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Issue iluwatar#469: Implementation of Event-based Asynchronous pattern
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‎event-asynchronous/README.md‎

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---
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layout: pattern
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title: Event-based Asynchronous
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folder: event-asynchronous
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permalink: /patterns/event-asynchronous/
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categories: Other
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tags:
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- Java
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---
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## Intent
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The Event-based Asynchronous Pattern makes available the advantages of multithreaded applications while hiding many
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of the complex issues inherent in multithreaded design. Using a class that supports this pattern can allow you to:-
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(1) Perform time-consuming tasks, such as downloads and database operations, "in the background," without interrupting your application.
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(2) Execute multiple operations simultaneously, receiving notifications when each completes.
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(3) Wait for resources to become available without stopping ("hanging") your application.
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(4) Communicate with pending asynchronous operations using the familiar events-and-delegates model.
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![alt text](./etc/event-asynchronous.png "Event-based Asynchronous")
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## Applicability
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Use the Event-based Asynchronous pattern(s) when
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* Time-consuming tasks are needed to run in the background without disrupting the current application.
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## Credits
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* [Event-based Asynchronous Pattern Overview](https://msdn.microsoft.com/en-us/library/wewwczdw%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396)

‎event-asynchronous/etc/event-asynchronous.png‎

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‎event-asynchronous/etc/event-asynchronous.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.10" icons="true" automaticImage="PNG" 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.event.asynchronous.App" project="event-asynchronous"
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file="/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/App.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="629" y="221"/>
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<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="false" package="false" protected="false" private="false" 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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<class id="2" language="java" name="com.iluwatar.event.asynchronous.Event" project="event-asynchronous"
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file="/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/Event.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="195" y="475"/>
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<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="false" package="false" protected="false" private="false" 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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<class id="3" language="java" name="com.iluwatar.event.asynchronous.EventManager" project="event-asynchronous"
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file="/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/EventManager.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="575" y="475"/>
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<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="false" package="false" protected="false" private="false" 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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<interface id="4" language="java" name="com.iluwatar.event.asynchronous.IEvent" project="event-asynchronous"
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file="/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/IEvent.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="196" y="197"/>
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<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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</display>
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</interface>
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<interface id="5" language="java" name="com.iluwatar.event.asynchronous.ThreadCompleteListener"
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project="event-asynchronous"
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file="/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/ThreadCompleteListener.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="396" y="229"/>
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<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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</display>
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</interface>
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<class id="6" language="java" name="com.iluwatar.event.asynchronous.EventAsynchronousTest"
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project="event-asynchronous"
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file="/event-asynchronous/src/test/java/com/iluwatar/event/asynchronous/EventAsynchronousTest.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="924" y="220"/>
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<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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</display>
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</class>
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<association id="7">
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<bendpoint x="433" y="475"/>
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<end type="SOURCE" refId="3" navigable="false">
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<attribute id="8" name="eventPool"/>
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<multiplicity id="9" minimum="0" maximum="2147483647"/>
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</end>
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<end type="TARGET" refId="2" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="10">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="11" name="eventListener">
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<position height="18" width="74" x="250" y="287"/>
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</attribute>
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<multiplicity id="12" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="5" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="13">
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<end type="SOURCE" refId="6" navigable="false">
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<attribute id="14" name="app"/>
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<multiplicity id="15" minimum="0" maximum="1">
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<position height="16" width="23" x="714" y="226"/>
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</multiplicity>
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</end>
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<end type="TARGET" refId="1" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<realization id="16">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="4"/>
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</realization>
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<realization id="17">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="5"/>
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</realization>
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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>

‎event-asynchronous/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 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 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>1.13.0-SNAPSHOT</version>
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</parent>
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<artifactId>event-asynchronous</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>

‎event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/App.java‎

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/**
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* The MIT License 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 of this software and associated
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* documentation files (the "Software"), to deal in the Software without restriction, including without limitation the
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* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
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* permit persons to whom the Software is 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 all copies or substantial portions of the
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* Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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* WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
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* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
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* OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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*/
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package com.iluwatar.event.asynchronous;
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import java.io.IOException;
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import java.io.InputStream;
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import java.util.Properties;
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import java.util.Scanner;
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/**
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*
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* This application demonstrates the <b>Event-based Asynchronous</b> pattern. Essentially, users (of the pattern) may
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* choose to run events in an Asynchronous or Synchronous mode. There can be multiple Asynchronous events running at
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* once but only one Synchronous event can run at a time. Asynchronous events are synonymous to multi-threads. The key
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* point here is that the threads run in the background and the user is free to carry on with other processes. Once an
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* event is complete, the appropriate listener/callback method will be called. The listener then proceeds to carry out
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* further processing depending on the needs of the user.
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*
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* The {@link EventManager} manages the events/threads that the user creates. Currently, the supported event operations
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* are: <code>start</code>, <code>stop</code>, <code>getStatus</code>. For Synchronous events, the user is unable to
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* start another (Synchronous) event if one is already running at the time. The running event would have to either be
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* stopped or completed before a new event can be started.
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*
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* The Event-based Asynchronous Pattern makes available the advantages of multithreaded applications while hiding many
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* of the complex issues inherent in multithreaded design. Using a class that supports this pattern can allow you to:-
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* (1) Perform time-consuming tasks, such as downloads and database operations, "in the background," without
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* interrupting your application. (2) Execute multiple operations simultaneously, receiving notifications when each
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* completes. (3) Wait for resources to become available without stopping ("hanging") your application. (4) Communicate
43+
* with pending asynchronous operations using the familiar events-and-delegates model.
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*
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* @see EventManager
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* @see Event
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*
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*/
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public class App {
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boolean interactiveMode = false;
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public static void main(String[] args) {
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App app = new App();
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app.setUp();
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app.run();
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}
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/**
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* App can run in interactive mode or not. Interactive mode == Allow user interaction with command line.
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* Non-interactive is a quick sequential run through the available {@link EventManager} operations.
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*/
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public void setUp() {
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Properties prop = new Properties();
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String propFileName = "config.properties";
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InputStream inputStream = App.class.getClassLoader().getResourceAsStream(propFileName);
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if (inputStream != null) {
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try {
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prop.load(inputStream);
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} catch (IOException e) {
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}
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String property = prop.getProperty("INTERACTIVE_MODE");
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if (property.equalsIgnoreCase("YES")) {
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interactiveMode = true;
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}
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}
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}
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public void run() {
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if (interactiveMode) {
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runInteractiveMode();
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} else {
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quickRun();
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}
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}
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public void quickRun() {
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EventManager eventManager = new EventManager();
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try {
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// Create an Asynchronous event.
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int aEventID = eventManager.createAsyncEvent(60);
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System.out.println("Event [" + aEventID + "] has been created.");
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eventManager.startEvent(aEventID);
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System.out.println("Event [" + aEventID + "] has been started.");
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// Create a Synchronous event.
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int sEventID = eventManager.createSyncEvent(60);
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System.out.println("Event [" + sEventID + "] has been created.");
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eventManager.startEvent(sEventID);
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System.out.println("Event [" + sEventID + "] has been started.");
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eventManager.getStatus(aEventID);
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eventManager.getStatus(sEventID);
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eventManager.stopEvent(aEventID);
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System.out.println("Event [" + aEventID + "] has been stopped.");
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eventManager.stopEvent(sEventID);
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System.out.println("Event [" + sEventID + "] has been stopped.");
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} catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException
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| InvalidOperationException e) {
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System.out.println(e.getMessage());
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}
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}
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public void runInteractiveMode() {
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EventManager eventManager = new EventManager();
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Scanner s = new Scanner(System.in);
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int option = 0;
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option = -1;
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while (option != 5) {
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System.out
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.println("(1) START_EVENT \n(2) STOP_EVENT \n(3) STATUS_OF_EVENT \n(4) STATUS_OF_ALL_EVENTS \n(5) EXIT");
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System.out.print("Choose [1,2,3,4,5]: ");
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option = s.nextInt();
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if (option == 1) {
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s.nextLine();
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System.out.print("(A)sync or (S)ync event?: ");
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String eventType = s.nextLine();
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System.out.print("How long should this event run for (in seconds)?: ");
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int eventTime = s.nextInt();
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if (eventType.equalsIgnoreCase("A")) {
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try {
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int eventID = eventManager.createAsyncEvent(eventTime);
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System.out.println("Event [" + eventID + "] has been created.");
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eventManager.startEvent(eventID);
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System.out.println("Event [" + eventID + "] has been started.");
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} catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException e) {
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System.out.println(e.getMessage());
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}
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} else if (eventType.equalsIgnoreCase("S")) {
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try {
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int eventID = eventManager.createSyncEvent(eventTime);
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System.out.println("Event [" + eventID + "] has been created.");
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eventManager.startEvent(eventID);
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System.out.println("Event [" + eventID + "] has been started.");
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} catch (MaxNumOfEventsAllowedException | InvalidOperationException | LongRunningEventException
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| EventDoesNotExistException e) {
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System.out.println(e.getMessage());
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}
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} else {
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System.out.println("Unknown event type.");
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}
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} else if (option == 2) {
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System.out.print("Event ID: ");
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int eventID = s.nextInt();
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try {
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eventManager.stopEvent(eventID);
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System.out.println("Event [" + eventID + "] has been stopped.");
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} catch (EventDoesNotExistException e) {
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System.out.println(e.getMessage());
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}
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} else if (option == 3) {
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System.out.print("Event ID: ");
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int eventID = s.nextInt();
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try {
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eventManager.getStatus(eventID);
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} catch (EventDoesNotExistException e) {
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System.out.println(e.getMessage());
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}
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} else if (option == 4) {
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eventManager.getStatusOfAllEvents();
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}
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}
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s.close();
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}
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}

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