Memory Leak When Injecting Prototype Beans into Spring Singleton Beans — Full Case Study

Umesh Kumar Yadav

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6 hours ago

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Interactive Interview Theme:
*“You’re in a senior backend interview.

The interviewer says:
‘We have a memory leak. A List in a base class keeps growing — even after we marked the beans as @Scope("prototype"). Why? And how do you fix it?’

Can you explain with logs, source-level insight, and two production fixes — all while staying calm?”*

Let’s simulate the real incident — and turn it into a learning masterpiece.

Problem Background

A stateful abstract service accumulates 1 million characters per call in a List<String>.
Subclasses are injected into a singleton controller via List<SayService>.

Expectation:
Each HTTP request → new SayHello and SayBye instances → fresh data list.

Reality:
data.size() keeps growing → OOM after 100+ requests.

The Code That Broke Production

// Stateful base class — DANGER!
public abstract class SayService {
    List<String> data = new ArrayList<>();  // ← Shared per instance

    public void say() {
        data.add(IntStream.rangeClosed(1, 1_000_000)…