OutOfMemoryError: Unable to Create New Native Thread

Why This Happens

Each Java thread maps to a native OS thread, and each thread consumes stack memory. When you create thousands of threads, the OS runs out of memory for thread stacks or hits the process thread limit. Use thread pools instead of creating unbounded threads.

The Problem

for (int i = 0; i < 100_000; i++) {
    new Thread(() -> {
        try { Thread.sleep(60_000); } catch (Exception e) {}
    }).start();
}

The Fix

ExecutorService executor = Executors.newFixedThreadPool(100);
for (int i = 0; i < 100_000; i++) {
    executor.submit(() -> {
        try { Thread.sleep(60_000); } catch (Exception e) {}
    });
}
executor.shutdown();

Step-by-Step Fix

1. 1

   Identify thread creation

   Use jstack or a thread dump to see how many threads are alive and what they are doing.

2. 2

   Find unbounded thread creation

   Search for new Thread() or Executors.newCachedThreadPool() that could create unlimited threads.

3. 3

   Use a bounded thread pool

   Replace direct thread creation with a fixed-size ExecutorService, or reduce stack size with -Xss if threads are using more stack than needed.