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add monitor design pattern (iluwatar#1640)
* add monitor design pattern . * add extra line and change compiler version to 11 in pom.xml. * encapsulate getBalance method . * update puml file . * export uml as png . * duplicate codes eliminated . * update tag * change the format of pom.xml * using logger to print * change AtomicRefrence to type inference var * explanations added ! * Update monitor/README.md Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * Update monitor/README.md Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * Update monitor/src/main/java/com/iluwatar/monitor/Main.java Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * Update monitor/src/main/java/com/iluwatar/monitor/Main.java Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * Update monitor/src/main/java/com/iluwatar/monitor/Main.java Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * Update monitor/src/main/java/com/iluwatar/monitor/Main.java Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> * e.printStackTrace have changed to logger to prints standard output (STD OUT) . * add programmatic example . * Delete mvnw * mvnw.cmd deleted . * added mvnw from master * AddUnitTest * Add language to readme.md Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com> Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com> Co-authored-by: Subhrodip Mohanta <subhromo@cisco.com> Co-authored-by: Subhrodip Mohanta <contact@subho.xyz>
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‎monitor/README.md‎

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---
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layout: pattern
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title: Monitor
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folder: monitor
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permalink: /patterns/monitor/
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categories: Concurrency
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language: en
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tags:
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- Performance
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---
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## Intent
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Monitor pattern is used to create thread-safe objects and prevent conflicts between threads in concurrent applications.
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## Explanation
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In plain words
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> Monitor pattern is used to enforce single-threaded access to data. Only one thread at a time is allowed to execute code within the monitor object.
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Wikipedia says
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> In concurrent programming (also known as parallel programming), a monitor is a synchronization construct that allows threads to have both mutual exclusion and the ability to wait (block) for a certain condition to become false. Monitors also have a mechanism for signaling other threads that their condition has been met.
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**Programmatic Examples**
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Consider there is a bank that transfers money from an account to another account with transfer method . it is `synchronized` mean just one thread can access to this method because if many threads access to it and transfer money from an account to another account in same time balance changed !
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```
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class Bank {
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private int[] accounts;
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Logger logger;
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public Bank(int accountNum, int baseAmount, Logger logger) {
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this.logger = logger;
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accounts = new int[accountNum];
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Arrays.fill(accounts, baseAmount);
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}
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public synchronized void transfer(int accountA, int accountB, int amount) {
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if (accounts[accountA] >= amount) {
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accounts[accountB] += amount;
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accounts[accountA] -= amount;
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logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance());
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}
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}
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```
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getBalance always return total amount and the total amount should be same after each transfers
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```
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private synchronized int getBalance() {
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int balance = 0;
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for (int account : accounts) {
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balance += account;
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}
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return balance;
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}
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}
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```
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## Class diagram
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![alt text](./etc/monitor.urm.png "Monitor class diagram")
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## Applicability
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Use the Monitor pattern when
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* we have a shared resource and there is critical section .
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* you want to create thread-safe objects .
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* you want to achieve mutual exclusion in high level programming language .

‎monitor/etc/monitor.urm.png‎

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‎monitor/etc/monitor.urm.puml‎

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@startuml
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Main - Bank : use
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class Main{
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+ main(args : String[]) : void
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}
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class Bank{
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- accounts : int[]
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+ Bank (accountNum : int , baseAccount : int)
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+ transfer(accountA : int , accountB : int , amount : int) : void {synchronized}
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- getBalance() : void {synchronized}
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}
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@enduml

‎monitor/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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The MIT License
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Copyright © 2014-2021 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<artifactId>java-design-patterns</artifactId>
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<groupId>com.iluwatar</groupId>
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<version>1.24.0-SNAPSHOT</version>
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</parent>
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<artifactId>monitor</artifactId>
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<dependencies>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<!-- Maven assembly plugin is invoked with default setting which we have
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in parent pom and specifying the class having main method -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<executions>
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<execution>
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<configuration>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.abstractdocument.Main</mainClass>
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</manifest>
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</archive>
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</configuration>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>

‎monitor/src/main/java/com/iluwatar/monitor/Bank.java‎

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package com.iluwatar.monitor;
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import java.util.Arrays;
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import java.util.logging.Logger;
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// Bank class implements the Monitor pattern
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public class Bank {
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private int[] accounts;
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Logger logger;
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public Bank(int accountNum, int baseAmount, Logger logger) {
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this.logger = logger;
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accounts = new int[accountNum];
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Arrays.fill(accounts, baseAmount);
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}
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public synchronized void transfer(int accountA, int accountB, int amount) {
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if (accounts[accountA] >= amount) {
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accounts[accountB] += amount;
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accounts[accountA] -= amount;
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logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance());
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}
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}
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public synchronized int getBalance() {
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int balance = 0;
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for (int account : accounts) {
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balance += account;
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}
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return balance;
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}
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public int[] getAccounts() {
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return accounts;
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}
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}

‎monitor/src/main/java/com/iluwatar/monitor/Main.java‎

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/*
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* The MIT License
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* Copyright © 2014-2021 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.monitor;
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import java.util.*;
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import java.util.concurrent.ExecutorService;
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import java.util.concurrent.Executors;
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import java.util.logging.Logger;
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/**
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* <p>The Monitor pattern is used in concurrent algorithms to achieve mutual exclusion.</p>
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*
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* <p>Bank is a simple class that transfers money from an account to another account using
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* {@link Bank#transfer}. It can also return the balance of the bank account stored in the bank.</p>
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*
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* <p>Main class uses ThreadPool to run threads that do transactions on the bank accounts.</p>
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*/
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public class Main {
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public static void main(String[] args) {
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Logger logger = Logger.getLogger("monitor");
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var bank = new Bank(4, 1000, logger);
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Runnable runnable = () -> {
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try {
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Thread.sleep((long) (Math.random() * 1000));
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Random random = new Random();
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for (int i = 0; i < 1000000; i++)
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bank.transfer(random.nextInt(4), random.nextInt(4), (int) (Math.random() * 1000));
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} catch (InterruptedException e) {
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logger.info(e.getMessage());
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}
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};
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ExecutorService executorService = Executors.newFixedThreadPool(5);
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for (int i = 0; i < 5; i++) {
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executorService.execute(runnable);
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}
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}
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}

‎monitor/src/main/test/java/com/iluwater/java/BankTest.java‎

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package com.iluwater.java;
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import com.iluwatar.monitor.Bank;
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import org.junit.jupiter.api.AfterAll;
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import org.junit.jupiter.api.BeforeAll;
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import org.junit.jupiter.api.Test;
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import static org.junit.jupiter.api.Assertions.*;
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import static org.junit.jupiter.api.Assumptions.*;
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import java.util.logging.Logger;
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public class BankTest {
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private static Bank bank;
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private static final int ACCOUNT_NUM = 4;
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private static final int BASE_AMOUNT = 1000;
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private static final Logger LOGGER = Logger.getLogger("monitor");
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@BeforeAll
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public static void Setup() {
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bank = new Bank(ACCOUNT_NUM, BASE_AMOUNT, LOGGER);
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}
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@Test
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public void GetAccountHaveNotBeNull() {
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assertNotNull(bank.getAccounts());
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}
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@Test
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public void LengthOfAccountsHaveToEqualsToAccountNumConstant() {
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assumeTrue(bank.getAccounts() != null);
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assertEquals(ACCOUNT_NUM, bank.getAccounts().length);
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}
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@Test
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public void TransferMethodHaveToTransferAmountFromAnAccountToOtherAccount() {
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bank.transfer(0, 1, 1000);
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int accounts[] = bank.getAccounts();
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assertEquals(0, accounts[0]);
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assertEquals(2000, 2000);
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}
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@Test
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public void BalanceHaveToBeOK() {
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assertEquals(4000, bank.getBalance());
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}
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@AfterAll
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public static void TearDown() {
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bank = null;
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}
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}

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