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#107 JavaDoc for Singleton

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This commit is contained in:
Ilkka Seppala
2015-08-21 23:09:51 +03:00
parent 743b6e69c5
commit 4bf2e3f16d
5 changed files with 88 additions and 74 deletions
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106
singleton/src/main/java/com/iluwatar/singleton/App.java
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@@ -1,51 +1,55 @@
package com.iluwatar.singleton;
/**
*
* Singleton pattern ensures that the class (IvoryTower) can have only one
* existing instance per java classloader instance and provides global access to it.
*
* http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
*
* The risk of this pattern is that bugs resulting from setting a singleton up in a distributed environment can
* be tricky to debug, since it will work fine if you debug with a single classloader. Additionally, these
* problems can crop up a while after the implementation of a singleton, since they may start out synchronous and
* only become async with time, so you it may not be clear why you are seeing certain changes in behaviour.
*
* http://stackoverflow.com/questions/17721263/singleton-across-jvm-or-application-instance-or-tomcat-instance
*/
public class App {
public static void main(String[] args) {
// eagerly initialized singleton
IvoryTower ivoryTower1 = IvoryTower.getInstance();
IvoryTower ivoryTower2 = IvoryTower.getInstance();
System.out.println("ivoryTower1=" + ivoryTower1);
System.out.println("ivoryTower2=" + ivoryTower2);
// lazily initialized singleton
ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower1 = ThreadSafeLazyLoadedIvoryTower
.getInstance();
ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower2 = ThreadSafeLazyLoadedIvoryTower
.getInstance();
System.out.println("threadSafeIvoryTower1=" + threadSafeIvoryTower1);
System.out.println("threadSafeIvoryTower2=" + threadSafeIvoryTower2);
// enum singleton
EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
System.out.println("enumIvoryTower1=" + enumIvoryTower1);
System.out.println("enumIvoryTower2=" + enumIvoryTower2);
InitializingOnDemandHolderIdiom demandHolderIdiom = InitializingOnDemandHolderIdiom.getInstance();
System.out.println(demandHolderIdiom);
InitializingOnDemandHolderIdiom demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
System.out.println(demandHolderIdiom2);
ThreadSafeDoubleCheckLocking dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
System.out.println(dcl1);
ThreadSafeDoubleCheckLocking dcl2 = ThreadSafeDoubleCheckLocking.getInstance();
System.out.println(dcl2);
}
}
package com.iluwatar.singleton;
/**
*
* Singleton pattern ensures that the class ({@link IvoryTower}) can have only one
* existing instance per Java classloader instance and provides global access to it.
* <p>
* http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
*<p>
* The risk of this pattern is that bugs resulting from setting a singleton up in a distributed environment can
* be tricky to debug, since it will work fine if you debug with a single classloader. Additionally, these
* problems can crop up a while after the implementation of a singleton, since they may start out synchronous and
* only become async with time, so you it may not be clear why you are seeing certain changes in behaviour.
* <p>
* http://stackoverflow.com/questions/17721263/singleton-across-jvm-or-application-instance-or-tomcat-instance
*/
public class App {
/**
* Program entry point
* @param args command line args
*/
public static void main(String[] args) {
// eagerly initialized singleton
IvoryTower ivoryTower1 = IvoryTower.getInstance();
IvoryTower ivoryTower2 = IvoryTower.getInstance();
System.out.println("ivoryTower1=" + ivoryTower1);
System.out.println("ivoryTower2=" + ivoryTower2);
// lazily initialized singleton
ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower1 = ThreadSafeLazyLoadedIvoryTower
.getInstance();
ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower2 = ThreadSafeLazyLoadedIvoryTower
.getInstance();
System.out.println("threadSafeIvoryTower1=" + threadSafeIvoryTower1);
System.out.println("threadSafeIvoryTower2=" + threadSafeIvoryTower2);
// enum singleton
EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
System.out.println("enumIvoryTower1=" + enumIvoryTower1);
System.out.println("enumIvoryTower2=" + enumIvoryTower2);
InitializingOnDemandHolderIdiom demandHolderIdiom = InitializingOnDemandHolderIdiom.getInstance();
System.out.println(demandHolderIdiom);
InitializingOnDemandHolderIdiom demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
System.out.println(demandHolderIdiom2);
ThreadSafeDoubleCheckLocking dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
System.out.println(dcl1);
ThreadSafeDoubleCheckLocking dcl2 = ThreadSafeDoubleCheckLocking.getInstance();
System.out.println(dcl2);
}
}

6
singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java
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@@ -4,12 +4,12 @@ import java.io.Serializable;
/**
* The Initialize-on-demand-holder idiom is a secure way of
* creating lazy initialize singleton Object in Java.
* creating lazy initialized singleton object in Java.
* refer to "The CERT Oracle Secure Coding Standard for Java"
* By Dhruv Mohindra, Robert C. Seacord p.378
*
* <p>
* Singleton objects usually are heavy to create and sometimes need to serialize them.
* This class also shows how to preserve singleton in Serialized version of singleton.
* This class also shows how to preserve singleton in serialized version of singleton.
*
* @author mortezaadi@gmail.com
*

5
singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java
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@@ -2,10 +2,11 @@ package com.iluwatar.singleton;
/**
* Double check locking
*
* <p>
* http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
*
* <p>
* Broken under Java 1.4.
*
* @author mortezaadi@gmail.com
*
*/

33
singleton/src/test/java/com/iluwatar/singleton/AppTest.java
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@@ -1,14 +1,19 @@
package com.iluwatar.singleton;
import org.junit.Test;
import com.iluwatar.singleton.App;
public class AppTest {
@Test
public void test() {
String[] args = {};
App.main(args);
}
}
package com.iluwatar.singleton;
import org.junit.Test;
import com.iluwatar.singleton.App;
/**
*
* Application test
*
*/
public class AppTest {
@Test
public void test() {
String[] args = {};
App.main(args);
}
}

12
singleton/src/test/java/com/iluwatar/singleton/LazyLoadedSingletonThreadSafetyTest.java
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@@ -5,9 +5,9 @@ import org.junit.Test;
/**
*
* This test case demonstrates thread safety issues of lazy loaded Singleton implementation.
*
* <p>
* Out of the box you should see the test output something like the following:
*
* <p>
* Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
* Thread=Thread-2 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
* Thread=Thread-0 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
@@ -15,13 +15,17 @@ import org.junit.Test;
* Thread=Thread-3 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
* Thread=Thread-1 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@60f330b0
* Thread=Thread-2 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@6fde356e
*
* <p>
* By changing the method signature of LazyLoadedIvoryTower#getInstance from
* <p><blockquote><pre>
* public static LazyLoadedIvoryTower getInstance()
* </pre></blockquote><p>
* into
* <p><blockquote><pre>
* public synchronized static LazyLoadedIvoryTower getInstance()
* </pre></blockquote><p>
* you should see the test output change to something like the following:
*
* <p>
* Thread=Thread-4 creating instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
* Thread=Thread-4 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490
* Thread=Thread-0 got instance=com.iluwatar.singleton.LazyLoadedSingletonThreadSafetyTest$LazyLoadedIvoryTower@3c688490