How does Java solve retain cycles in garbage collection?

Question

I know that a retain cycle (at least in Objective-C and Swift) is when two objects claim ownership of one another (they have references to each other). And in Objective-C we can

Answer 1

The Java (JVM) garbage collector works by looking for "reachable" objects - from the root(s) of the object tree. If they can't be reached (if they have no outside object references) then entire object graphs can be discarded.

Essentially it just just traverses the tree from root(s) to leaf nodes and marks all objects it encounters. Any memory not taken up by marked objects in the heap is swept (marked as free). This is called mark and sweep. img src

This can't be done easily in objective-c because it uses reference counting, not mark and sweep which has it's flaws

The reason there can be no retain cycles is because if they aren't linked to the "tree" anywhere, they aren't marked and can be discarded.

Answer 2

As the name suggests, Garbage Collection refers to removing of objects which are no longer in use. It is a well known fact that irrespective of their scope objects, Java stores objects in heap. Thus, if we keep on creating objects without clearing the heap, our computers might run out of heap space and we get ‘Out of Memory’ error. Garbage Collection in Java is a mechanism which is controlled and executed by the Java Virtual Machine (JVM) to release the heap space occupied by the objects which are no more in use. In contrast to C++, garbage collection in java relives the developer from the Memory Management related activities. The JVM executes this process with the help of a demon thread called the ‘Garbage Collector’. The garbage collector thread first invokes the finalize method of the object. This performs the cleanup activity on the said object. As a developer we cannot force the JVM to run the garbage collector thread. Though there are methods e.g Runtime.gc () or System.gc(), but none of these assures the execution of garbage collector thread. These methods are used to send garbage collection requests to the JVM. It is up to the Java Virtual machine when it will initiate the garbage collection process.

Take a look at this stuff

How Garbage Collection works in Java

Answer 3

In basic terms, Garbage Collection works by walking the object graphs from a number of predefined roots. Anything not accessible from those roots is garbage, therefore one object referencing another is irrelevant unless either can be accessed from one or more roots.

It's all explained in more detail in How Garbage Collection Really Works.

Answer 4

The garbage collector looks for reachable objects, starting from the roots (typically: variables on the call stack or global variables). So if two objects reference each other but are not otherwise reachable they won't be flagged as "live" and will be collected.