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Merge pull request iluwatar#397 from gwildor28/master
Lock Pattern iluwatar#71: Added mutex and semaphore modules to demonstrate locks
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‎mutex/README.md

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---
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layout: pattern
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title: Mutex
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folder: mutex
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permalink: /patterns/mutex/
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categories: Lock
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tags:
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- Java
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- Difficulty-Beginner
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---
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## Also known as
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Mutual Exclusion Lock
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Binary Semaphore
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## Intent
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Create a lock which only allows a single thread to access a resource at any one instant.
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![alt text](./etc/mutex.png "Mutex")
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## Applicability
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Use a Mutex when
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* you need to prevent two threads accessing a critical section at the same time
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* concurrent access to a resource could lead to a race condition
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## Credits
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* [Lock (computer science)] (http://en.wikipedia.org/wiki/Lock_(computer_science))
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* [Semaphores] (http://tutorials.jenkov.com/java-concurrency/semaphores.html)

‎mutex/etc/mutex.png

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‎mutex/pom.xml

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<?xml version="1.0"?>
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<!--
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The MIT License
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Copyright (c) 2014 Ilkka Sepp�l�
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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 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"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.12.0-SNAPSHOT</version>
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</parent>
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<artifactId>mutex</artifactId>
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<dependencies>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

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

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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/**
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* A Mutex prevents multiple threads from accessing a resource simultaneously.
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* <p>
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* In this example we have two thieves who are taking beans from a jar.
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* Only one thief can take a bean at a time. This is ensured by a Mutex lock
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* which must be acquired in order to access the jar. Each thief attempts to
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* acquire the lock, take a bean and then release the lock. If the lock has
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* already been acquired, the thief will be prevented from continuing (blocked)
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* until the lock has been released. The thieves stop taking beans once there
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* are no beans left to take.
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*/
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public class App {
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/**
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* main method
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*/
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public static void main(String[] args) {
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Mutex mutex = new Mutex();
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Jar jar = new Jar(1000, mutex);
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Thief peter = new Thief("Peter", jar);
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Thief john = new Thief("John", jar);
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peter.start();
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john.start();
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}
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}

‎mutex/src/main/java/com/iluwatar/mutex/Jar.java

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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/**
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* A Jar has a resource of beans which can only be accessed by a single Thief
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* (thread) at any one time. A Mutex lock is used to prevent more than one Thief
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* taking a bean simultaneously.
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*/
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public class Jar {
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/**
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* The lock which must be acquired to access the beans resource.
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*/
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private final Lock lock;
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/**
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* The resource within the jar.
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*/
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private int beans;
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public Jar(int beans, Lock lock) {
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this.beans = beans;
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this.lock = lock;
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}
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/**
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* Method for a thief to take a bean.
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*/
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public boolean takeBean() {
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boolean success = false;
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try {
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lock.acquire();
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success = beans > 0;
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if (success) {
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beans = beans - 1;
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}
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} catch (Exception e) {
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e.printStackTrace();
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} finally {
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lock.release();
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}
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return success;
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}
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}

‎mutex/src/main/java/com/iluwatar/mutex/Lock.java

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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/**
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* Lock is an interface for a lock which can be acquired and released.
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*/
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public interface Lock {
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void acquire() throws InterruptedException;
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void release();
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}

‎mutex/src/main/java/com/iluwatar/mutex/Mutex.java

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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/**
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* Mutex is an implementation of a mutual exclusion lock.
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*/
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public class Mutex implements Lock {
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/**
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* The current owner of the lock.
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*/
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private Object owner;
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/**
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* Returns the current owner of the Mutex, or null if available
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*/
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public Object getOwner() {
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return owner;
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}
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/**
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* Method called by a thread to acquire the lock. If the lock has already
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* been acquired this will wait until the lock has been released to
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* re-attempt the acquire.
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*/
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@Override
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public synchronized void acquire() throws InterruptedException {
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while (owner != null) {
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wait();
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}
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owner = Thread.currentThread();
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}
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/**
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* Method called by a thread to release the lock.
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*/
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@Override
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public synchronized void release() {
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if (Thread.currentThread() == owner) {
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owner = null;
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notify();
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}
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}
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}

‎mutex/src/main/java/com/iluwatar/mutex/Thief.java

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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/**
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* Thief is a class which continually tries to acquire a jar and take a bean
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* from it. When the jar is empty the thief stops.
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*/
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public class Thief extends Thread {
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/**
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* The name of the thief.
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*/
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private final String name;
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/**
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* The jar
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*/
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private final Jar jar;
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public Thief(String name, Jar jar) {
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this.name = name;
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this.jar = jar;
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}
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/**
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* In the run method the thief repeatedly tries to take a bean until none
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* are left.
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*/
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@Override
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public void run() {
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int beans = 0;
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while (jar.takeBean()) {
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beans = beans + 1;
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System.out.println(name + " took a bean.");
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}
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System.out.println(name + " took " + beans + " beans.");
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}
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}

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

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/**
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* The MIT License
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* Copyright (c) 2014 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.mutex;
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import org.junit.Test;
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import java.io.IOException;
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public class AppTest{
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@Test
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public void test() throws IOException {
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String[] args = {};
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App.main(args);
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}
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}

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