649b14adf6
additional explanation about the risks of using the singleton pattern, as requested by @iluwatar
52 lines
2.3 KiB
Java
52 lines
2.3 KiB
Java
package com.iluwatar;
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/**
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*
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* Singleton pattern ensures that the class (IvoryTower) can have only one
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* existing instance per java classloader instance and provides global access to it.
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*
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* http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
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*
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* The risk of this pattern is that bugs resulting from setting a singleton up in a distributed environment can
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* be tricky to debug, since it will work fine if you debug with a single classloader. Additionally, these
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* problems can crop up a while after the implementation of a singleton, since they may start out synchronous and
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* only become async with time, so you it may not be clear why you are seeing certain changes in behaviour.
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*
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* http://stackoverflow.com/questions/17721263/singleton-across-jvm-or-application-instance-or-tomcat-instance
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*/
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public class App {
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public static void main(String[] args) {
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// eagerly initialized singleton
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IvoryTower ivoryTower1 = IvoryTower.getInstance();
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IvoryTower ivoryTower2 = IvoryTower.getInstance();
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System.out.println("ivoryTower1=" + ivoryTower1);
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System.out.println("ivoryTower2=" + ivoryTower2);
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// lazily initialized singleton
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ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower1 = ThreadSafeLazyLoadedIvoryTower
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.getInstance();
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ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower2 = ThreadSafeLazyLoadedIvoryTower
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.getInstance();
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System.out.println("threadSafeIvoryTower1=" + threadSafeIvoryTower1);
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System.out.println("threadSafeIvoryTower2=" + threadSafeIvoryTower2);
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// enum singleton
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EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
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EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
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System.out.println("enumIvoryTower1=" + enumIvoryTower1);
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System.out.println("enumIvoryTower2=" + enumIvoryTower2);
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InitializingOnDemandHolderIdiom demandHolderIdiom = InitializingOnDemandHolderIdiom.getInstance();
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System.out.println(demandHolderIdiom);
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InitializingOnDemandHolderIdiom demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
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System.out.println(demandHolderIdiom2);
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ThreadSafeDoubleCheckLocking dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
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System.out.println(dcl1);
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ThreadSafeDoubleCheckLocking dcl2 = ThreadSafeDoubleCheckLocking.getInstance();
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System.out.println(dcl2);
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}
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}
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