JIT optimization – Can we have mix of machine code and byte code

At runtime, for the same class, the JVM can execute some methods as machine code and others as bytecode.

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🧠 How it works:

The JVM (including GraalVM) treats each method independently:

• If a method becomes “hot” (called frequently), the JIT compiler compiles it to machine code.
• If a method is rarely called, it remains in interpreted mode (bytecode).
• The JVM can call between them seamlessly.

💡 So yes — for the same class, some methods will run as compiled native code, others will still run as interpreted bytecode.

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📦 Example

javaCopyEditpublic class MixedMode {

    public static int hotMethod(int x) {
        return x * 2;
    }

    public static int coldMethod(int x) {
        return x * 1000000;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 1_000_000; i++) {
            hotMethod(i); // gets compiled to machine code
        }

        coldMethod(42); // stays interpreted
    }
}

Runtime Execution:

• hotMethod() becomes machine code after it hits a threshold (e.g., 10,000 invocations).
• coldMethod() is executed as bytecode, since it’s only called once.