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guarded suspension pattern iluwatar#69
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‎guarded-suspension/README.md

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---
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layout: pattern
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title: Guarded Suspension
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folder: guarded-suspension
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permalink: /patterns/guarded-suspension/
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categories: Concurrency
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tags:
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- Java
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- Difficulty-Beginner
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---
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## Intent
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Use Guarded suspension pattern to handle a situation when you want to execute a method on object which is not in a proper state.
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![Guarded Suspension diagram](./etc/guarded-suspension.png)
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## Applicability
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Use Guarded Suspension pattern when the developer knows that the method execution will be blocked for a finite period of time
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## Related patterns
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* Balking

‎guarded-suspension/etc/guarded-suspension.png

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‎guarded-suspension/etc/guarded-suspension.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.13" icons="true" automaticImage="JPEG" always-add-relationships="false"
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generalizations="true" realizations="true" associations="true" dependencies="false" nesting-relationships="true"
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router="FAN">
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<class id="1" language="java" name="com.iluwatar.guarded.suspension.GuardedQueue"
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project="java-design-patterns_guarded-suspension"
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file="/java-design-patterns_guarded-suspension/src/main/java/com/iluwatar/guarded/suspension/GuardedQueue.java"
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binary="false" corner="BOTTOM_RIGHT">
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<position height="277" width="326" x="301" y="172"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>

‎guarded-suspension/etc/guarded-suspension.urm.puml

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@startuml
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package com.iluwatar.guarded.suspension {
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class GuardedQueue {
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- LOGGER : Logger {static}
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- sourceList : Queue<Integer>
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+ GuardedQueue()
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+ get() : Integer
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+ put(e : Integer)
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}
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}
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@enduml

‎guarded-suspension/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 (c) 2014 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" 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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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.15.0-SNAPSHOT</version>
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</parent>
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<packaging>jar</packaging>
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<artifactId>guarded-suspension</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>

‎guarded-suspension/src/main/java/com/iluwatar/guarded/suspension/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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/**
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* Guarded-suspension is a concurrent design pattern for handling situation when to execute some action we need
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* condition to be satisfied.
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* <p>
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* Implementation is based on GuardedQueue, which has two methods: get and put,
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* the condition is that we cannot get from empty queue so when thread attempt
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* to break the condition we invoke Object's wait method on him and when other thread put an element
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* to the queue he notify the waiting one that now he can get from queue.
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*/
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package com.iluwatar.guarded.suspension;
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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.concurrent.TimeUnit;
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/**
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* Created by robertt240 on 1/26/17.
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*/
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public class App {
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/**
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* Example pattern execution
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*
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* @param args - command line args
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*/
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public static void main(String[] args) {
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GuardedQueue guardedQueue = new GuardedQueue();
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ExecutorService executorService = Executors.newFixedThreadPool(3);
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//here we create first thread which is supposed to get from guardedQueue
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executorService.execute(() -> {
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guardedQueue.get();
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}
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);
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//here we wait two seconds to show that the thread which is trying to get from guardedQueue will be waiting
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try {
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Thread.sleep(2000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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//now we execute second thread which will put number to guardedQueue and notify first thread that it could get
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executorService.execute(() -> {
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guardedQueue.put(20);
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}
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);
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executorService.shutdown();
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try {
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executorService.awaitTermination(30, TimeUnit.SECONDS);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}

‎guarded-suspension/src/main/java/com/iluwatar/guarded/suspension/GuardedQueue.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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* <p>
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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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* <p>
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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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* <p>
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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.guarded.suspension;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import java.util.LinkedList;
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import java.util.Queue;
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public class GuardedQueue {
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private static final Logger LOGGER = LoggerFactory.getLogger(GuardedQueue.class);
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private final Queue<Integer> sourceList;
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public GuardedQueue() {
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this.sourceList = new LinkedList<>();
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}
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/**
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* @return last element of a queue if queue is not empty
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*/
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public synchronized Integer get() {
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while (sourceList.isEmpty()) {
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try {
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LOGGER.info("waiting");
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wait();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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LOGGER.info("getting");
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return sourceList.peek();
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}
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/**
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* @param e number which we want to put to our queue
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*/
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public synchronized void put(Integer e) {
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LOGGER.info("putting");
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sourceList.add(e);
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LOGGER.info("notifying");
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notify();
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}
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}

‎guarded-suspension/src/test/java/com/iluwatar/guarded/suspension/GuardedQueueTest.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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* <p>
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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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* <p>
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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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* <p>
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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.guarded.suspension;
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import org.junit.Assert;
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import org.junit.Test;
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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.concurrent.TimeUnit;
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public class GuardedQueueTest {
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private volatile Integer value;
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@Test
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public void testGet() {
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GuardedQueue g = new GuardedQueue();
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ExecutorService executorService = Executors.newFixedThreadPool(2);
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executorService.submit(() -> value = g.get());
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executorService.submit(() -> g.put(Integer.valueOf(10)));
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executorService.shutdown();
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try {
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executorService.awaitTermination(30, TimeUnit.SECONDS);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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Assert.assertEquals(Integer.valueOf(10), value);
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}
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@Test
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public void testPut() {
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GuardedQueue g = new GuardedQueue();
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g.put(12);
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Assert.assertEquals(Integer.valueOf(12), g.get());
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}
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}

‎pom.xml

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<module>factory-kit</module>
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<module>feature-toggle</module>
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<module>value-object</module>
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<module>module</module>
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<module>module</module>
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<module>monad</module>
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<module>mute-idiom</module>
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<module>mutex</module>
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<module>event-asynchronous</module>
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<module>queue-load-leveling</module>
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<module>object-mother</module>
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<module>guarded-suspension</module>
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</modules>
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<dependencyManagement>

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