Thread-Safe Singleton

Problem Context

In many real-world applications, certain objects should only be instantiated once during the lifecycle of the program — for example, a database connection pool, a configuration manager, or a logging service. The Singleton design pattern ensures that a class has only one instance and provides a global point of access to it.

However, in a multi-threaded environment, naïve implementations can lead to race conditions where multiple threads simultaneously create separate instances. Your task is to implement a thread-safe Singleton class in Java.

Task

Implement a Singleton class with the following requirements:

1. Only one instance of the class should ever be created, even when multiple threads request it concurrently.
2. The instance should be created lazily (i.e., only when first requested, not at class load time).
3. All threads must receive the same instance reference.
4. Your implementation must avoid unnecessary synchronization after the instance is created (for performance).

You must implement the following method:

public static Singleton getInstance()

Input / Output Format

For testing purposes, a simulation harness will:

• Spawn N threads simultaneously, each calling Singleton.getInstance().
• Collect all returned instance hash codes (via System.identityHashCode()).
• Return "PASS" if all hash codes are identical, "FAIL" otherwise.

Input: An integer N — the number of concurrent threads requesting the instance.

Output: "PASS" if all threads received the same instance, "FAIL" otherwise.

Examples

Example 1

Input: N = 1
Output: "PASS"
Explanation: Only one thread requests the instance. Trivially, one unique instance is created and returned.

Example 2

Input: N = 10
Output: "PASS"
Explanation: Ten threads concurrently call getInstance(). A correct thread-safe implementation ensures all threads receive the same instance reference.

Example 3

Input: N = 100 Even under heavy concurrency with 100 threads racing to get the instance, only one instance is ever created.