Bill Pugh Singleton Solution or Holder Singleton Pattern

Prior to Java 5, the Synchronized method and block Synchronization has a lot of issues and were failing in multiple scenarios in multi-threaded environments. So Bill Pugh suggested using static inner class to create Singleton classes.

The static inner classes will only get loaded in JVM memory when the getInstance method is called which makes it lazy-loaded. The static inner class will be invoked sequentially in JVM ie non-concurrent, so these are not required to be Synchronized, in a multi-threaded or concurrent environment, the threads will invoke the static class in order hence don't need external Synchronization which makes it thread-safe.

package org.wesome.dsalgo.design.pattern.singleton;

public class Apple {

    public Apple() {
        System.out.println("Apple default constructor");
    }
 
    public static void favoriteFruitUtilityFunc() {
        System.out.println("Apple is my favorite fruit.");
    }

    private static class BillPughSingletonClass {
        static Apple appleInstance = new Apple();
    }

    public static Apple getAppleInstance() {
        System.out.println("Apple getAppleInstance");
        return BillPughSingletonClass.appleInstance;
    }
}

package org.wesome.dsalgo.design.pattern.singleton;

public class SingletonClass {
    public static void main(String[] args) {
        Apple.favoriteFruitUtilityFunc();
        System.out.println(Apple.getAppleInstance());
    }
}

plugins {
    id 'java'
    id "io.freefair.lombok" version "6.4.1"
}

group = 'org.wesome'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = JavaVersion.VERSION_1_8

configurations {
    compileOnly {
        extendsFrom annotationProcessor
    }
}

repositories {
    mavenCentral()
}

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter:5.6.2'
}

test {
    useJUnitPlatform()
}