+public class LoadBalancer {
+  private static List<Server> servers = new ArrayList<>();
+  private static int id = 0;
+  private static int lastServedId = 0;
+
+  static {
+    servers.add(new Server("localhost", 8081, ++id));
+    servers.add(new Server("localhost", 8080, ++id));
+    servers.add(new Server("localhost", 8082, ++id));
+    servers.add(new Server("localhost", 8083, ++id));
+    servers.add(new Server("localhost", 8084, ++id));
+  }
+
+  public final void addServer(Server server) {
+    synchronized (servers) {
+      servers.add(server);
+    }
+
+  }
+
+  public final int getNoOfServers() {
+    return servers.size();
+  }
+
+  public static int getLastServedId() {
+    return lastServedId;
+  }
+
+  public void serverequest(Request request) {
+    if (lastServedId >= servers.size()) {
+      lastServedId = 0;
+    }
+    Server server = servers.get(lastServedId++);
+    server.serve(request);
+  }
+  
+
+
+}

monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java

@@ -0,0 +1,15 @@
+package com.iluwatar.monostate;
+
+/**
+ * 
+ * The Request class. A {@link Server} can handle an instance of a Request. 
+ * 
+ */
+
+public class Request {
+  public final String value;
+
+  public Request(String value) {
+    this.value = value;
+  }
+}

monostate/src/main/java/com/iluwatar/monostate/Request.java

@@ -0,0 +1,31 @@
+package com.iluwatar.monostate;
+
+/**
+ * 
+ * The Server class. Each Server sits behind a LoadBalancer which delegates the call to the 
+ * servers in a simplistic Round Robin fashion.
+ *
+ */
+public class Server {
+  public final String host;
+  public final int port;
+  public final int id;
+
+  public Server(String host, int port, int id) {
+    this.host = host;
+    this.port = port;
+    this.id = id;
+  }
+
+  public String getHost() {
+    return host;
+  }
+
+  public int getPort() {
+    return port;
+  }
+
+  public final void serve(Request request) {
+    System.out.println("Server ID " + id + " associated to host : " + getHost() + " and Port " + getPort() +" Processed request with value  " + request.value);
+  }
+}

monostate/src/main/java/com/iluwatar/monostate/Server.java

@@ -0,0 +1,26 @@
+package com.iluwatar.monostate;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class AppTest {
+
+  @Test
+  public void testSameStateAmonstAllInstances() {
+    LoadBalancer balancer = new LoadBalancer();
+    LoadBalancer balancer2 = new LoadBalancer();
+    balancer.addServer(new Server("localhost", 8085, 6));
+    // Both should have the same number of servers.
+    Assert.assertTrue(balancer.getNoOfServers() == balancer2.getNoOfServers());
+    // Both Should have the same LastServedId
+    Assert.assertTrue(balancer.getLastServedId() == balancer2.getLastServedId());
+  }
+  
+  @Test
+  public void testMain() {
+    String[] args = {};
+    App.main(args);
+    Assert.assertTrue(LoadBalancer.getLastServedId() == 2);
+  }
+
+}

monostate/src/test/java/com/iluwatar/monostate/AppTest.java

@@ -73,9 +73,10 @@
 		<module>front-controller</module>
 		<module>repository</module>
 		<module>async-method-invocation</module>
-	    	<module>business-delegate</module>
-    		<module>half-sync-half-async</module>
+                <module>monostate</module>
 		<module>step-builder</module>
+    	       <module>business-delegate</module>
+    	       <module>half-sync-half-async</module>
 		<module>layers</module>
 		<module>message-channel</module>
 		<module>fluentinterface</module>