When Is an Object Eligible for GC?

Direct Answer

An object is eligible for garbage collection when no chain of strong references from a GC root can reach it. Setting one variable to null or leaving a scope is enough only if no other reachable path still points to that object. Eligibility does not promise when—or whether before process exit—the JVM will reclaim it.

Code Example

public final class ReachabilityExample {
    static Object retained;

    static void references() {
        Object first = new Object();
        Object alias = first;

        first = null; // the object is still reachable through alias
        retained = alias;
        alias = null; // the object is still reachable through retained

        retained = null; // now it may be eligible if no other path exists
    }

    static void unreachableCycle() {
        Node left = new Node();
        Node right = new Node();
        left.next = right;
        right.next = left;
        // If neither node escapes this call, the cycle can become unreachable.
    }

    static final class Node { Node next; }
}

These comments illustrate reference paths, not a guaranteed collection instant. The JIT may also prove that a local is no longer needed before the source scope ends.

Common Mistakes & Pitfalls

• Thinking null means instant deletion: Setting a variable to null removes one path. The object becomes eligible only when no strong-reference path from a GC root reaches it.
• Forgetting long-lived roots: A collection reachable through static state or a live thread can retain every object it contains until that path is cleared or its class loader or thread can end.
• Misunderstanding circular references: Two objects may point to each other and still be eligible when no root can reach either one. Reachability tracing handles such cycles.