Understand the Singleton pattern by comparing Java and JavaScript code


Details and other patterns will be written in ** Understanding design patterns by comparing implementations in JavaScript and Java **.

Singleton pattern

Software usually creates or prepares multiple instances from a class However, there are times when "I only create one instance of this class and I don't want to." Like the legendary sword in RPGs If you get many legendary swords in one game, online game, etc., the value will decrease.

A putter that guarantees that only one such instance exists is called the ** Singleton pattern **.

Implementation example in Java

Class diagram

The constructor is private so that if you write `new Singleton ()`, you will get an error. Singleton.png



public class Main {
    public static void main(String[] args) {
        Singleton obj1 = Singleton.getInstance();
        Singleton obj2 = Singleton.getInstance();
        if (obj1 == obj2) {
            System.out.println("obj1 and ojb2 are the same instance.");
        } else {
            System.out.println("obj1 and obj2 are not the same instance.");
        //I get an error when I uncomment the following
        // Singleton obj3 = new Singleton();


public class Singleton {
    private static Singleton singleton = new Singleton();

    private Singleton() {
        System.out.println("You have created an instance.");
    public static Singleton getInstance() {
        return singleton;

** Instance generation timing **

I think the execution result will be as follows

You have created an instance.
obj1 and obj2 are the same instance.

As you can see above, you can see that the instance was created the first time the getInstance method was called. The Singleton class is loaded when the getInstance static method is called.

Implementation example in JavaScript



<!DOCTYPE html>
<html lang="ja">
    <meta charset="UTF-8">
    <script src="Main.js"></script>
    <script src="Singleton.js"></script>


var MAIN = {};

MAIN.init = function() {
     *The book said the following, but I changed it to make it easier to compare
     * var obj1 = SINGLETON.singleton
     * var obj2 = SINGLETON.singleton
    var obj1 = SINGLETON.singleton.getInstance();
    var obj2 = SINGLETON.singleton.getInstance();
    if (obj1 === obj2) {
        console.log("obj1 and obj2 are the same instance.");
    } else {
        console.log("obj1 and obj2 are not the same instance.");

window.addEventListener("load", MAIN.init);


var SINGLETON = {};

SINGLETON.singleton = (function() {
    var instance;

    function init() {
        console.log("You have created an instance.");
        var privateVariable = "I am also private";
        function privateMethod() {
            console.log("I am private");

        return {
            publicProperty: "I am also public",
            publicMethod: function() {
                console.log("The public can ses me!");

    return {
        getInstance: function() {
            if (!instance) {
                instance = init();

            return instance;

Characters in the Singleton pattern

** The role of Singleton **

Has a static method to get only one instance This method always returns the same instance

Singleton class diagram


Singleton pattern need

Guarantee when you only want to create one instance as described above In addition, delayed execution is important for singletons. Doesn't consume memory unless a static instance is needed as a reason In the case of C ++, the order of dynamic initialization is unpredictable, so using a singleton has the advantage that the programmer can decide the timing himself. In addition to wanting to make only one, use it for things that do not make sense even if you can make two or more

Example of use When you want to use a class like a global variable

Related patterns


[Introduction to Design Patterns Learned in the Enhanced and Revised Java Language](https://www.amazon.co.jp/%E5%A2%97%E8%A3%9C%E6%94%B9%E8%A8%82% E7% 89% 88Java% E8% A8% 80% E8% AA% 9E% E3% 81% A7% E5% AD% A6% E3% 81% B6% E3% 83% 87% E3% 82% B6% E3% 82% A4% E3% 83% B3% E3% 83% 91% E3% 82% BF% E3% 83% BC% E3% 83% B3% E5% 85% A5% E9% 96% 80-% E7% B5 % 90% E5% 9F% 8E-% E6% B5% A9 / dp / 4797327030) JavaScript design pattern

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