Java.Multithreading.What are “green threads” and do they exist in Java?

🌱 What Are Green Threads?

Green threads are threads that are scheduled and managed entirely by the JVM (or user-level runtime), not the OS.

That means:

• The JVM simulates threading using only one OS thread (or very few)
• Thread switching is handled inside the JVM, not by the OS
• Good for platforms where OS threads are expensive or unavailable

🧠 Think of green threads as “user-mode” threads.

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💡 Characteristics of Green Threads

Feature	Green Threads
Scheduled by	JVM or runtime (user-space)
Number of OS threads	Usually just 1
Portability	High (doesn’t depend on OS threading)
True parallelism	❌ No (on multi-core systems)
Performance on IO	✅ Often faster for lightweight tasks
Blocking behavior	❌ One thread blocks = all block

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☕ Did Java Ever Use Green Threads?

✅ Yes! Java originally used green threads, especially in:

• Early JVMs (like on Solaris or old Linux systems)
• Java 1.1 and some embedded environments

But…

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❌ Do Green Threads Exist in Java Today?

🔥 No — modern Java uses native OS threads by default.

Since Java 1.2, the HotSpot JVM uses native threads, where:

• Each Java thread = a real OS thread
• The OS scheduler manages them
• You get true parallelism on multi-core CPUs

This makes Java apps faster and more scalable on modern machines.

🚀 But Wait — Green Threads Are Coming Back?

Yes! In the form of:

🧵 Project Loom (since Java 19+ as preview)

Loom introduces virtual threads, which are kind of like modern green threads.

• Scheduled by JVM, not OS
• Thousands of threads with tiny memory footprint
• Work cooperatively unless they block on native code
• Enable massive concurrency (millions of threads!)

Thread.startVirtualThread(() -> {
    System.out.println("Hello from a virtual thread!");
});

✅ These are green threads, reborn — and they’re amazing for lightweight, IO-heavy applications like web servers.

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🧠 Summary Table

Feature	Green Threads	Native Threads	Virtual Threads (Loom)
Managed by	JVM	OS	JVM
Introduced in Java	Java 1.0–1.1	Java 1.2+	Java 19+ (preview)
True parallelism	❌ No	✅ Yes	✅ Yes (cooperatively)
Memory overhead	🟢 Very low	🔴 Higher	🟢 Very low
Blocking behavior	❌ Blocks all	✅ Thread-local blocking	🟢 Handles well with Loom