Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break the modularity of your code.
You can’t just use a class that depends on a Singleton in some other context, without carrying over the Singleton to the other context. Most of the time, this limitation comes up during the creation of unit tests.
Identification: Singleton can be recognized by a static creation method, which returns the same cached object.
Naïve Singleton (single-threaded)
It’s pretty easy to implement a sloppy Singleton. You just need to hide the constructor and implement a static creation method.
Singleton.java: Singleton
package refactoring_guru.singleton.example.non_thread_safe; public final class Singleton { private static Singleton instance; public String value; private Singleton(String value) { // The following code emulates slow initialization. try { Thread.sleep(1000); } catch (InterruptedException ex) { ex.printStackTrace(); } this.value = value; } public static Singleton getInstance(String value) { if (instance == null) { instance = new Singleton(value); } return instance; } }
DemoSingleThread.java: Client code
package refactoring_guru.singleton.example.non_thread_safe; public class DemoSingleThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Singleton singleton = Singleton.getInstance("FOO"); Singleton anotherSingleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); System.out.println(anotherSingleton.value); } }
OutputDemoSingleThread.txt: Execution result
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO FOO
Naïve Singleton (multithreaded)
The same class behaves incorrectly in a multithreaded environment. Multiple threads can call the creation method simultaneously and get several instances of Singleton class.
Singleton.java: Singleton
package refactoring_guru.singleton.example.non_thread_safe; public final class Singleton { private static Singleton instance; public String value; private Singleton(String value) { // The following code emulates slow initialization. try { Thread.sleep(1000); } catch (InterruptedException ex) { ex.printStackTrace(); } this.value = value; } public static Singleton getInstance(String value) { if (instance == null) { instance = new Singleton(value); } return instance; } }
DemoMultiThread.java: Client code
package refactoring_guru.singleton.example.non_thread_safe; public class DemoMultiThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Thread threadFoo = new Thread(new ThreadFoo()); Thread threadBar = new Thread(new ThreadBar()); threadFoo.start(); threadBar.start(); } static class ThreadFoo implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("FOO"); System.out.println(singleton.value); } } static class ThreadBar implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); } } }
OutputDemoMultiThread.txt: Execution result
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO BAR
Thread-safe Singleton with lazy loading
To fix the problem, you have to synchronize threads during first creation of the Singleton object.
Singleton.java: Singleton
package refactoring_guru.singleton.example.thread_safe; public final class Singleton { // The field must be declared volatile so that double check lock would work // correctly. private static volatile Singleton instance; public String value; private Singleton(String value) { this.value = value; } public static Singleton getInstance(String value) { // The approach taken here is called double-checked locking (DCL). It // exists to prevent race condition between multiple threads that may // attempt to get singleton instance at the same time, creating separate // instances as a result. // // It may seem that having the `result` variable here is completely // pointless. There is, however, a very important caveat when // implementing double-checked locking in Java, which is solved by // introducing this local variable. // // You can read more info DCL issues in Java here: // https://refactoring.guru/java-dcl-issue Singleton result = instance; if (result != null) { return result; } synchronized(Singleton.class) { if (instance == null) { instance = new Singleton(value); } return instance; } } }
DemoMultiThread.java: Client code
package refactoring_guru.singleton.example.thread_safe; public class DemoMultiThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Thread threadFoo = new Thread(new ThreadFoo()); Thread threadBar = new Thread(new ThreadBar()); threadFoo.start(); threadBar.start(); } static class ThreadFoo implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("FOO"); System.out.println(singleton.value); } } static class ThreadBar implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); } } }
OutputDemoMultiThread.txt: Execution result
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: BAR BAR
Want more?
There are even more special flavors of the Singleton pattern in Java. Take a look at this article to find out more:
