javaAsync race conditionHard

Singleton Instance Inconsistency in Multi-threaded Environment

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What it should do

Return the single instance of the Singleton class. Example: getInstance() -> Singleton instance.

What happens instead

Inconsistent singleton instance returned across different threads.

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the async and race conditions pattern

A missing await, unbuffered channel deadlocks, ConcurrentModificationException, and data races under load.

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public class Singleton { private static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance == null) { synchronized (Singleton.class) { if (instance == null) { instance = new Singleton(); } } } return instance; } }

Singleton Instance Inconsistency in Multi-threaded Environment

What it should do

What happens instead

More async and race conditions to practise

Singleton Instance Inconsistency in Multi-threaded Environment

What it should do

What happens instead

More async and race conditions to practise