Thread-Safe Singleton Logger

Problem Context

You are building a distributed logging system for a high-traffic web application. The system requires a single, shared Logger instance that is safely created and accessed across multiple concurrent threads. Your task is to implement a thread-safe Singleton pattern in Java that:

1. Ensures only one instance of Logger is ever created, even under heavy concurrent access.
2. Is lazily initialized (the instance is not created until it is first needed).
3. Is efficient — once initialized, threads should not unnecessarily block each other.

Requirements

Implement a Logger class with the following behavior:

• Logger.getInstance() — Returns the single shared instance of Logger. Must be thread-safe.
• void log(String message) — Appends the message (with a newline) to an internal log.
• String getLogs() — Returns all logged messages as a single concatenated string.
• void reset() — For testing only. Clears all logs and resets the instance (simulates a fresh start).

Input / Output Format

You will be given a sequence of operations as two arrays:

• operations: a list of method names to call ("getInstance", "log", "getLogs", "reset")
• arguments: a list of arguments for each operation (empty string "" if no argument is needed)

Return a list of results for each operation:

• "getInstance" → returns "Logger@singleton" (confirms same instance)
• "log" → returns "null"
• "getLogs" → returns the full log string
• "reset" → returns "null"

Examples

Example 1

Input: operations = ["getInstance", "log", "log", "getLogs"], arguments = ["", "Server started", "Request received", ""]

Output: ["Logger@singleton", "null", "null", "Server started\nRequest received\n"]

Explanation: getInstance returns the singleton. Two messages are logged. getLogs returns them in order.

Example 2

Input: operations = ["getInstance", "getInstance"], arguments = ["", ""]

Output: ["Logger@singleton", "Logger@singleton"]

Explanation: Both calls return the exact same singleton instance, confirming only one object is ever created.

Example 3

Input: operations = ["log", "getLogs", "reset", "getLogs"], arguments = ["Error 500", "", "", ""]

Output: ["null", "Error 500\n", "null", ""]

Explanation: After reset, the log is cleared and getLogs returns an empty string.

Approach Hints

• Consider using the double-checked locking idiom with a volatile field for lazy initialization.
• Alternatively, explore the Bill Pugh Singleton (static inner holder class) for a clean, thread-safe approach without explicit synchronization.
• Avoid naive synchronized getInstance() which locks on every call — this is inefficient.
• Think about what volatile guarantees in the Java Memory Model and why it matters here.