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* 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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--- | ||
layout: pattern | ||
title: Monitor | ||
folder: monitor | ||
permalink: /patterns/monitor/ | ||
categories: Concurrency | ||
language: en | ||
tags: | ||
- Performance | ||
--- | ||
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## Intent | ||
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. | ||
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. | ||
**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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``` | ||
class Bank { | ||
private int[] accounts; | ||
Logger logger; | ||
public Bank(int accountNum, int baseAmount, Logger logger) { | ||
this.logger = logger; | ||
accounts = new int[accountNum]; | ||
Arrays.fill(accounts, baseAmount); | ||
} | ||
public synchronized void transfer(int accountA, int accountB, int amount) { | ||
if (accounts[accountA] >= amount) { | ||
accounts[accountB] += amount; | ||
accounts[accountA] -= amount; | ||
logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance()); | ||
} | ||
} | ||
``` | ||
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getBalance always return total amount and the total amount should be same after each transfers | ||
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``` | ||
private synchronized int getBalance() { | ||
int balance = 0; | ||
for (int account : accounts) { | ||
balance += account; | ||
} | ||
return balance; | ||
} | ||
} | ||
``` | ||
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## Class diagram | ||
![alt text](./etc/monitor.urm.png "Monitor class diagram") | ||
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## Applicability | ||
Use the Monitor pattern when | ||
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* we have a shared resource and there is critical section . | ||
* you want to create thread-safe objects . | ||
* you want to achieve mutual exclusion in high level programming language . |
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@startuml | ||
Main - Bank : use | ||
class Main{ | ||
+ main(args : String[]) : void | ||
} | ||
class Bank{ | ||
- accounts : int[] | ||
+ Bank (accountNum : int , baseAccount : int) | ||
+ transfer(accountA : int , accountB : int , amount : int) : void {synchronized} | ||
- getBalance() : void {synchronized} | ||
} | ||
@enduml |
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<?xml version="1.0" encoding="UTF-8"?> | ||
<!-- | ||
The MIT License | ||
Copyright © 2014-2021 Ilkka Seppälä | ||
Permission is hereby granted, free of charge, to any person obtaining a copy | ||
of this software and associated documentation files (the "Software"), to deal | ||
in the Software without restriction, including without limitation the rights | ||
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
copies of the Software, and to permit persons to whom the Software is | ||
furnished to do so, subject to the following conditions: | ||
The above copyright notice and this permission notice shall be included in | ||
all copies or substantial portions of the Software. | ||
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
THE SOFTWARE. | ||
--> | ||
<project xmlns="http://maven.apache.org/POM/4.0.0" | ||
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" | ||
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> | ||
<modelVersion>4.0.0</modelVersion> | ||
<parent> | ||
<artifactId>java-design-patterns</artifactId> | ||
<groupId>com.iluwatar</groupId> | ||
<version>1.24.0-SNAPSHOT</version> | ||
</parent> | ||
<artifactId>monitor</artifactId> | ||
<dependencies> | ||
<dependency> | ||
<groupId>org.junit.jupiter</groupId> | ||
<artifactId>junit-jupiter-engine</artifactId> | ||
<scope>test</scope> | ||
</dependency> | ||
</dependencies> | ||
<build> | ||
<plugins> | ||
<!-- Maven assembly plugin is invoked with default setting which we have | ||
in parent pom and specifying the class having main method --> | ||
<plugin> | ||
<groupId>org.apache.maven.plugins</groupId> | ||
<artifactId>maven-assembly-plugin</artifactId> | ||
<executions> | ||
<execution> | ||
<configuration> | ||
<archive> | ||
<manifest> | ||
<mainClass>com.iluwatar.abstractdocument.Main</mainClass> | ||
</manifest> | ||
</archive> | ||
</configuration> | ||
</execution> | ||
</executions> | ||
</plugin> | ||
</plugins> | ||
</build> | ||
</project> |
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package com.iluwatar.monitor; | ||
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import java.util.Arrays; | ||
import java.util.logging.Logger; | ||
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// Bank class implements the Monitor pattern | ||
public class Bank { | ||
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private int[] accounts; | ||
Logger logger; | ||
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public Bank(int accountNum, int baseAmount, Logger logger) { | ||
this.logger = logger; | ||
accounts = new int[accountNum]; | ||
Arrays.fill(accounts, baseAmount); | ||
} | ||
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public synchronized void transfer(int accountA, int accountB, int amount) { | ||
if (accounts[accountA] >= amount) { | ||
accounts[accountB] += amount; | ||
accounts[accountA] -= amount; | ||
logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance()); | ||
} | ||
} | ||
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public synchronized int getBalance() { | ||
int balance = 0; | ||
for (int account : accounts) { | ||
balance += account; | ||
} | ||
return balance; | ||
} | ||
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public int[] getAccounts() { | ||
return accounts; | ||
} | ||
} |
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/* | ||
* The MIT License | ||
* Copyright © 2014-2021 Ilkka Seppälä | ||
* | ||
* Permission is hereby granted, free of charge, to any person obtaining a copy | ||
* of this software and associated documentation files (the "Software"), to deal | ||
* in the Software without restriction, including without limitation the rights | ||
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
* copies of the Software, and to permit persons to whom the Software is | ||
* furnished to do so, subject to the following conditions: | ||
* | ||
* The above copyright notice and this permission notice shall be included in | ||
* all copies or substantial portions of the Software. | ||
* | ||
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
* THE SOFTWARE. | ||
*/ | ||
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package com.iluwatar.monitor; | ||
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import java.util.*; | ||
import java.util.concurrent.ExecutorService; | ||
import java.util.concurrent.Executors; | ||
import java.util.logging.Logger; | ||
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/** | ||
* <p>The Monitor pattern is used in concurrent algorithms to achieve mutual exclusion.</p> | ||
* | ||
* <p>Bank is a simple class that transfers money from an account to another account using | ||
* {@link Bank#transfer}. It can also return the balance of the bank account stored in the bank.</p> | ||
* | ||
* <p>Main class uses ThreadPool to run threads that do transactions on the bank accounts.</p> | ||
*/ | ||
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public class Main { | ||
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public static void main(String[] args) { | ||
Logger logger = Logger.getLogger("monitor"); | ||
var bank = new Bank(4, 1000, logger); | ||
Runnable runnable = () -> { | ||
try { | ||
Thread.sleep((long) (Math.random() * 1000)); | ||
Random random = new Random(); | ||
for (int i = 0; i < 1000000; i++) | ||
bank.transfer(random.nextInt(4), random.nextInt(4), (int) (Math.random() * 1000)); | ||
} catch (InterruptedException e) { | ||
logger.info(e.getMessage()); | ||
} | ||
}; | ||
ExecutorService executorService = Executors.newFixedThreadPool(5); | ||
for (int i = 0; i < 5; i++) { | ||
executorService.execute(runnable); | ||
} | ||
} | ||
} |
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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; | ||
import org.junit.jupiter.api.AfterAll; | ||
import org.junit.jupiter.api.BeforeAll; | ||
import org.junit.jupiter.api.Test; | ||
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import static org.junit.jupiter.api.Assertions.*; | ||
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; | ||
private static final int ACCOUNT_NUM = 4; | ||
private static final int BASE_AMOUNT = 1000; | ||
private static final Logger LOGGER = Logger.getLogger("monitor"); | ||
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@BeforeAll | ||
public static void Setup() { | ||
bank = new Bank(ACCOUNT_NUM, BASE_AMOUNT, LOGGER); | ||
} | ||
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@Test | ||
public void GetAccountHaveNotBeNull() { | ||
assertNotNull(bank.getAccounts()); | ||
} | ||
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@Test | ||
public void LengthOfAccountsHaveToEqualsToAccountNumConstant() { | ||
assumeTrue(bank.getAccounts() != null); | ||
assertEquals(ACCOUNT_NUM, bank.getAccounts().length); | ||
} | ||
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@Test | ||
public void TransferMethodHaveToTransferAmountFromAnAccountToOtherAccount() { | ||
bank.transfer(0, 1, 1000); | ||
int accounts[] = bank.getAccounts(); | ||
assertEquals(0, accounts[0]); | ||
assertEquals(2000, 2000); | ||
} | ||
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@Test | ||
public void BalanceHaveToBeOK() { | ||
assertEquals(4000, bank.getBalance()); | ||
} | ||
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@AfterAll | ||
public static void TearDown() { | ||
bank = null; | ||
} | ||
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} |