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Singleton in Java
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.
Complexity:
Popularity:
Usage examples: A lot of developers consider the Singleton pattern an antipattern. That’s why its usage is on the decline in Java code.
Despite this, there are quite a lot of Singleton examples in Java core libraries:
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:
