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Commitbe1c0b8

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Fix issueiluwatar#179: Leader Followers Pattern (iluwatar#1189)
* add leader followers pattern* use var and streams instead in App::execute* use logger instead of printing to system output stream
1 parent6ce33ed commitbe1c0b8

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‎leader-followers/README.md

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---
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layout:pattern
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title:Leader/Followers
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folder:leader-followers
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permalink:/patterns/leader-followers/
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categories:Concurrency
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tags:
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-Performance
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---
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##Intent
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The Leader/Followers pattern provides a concurrency model where multiple
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threads can efficiently de-multiplex events and dispatch event handlers
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that process I/O handles shared by the threads.
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##Class diagram
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![Leader/Followers class diagram](./etc/leader-followers.png)
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##Applicability
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Use Leader-Followers pattern when
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* multiple threads take turns sharing a set of event sources in order to detect, de-multiplex, dispatch and process service requests that occur on the event sources.
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##Real world examples
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*[ACE Thread Pool Reactor framework](https://www.dre.vanderbilt.edu/~schmidt/PDF/HPL.pdf)
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*[JAWS](http://www.dre.vanderbilt.edu/~schmidt/PDF/PDCP.pdf)
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*[Real-time CORBA](http://www.dre.vanderbilt.edu/~schmidt/PDF/RTS.pdf)
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##Credits
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*[Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers](http://www.kircher-schwanninger.de/michael/publications/lf.pdf)
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@startuml
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packagecom.iluwatar.leaderfollowers {
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classApp {
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+App()
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-addTasks(taskSet : TaskSet) {static}
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-execute(workCenter : WorkCenter, taskSet : TaskSet) {static}
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+main(args : String[]) {static}
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}
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classTask {
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-finished :boolean
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-time :int
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+Task(time : int)
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+getTime() :int
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+isFinished() :boolean
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+setFinished()
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}
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classTaskHandler {
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+TaskHandler()
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+handleTask(task : Task)
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}
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classTaskSet {
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-queue : BlockingQueue<Task>
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+TaskSet()
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+addTask(task : Task)
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+getSize() :int
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+getTask() :Task
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}
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classWorkCenter {
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-leader :Worker
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-workers : List<Worker>
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+WorkCenter()
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+addWorker(worker : Worker)
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+createWorkers(numberOfWorkers : int, taskSet : TaskSet, taskHandler : TaskHandler)
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+getLeader() :Worker
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+getWorkers() : List<Worker>
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+promoteLeader()
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+removeWorker(worker : Worker)
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}
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classWorker {
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-id :long
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-taskHandler :TaskHandler
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-taskSet :TaskSet
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-workCenter :WorkCenter
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+Worker(id : long, workCenter : WorkCenter, taskSet : TaskSet, taskHandler : TaskHandler)
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+equals(o : Object) : boolean
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+hashCode() :int
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+run()
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}
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}
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Worker--> "-taskSet"TaskSet
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Worker--> "-taskHandler"TaskHandler
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TaskSet--> "-queue"Task
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Worker--> "-workCenter"WorkCenter
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@enduml

‎leader-followers/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 © 2014-2019 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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<projectxsi: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"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>1.23.0-SNAPSHOT</version>
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</parent>
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<artifactId>leader-followers</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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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-core</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>
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/*
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* The MIT License
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* Copyright © 2014-2019 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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packagecom.iluwatar.leaderfollowers;
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importjava.util.Random;
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importjava.util.concurrent.Executors;
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importjava.util.concurrent.TimeUnit;
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/**
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* Leader/Followers pattern is a concurrency pattern. This pattern behaves like a taxi stand where
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* one of the threads acts as leader thread which listens for event from event sources,
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* de-multiplexes, dispatches and handles the event. It promotes the follower to be the new leader.
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* When processing completes the thread joins the followers queue, if there are no followers then it
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* becomes the leader and cycle repeats again.
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*
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* <p>In this example, one of the workers becomes Leader and listens on the {@link TaskSet} for
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* work. {@link TaskSet} basically acts as the source of input events for the {@link Worker}, who
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* are spawned and controlled by the {@link WorkCenter} . When {@link Task} arrives then the leader
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* takes the work and calls the {@link TaskHandler}. It also calls the {@link WorkCenter} to
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* promotes one of the followers to be the new leader, who can then process the next work and so
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* on.
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*
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* <p>The pros for this pattern are:
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* It enhances CPU cache affinity and eliminates unbound allocation and data buffer sharing between
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* threads by reading the request into buffer space allocated on the stack of the leader or by using
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* the Thread-Specific Storage pattern [22] to allocate memory. It minimizes locking overhead by not
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* exchanging data between threads, thereby reducing thread synchronization. In bound handle/thread
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* associations, the leader thread dispatches the event based on the I/O handle. It can minimize
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* priority inversion because no extra queuing is introduced in the server. It does not require a
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* context switch to handle each event, reducing the event dispatching latency. Note that promoting
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* a follower thread to fulfill the leader role requires a context switch. Programming simplicity:
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* The Leader/Followers pattern simplifies the programming of concurrency models where multiple
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* threads can receive requests, process responses, and de-multiplex connections using a shared
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* handle set.
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*/
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publicclassApp {
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/**
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* The main method for the leader followers pattern.
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*/
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publicstaticvoidmain(String[]args)throwsInterruptedException {
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vartaskSet =newTaskSet();
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vartaskHandler =newTaskHandler();
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varworkCenter =newWorkCenter();
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workCenter.createWorkers(4,taskSet,taskHandler);
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execute(workCenter,taskSet);
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}
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/**
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* Start the work, dispatch tasks and stop the thread pool at last.
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*/
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privatestaticvoidexecute(WorkCenterworkCenter,TaskSettaskSet)throwsInterruptedException {
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varworkers =workCenter.getWorkers();
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varexec =Executors.newFixedThreadPool(workers.size());
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workers.forEach(exec::submit);
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Thread.sleep(1000);
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addTasks(taskSet);
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exec.awaitTermination(2,TimeUnit.SECONDS);
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exec.shutdownNow();
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}
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/**
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* Add tasks.
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*/
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privatestaticvoidaddTasks(TaskSettaskSet)throwsInterruptedException {
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varrand =newRandom();
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for (vari =0;i <5;i++) {
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vartime =Math.abs(rand.nextInt(1000));
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taskSet.addTask(newTask(time));
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}
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}
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}
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/*
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* The MIT License
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* Copyright © 2014-2019 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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packagecom.iluwatar.leaderfollowers;
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/**
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* A unit of work to be processed by the Workers.
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*/
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publicclassTask {
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privatefinalinttime;
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privatebooleanfinished;
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publicTask(inttime) {
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this.time =time;
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}
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publicintgetTime() {
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returntime;
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}
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publicvoidsetFinished() {
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this.finished =true;
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}
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publicbooleanisFinished() {
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returnthis.finished;
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}
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}
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/*
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* The MIT License
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* Copyright © 2014-2019 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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packagecom.iluwatar.leaderfollowers;
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importorg.slf4j.Logger;
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importorg.slf4j.LoggerFactory;
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/**
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* The TaskHandler is used by the {@link Worker} to process the newly arrived task.
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*/
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publicclassTaskHandler {
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privatestaticfinalLoggerLOGGER =LoggerFactory.getLogger(TaskHandler.class);
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/**
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* This interface handles one task at a time.
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*/
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publicvoidhandleTask(Tasktask)throwsInterruptedException {
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vartime =task.getTime();
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Thread.sleep(time);
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LOGGER.info("It takes " +time +" milliseconds to finish the task");
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task.setFinished();
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}
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}
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/*
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* The MIT License
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* Copyright © 2014-2019 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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packagecom.iluwatar.leaderfollowers;
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importjava.util.concurrent.ArrayBlockingQueue;
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importjava.util.concurrent.BlockingQueue;
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/**
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* A TaskSet is a collection of the tasks, the leader receives task from here.
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*/
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publicclassTaskSet {
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privateBlockingQueue<Task>queue =newArrayBlockingQueue<>(100);
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publicvoidaddTask(Tasktask)throwsInterruptedException {
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queue.put(task);
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}
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publicTaskgetTask()throwsInterruptedException {
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returnqueue.take();
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}
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publicintgetSize() {
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returnqueue.size();
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}
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}

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