How Does a Java Program Run Inside the JVM? A Step‑by‑Step Walkthrough

We all know Java programs run on the JVM, but many are unaware of how the execution actually proceeds. This article explores that process.

Program Counter: The bytecode execution indicator that records the current line number, private to each thread.

JVM Stack: Also thread‑private, it provides a stack frame for each method, storing the local variable table, method return address, etc. When a method is invoked, a frame is pushed; when it returns, the frame is popped.

Native Method Stack: Similar to the JVM stack but serves native (C/C++) methods.

Heap: The largest memory area in the JVM, storing objects; garbage collection primarily targets this region.

For developers, the heap and stack are the most relevant parts of the JVM because they hold objects and their references.

Consider the following simple code:

public class Example {
    public static void main(String[] args) {
        a();
    }

    public static void a() {
        int a = 1;
        b();
    }

    public static void b() {
        User b = new User();
    }
}

The code defines a class with a main() method that calls a(), which defines an int variable and calls b(); b() creates a new User object.

To understand the execution, you need a bit of prior knowledge:

The local variable table in a stack frame stores Java primitive and reference types.

Instance objects reside in the heap, while the stack holds references to those instances.

The following steps illustrate how the above code runs inside the JVM:

Program starts.

main() method is pushed onto the stack.

a() is invoked and pushed.

Primitive variable a is created in the stack.

b() is invoked and pushed.

Reference variable b is created in the stack.

An instance of User is allocated in the heap.

The heap instance’s address is stored in the reference b.

b() finishes, b is popped and the reference is cleared.

a() finishes, a is popped.

Garbage collection reclaims the now‑unreferenced User object.

main() finishes and is popped.

Program execution ends.