How Does Java Garbage Collection Handle Circular References to Prevent Memory Leaks?

How Does Java Garbage Collection Handle Circular References?

In Java, garbage collection (GC) removes unused objects from memory. However, circular references, where objects reference each other, can prevent GC from functioning correctly.

Consider the following example:

class Node {
    public Object value;
    public Node next;

    public Node(Object o, Node n) {
        value = o;
        next = n;
    }
}

//...some code
{
    Node a = new Node("a", null),
            b = new Node("b", a),
            c = new Node("c", b);
    a.next = c;
} //end of scope
//...other code

In this example, a, b, and c form a cycle of references. As a result, the GC cannot reclaim these objects because there is always a reference leading back to them.

How does Java GC deal with this?

Java's GC: Unreachable Objects

Java GC identifies objects as garbage only if they are unreachable from any GC root. GC roots include global variables, static variables, thread stacks, and objects pointed to by other reachable objects.

In our example, since a, b, and c are not referenced by any GC roots, they are considered unreachable and thus garbage. Despite forming a cycle, GC will reclaim these objects.

Implications for Memory Management

Circular references do not inherently cause memory leaks in Java. As long as the objects in the circular reference are unreachable from any GC root, they will still be collected. However, it's important to be aware of circular references if they can lead to unexpected behavior or performance issues.

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