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Issue #273:Caching Patterns [new pattern]
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‎caching/.gitignore‎

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/target/

‎caching/etc/caching.png‎

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‎caching/etc/caching.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true">
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<class id="1" language="java" name="main.java.com.wssia.caching.App" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/App.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="249" y="150"/>
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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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<class id="2" language="java" name="main.java.com.wssia.caching.AppManager" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/AppManager.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="502" y="163"/>
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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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<class id="3" language="java" name="main.java.com.wssia.caching.CacheStore" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/CacheStore.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="537" y="436"/>
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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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<enumeration id="4" language="java" name="main.java.com.wssia.caching.CachingPolicy" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/CachingPolicy.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="789" y="162"/>
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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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</enumeration>
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<class id="5" language="java" name="main.java.com.wssia.caching.DBManager" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/DBManager.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="1137" y="134"/>
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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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<class id="6" language="java" name="main.java.com.wssia.caching.LRUCache" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/LRUCache.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="884" y="435"/>
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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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<class id="7" language="java" name="main.java.com.wssia.caching.UserAccount" project="CachingPatterns"
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file="/CachingPatterns/src/main/java/com/wssia/caching/UserAccount.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="1137" y="382"/>
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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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<class id="8" language="java" name="test.java.com.wssia.caching.AppTest" project="CachingPatterns"
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file="/CachingPatterns/src/test/java/com/wssia/caching/AppTest.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="251" y="374"/>
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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="9">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="10" name="cachingPolicy"/>
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<multiplicity id="11" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="4" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="12">
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<end type="SOURCE" refId="8" navigable="false">
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<attribute id="13" name="app"/>
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<multiplicity id="14" minimum="0" maximum="1"/>
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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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<association id="15">
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<end type="SOURCE" refId="3" navigable="false">
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<attribute id="16" name="cache"/>
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<multiplicity id="17" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="6" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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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>

‎caching/index.md‎

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---
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layout: pattern
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title: Caching
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folder: caching
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permalink: /patterns/caching/
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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:** To avoid expensive re-acquisition of resources by not releasing
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the resources immediately after their use. The resources retain their identity, are kept in some
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fast-access storage, and are re-used to avoid having to acquire them again.
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![alt text](./etc/caching.png "Caching")
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**Applicability:** Use the Caching pattern(s) when
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* Repetitious acquisition, initialization, and release of the same resource causes unnecessary performance overhead.
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**Credits**
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* [Write-through, write-around, write-back: Cache explained](http://www.computerweekly.com/feature/Write-through-write-around-write-back-Cache-explained)
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* [Read-Through, Write-Through, Write-Behind, and Refresh-Ahead Caching](https://docs.oracle.com/cd/E15357_01/coh.360/e15723/cache_rtwtwbra.htm#COHDG5177)

‎caching/pom.xml‎

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<?xml version="1.0"?>
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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>1.7.0</version>
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</parent>
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<artifactId>caching</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.mongodb</groupId>
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<artifactId>mongodb-driver</artifactId>
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<version>3.0.4</version>
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</dependency>
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<dependency>
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<groupId>org.mongodb</groupId>
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<artifactId>mongodb-driver-core</artifactId>
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<version>3.0.4</version>
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</dependency>
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<dependency>
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<groupId>org.mongodb</groupId>
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<artifactId>bson</artifactId>
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<version>3.0.4</version>
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</dependency>
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</dependencies>
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<!--
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Due to the use of MongoDB in the test of this pattern, TRAVIS and/or MAVEN might fail if the DB connection is
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not open for the JUnit test. To avoid disrupting the compilation process, the surefire plug-in was used
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to SKIP the running of the JUnit tests for this pattern. To RE-ACTIVATE the running of the tests, change the
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skipTests (below) flag to 'false'.
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-->
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-surefire-plugin</artifactId>
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<version>2.19</version>
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<configuration>
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<skipTests>true</skipTests>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</project>

‎caching/src/main/java/com/wssia/caching/App.java‎

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package main.java.com.wssia.caching;
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/**
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*
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* The Caching pattern describes how to avoid expensive re-acquisition of resources by not releasing
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* the resources immediately after their use. The resources retain their identity, are kept in some
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* fast-access storage, and are re-used to avoid having to acquire them again. There are three main
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* caching strategies/techniques in this pattern; each with their own pros and cons. They are:
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* <code>write-through</code> which writes data to the cache and DB in a single transaction,
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* <code>write-around</code> which writes data immediately into the DB instead of the cache, and
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* <code>write-behind</code> which writes data into the cache initially whilst the data is only
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* written into the DB when the cache is full. The <code>read-through</code> strategy is also
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* included in the mentioned three strategies -- returns data from the cache to the caller <b>if</b>
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* it exists <b>else</b> queries from DB and stores it into the cache for future use. These
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* strategies determine when the data in the cache should be written back to the backing store (i.e.
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* Database) and help keep both data sources synchronized/up-to-date. This pattern can improve
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* performance and also helps to maintain consistency between data held in the cache and the data in
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* the underlying data store.
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* <p>
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* In this example, the user account ({@link UserAccount}) entity is used as the underlying
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* application data. The cache itself is implemented as an internal (Java) data structure. It adopts
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* a Least-Recently-Used (LRU) strategy for evicting data from itself when its full. The three
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* strategies are individually tested. The testing of the cache is restricted towards saving and
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* querying of user accounts from the underlying data store ( {@link DBManager}). The main class (
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* {@link App} is not aware of the underlying mechanics of the application (i.e. save and query) and
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* whether the data is coming from the cache or the DB (i.e. separation of concern). The AppManager
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* ({@link AppManager}) handles the transaction of data to-and-from the underlying data store
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* (depending on the preferred caching policy/strategy).
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*
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* <i>App --> AppManager --> CacheStore/LRUCache/CachingPolicy --> DBManager</i>
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* <p>
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*
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* @see CacheStore
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* @See LRUCache
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* @see CachingPolicy
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*
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*/
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public class App {
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/**
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* Read-through and write-through
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*/
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public void useReadAndWriteThroughStrategy() {
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System.out.println("# CachingPolicy.THROUGH");
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AppManager.initCachingPolicy(CachingPolicy.THROUGH);
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UserAccount userAccount1 = new UserAccount("001", "John", "He is a boy.");
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AppManager.save(userAccount1);
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System.out.println(AppManager.printCacheContent());
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userAccount1 = AppManager.find("001");
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userAccount1 = AppManager.find("001");
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}
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/**
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* Read-through and write-around
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*/
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public void useReadThroughAndWriteAroundStrategy() {
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System.out.println("# CachingPolicy.AROUND");
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AppManager.initCachingPolicy(CachingPolicy.AROUND);
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UserAccount userAccount2 = new UserAccount("002", "Jane", "She is a girl.");
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AppManager.save(userAccount2);
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System.out.println(AppManager.printCacheContent());
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userAccount2 = AppManager.find("002");
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System.out.println(AppManager.printCacheContent());
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userAccount2 = AppManager.find("002");
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userAccount2.setUserName("Jane G.");
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AppManager.save(userAccount2);
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System.out.println(AppManager.printCacheContent());
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userAccount2 = AppManager.find("002");
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System.out.println(AppManager.printCacheContent());
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userAccount2 = AppManager.find("002");
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}
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/**
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* Read-through and write-behind
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*/
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public void useReadThroughAndWriteBehindStrategy() {
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System.out.println("# CachingPolicy.BEHIND");
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AppManager.initCachingPolicy(CachingPolicy.BEHIND);
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UserAccount userAccount3 = new UserAccount("003", "Adam", "He likes food.");
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UserAccount userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
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UserAccount userAccount5 = new UserAccount("005", "Isaac", "He is allergic to mustard.");
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AppManager.save(userAccount3);
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AppManager.save(userAccount4);
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AppManager.save(userAccount5);
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System.out.println(AppManager.printCacheContent());
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userAccount3 = AppManager.find("003");
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System.out.println(AppManager.printCacheContent());
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UserAccount userAccount6 = new UserAccount("006", "Yasha", "She is an only child.");
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AppManager.save(userAccount6);
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System.out.println(AppManager.printCacheContent());
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userAccount4 = AppManager.find("004");
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System.out.println(AppManager.printCacheContent());
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}
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}

‎caching/src/main/java/com/wssia/caching/AppManager.java‎

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package main.java.com.wssia.caching;
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import java.text.ParseException;
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/**
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*
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* AppManager helps to bridge the gap in communication between the main class and the application's
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* back-end. DB connection is initialized through this class. The chosen caching strategy/policy is
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* also initialized here. Before the cache can be used, the size of the cache has to be set.
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* Depending on the chosen caching policy, AppManager will call the appropriate function in the
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* CacheStore class.
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*
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*/
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public class AppManager {
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private static CachingPolicy cachingPolicy;
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public static void init() {
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try {
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DBManager.connect();
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} catch (ParseException e) {
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e.printStackTrace();
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}
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}
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public static void initCachingPolicy(CachingPolicy policy) {
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cachingPolicy = policy;
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if (cachingPolicy == CachingPolicy.BEHIND) {
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Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
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@Override
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public void run() {
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CacheStore.flushCache();
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}
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}));
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}
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CacheStore.clearCache();
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}
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public static void initCacheCapacity(int capacity) {
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CacheStore.initCapacity(capacity);
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}
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public static UserAccount find(String userID) {
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if (cachingPolicy == CachingPolicy.THROUGH || cachingPolicy == CachingPolicy.AROUND) {
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return CacheStore.readThrough(userID);
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} else if (cachingPolicy == CachingPolicy.BEHIND) {
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return CacheStore.readThroughWithWriteBackPolicy(userID);
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}
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return null;
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}
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public static void save(UserAccount userAccount) {
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if (cachingPolicy == CachingPolicy.THROUGH) {
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CacheStore.writeThrough(userAccount);
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} else if (cachingPolicy == CachingPolicy.AROUND) {
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CacheStore.writeAround(userAccount);
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} else if (cachingPolicy == CachingPolicy.BEHIND) {
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CacheStore.writeBehind(userAccount);
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}
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}
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public static String printCacheContent() {
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return CacheStore.print();
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}
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}

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