JEP 490: ZGC: Remove the Non-Generational Mode

🧠 JEP 490: ZGC – Remove the Non-Generational Mode

🔍 What is ZGC?

• ZGC (Z Garbage Collector) is a low-latency, concurrent, and scalable garbage collector introduced in JDK 11.

• It was originally non-generational — treating all objects the same regardless of age.

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🆕 What Does JEP 490 Do?

It removes the old non-generational mode of ZGC and makes the generational mode the only option.

✅ Now, ZGC is always generational.

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🎯 Why Remove the Non-Generational Mode?

Problem With Old Mode	Benefit of Generational ZGC
Treated all objects equally	Separates young vs old objects
More CPU/memory overhead	Faster minor collections
Less predictable performance	Lower pause times, better throughput

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🧪 Key Benefit in Practice:

Generational GC improves performance by:

• Focusing frequent collections on short-lived (young) objects.

• Delaying collection of long-lived (old) objects, which change less often.

This reduces the cost of GC and improves overall application responsiveness.

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📅 Timeline:

JDK Version	ZGC Status
JDK 11–20	Non-generational (default)
JDK 21	Generational (preview)
JDK 22+	Generational becomes default
🔥 JDK 24	Non-generational is removed (JEP 490)

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🧰 Impact for Developers

• ✅ No config needed to enable generational ZGC — it's default.

• 🧹 The option -XX:+ZGenerational is no longer required.

• ❌ The old non-generational ZGC is no longer available.