Java Garbage Collection: GC Roots, Generational Collectors & ZGC

In languages like C and C++, developers must manually allocate and deallocate memory using malloc() and free(). One forgotten free creates memory leaks; a premature free causes fatal segmentation faults. In Java, Garbage Collection (GC) is entirely automated, continuously tracking object lifecycles and reclaiming memory silently in the background.

1. Reachability Analysis & GC Roots

An object is eligible for garbage collection when there is no path of references linking it to a GC Root. Primary GC Roots include:

• Thread Stack Local Variables: Parameters and local variables inside currently executing method frames.
• Static Variables: References held by loaded classes in Metaspace.
• JNI (Java Native Interface) Handles: Global and local native pointers in C/C++ code.
• Active Java Threads: Running Thread objects.

2. The Three Core Phases of Garbage Collection

1. Mark: The GC traverses the reference graph starting from GC Roots, marking all reachable (live) objects.
2. Sweep: Unmarked memory slots containing dead objects are cleared and marked as available for future allocations.
3. Compact: Live objects are shifted together to contiguous memory addresses to eliminate fragmented empty holes.

3. Generational GC Lifecycle

4. Modern Java Garbage Collectors Compared

Making Objects Eligible for Garbage Collection

public class GCDemo {
    public static void main(String[] args) {
        // Case 1: Nullifying reference
        String s1 = new String("Veewom");
        s1 = null; // Original String object is now unreachable and eligible for GC

        // Case 2: Reassigning reference
        String a = new String("First");
        String b = new String("Second");
        a = b; // "First" object is now orphaned and eligible for GC

        // Case 3: Objects created inside local method scope
        createTempData(); // Any objects inside createTempData() without escaping are collected
        
        // Suggesting GC (Never guaranteed in production!)
        System.gc();
    }

    private static void createTempData() {
        byte[] temp = new byte[1024]; // dies when method stack frame pops
    }
}