Virtual Method Table (VMT) & Dynamic Method Dispatch

1Concept

In Java, all non-final, non-private instance methods are virtual by default. The JVM resolves overridden method calls dynamically at runtime using a Virtual Method Table (vtable). Each class has a vtable pointing to the actual executable function pointers in memory.

2Architecture Diagram

[ PaymentService Ref ] ---> Runtime points to [ CryptoPaymentService Object ]
                                              |
                                              v [ CryptoPaymentService vtable ]
                                              processPayment() -> points to Crypto logic

3Code Example

public class DynamicDispatchDemo {
    abstract static class Notification {
        abstract void send(String msg);
    }

    static class EmailNotification extends Notification {
        void send(String msg) { System.out.println("Email Dispatch: " + msg); }
    }

    static class SmsNotification extends Notification {
        void send(String msg) { System.out.println("SMS Gateway Dispatch: " + msg); }
    }

    public static void main(String[] args) {
        Notification[] channels = {
            new EmailNotification(),
            new SmsNotification()
        };

        for (Notification channel : channels) {
            channel.send("Deployment Completed Successfully"); // Dynamic vtable lookup
        }
    }
}

4Expected Output

Email Dispatch: Deployment Completed Successfully
SMS Gateway Dispatch: Deployment Completed Successfully

5Key Takeaways

• ✓Dynamic dispatch executes via bytecode `invokevirtual` opcode.
• ✓HotSpot JIT devirtualizes monomorphic call sites to inline methods directly.
• ✓Variables are bound by reference type at compile time, but executed by object type at runtime.