How Does Java's Garbage Collector Handle Circular References?

Understanding Java's Garbage Collection with Circular References

Garbage collection in Java is responsible for automatically freeing up memory occupied by objects that are no longer in use by any live references. While this mechanism generally eliminates unnecessary memory consumption, it can encounter challenges when dealing with circular references.

Consider the following code:

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 scenario, objects a, b, and c form a circular reference (a points to c, c points to b, and b points to a). According to traditional garbage collection principles, these objects should be marked as unreachable, since no live references exist outside the scope where they were created. However, they appear to be referenced within the circular loop.

Java's Solution to Circular References

Java's garbage collector addresses circular references by employing a more sophisticated reachability analysis. It leverages the idea of "reachable" and "unreachable" objects.

An object is considered reachable if it is directly or indirectly referenced from a "garbage collection root". Garbage collection roots include global variables, references stored in thread registers, and references held by the JVM itself.

In the given example, objects a, b, and c are initially reachable when created within the scope. However, once the scope ends (when the curly braces close), all references to these objects from outside are severed. They become unreachable from the garbage collection root.

Reclaiming Unreachable Circular References

Now that a, b, and c are unreachable, the garbage collector can identify them as potential garbage. It triggers a mark-and-sweep operation where:

• Mark: The GC traverses the entire object graph, marking reachable objects. It identifies that a, b, and c are not reachable.
• Sweep: The GC removes all marked objects from the heap, reclaiming their memory.

In this manner, the Java garbage collector breaks the circular reference cycle and frees up the memory occupied by a, b, and c, even though they point to each other.

The above is the detailed content of How Does Java's Garbage Collector Handle Circular References?. For more information, please follow other related articles on the PHP Chinese website!