Marking by the garbage collector
Marking marks any live objects and anything not marked as ready to be garbage collected. The objects keep a special bit that determines whether they are marked or not. Upon creation, the bit is 0. In the mark phase, if an object is still in use and should not be removed, it gets set to 1.
The heap is constantly changing and so is the stack. Objects on the heap that do not have a connection to the step are eligible for GC. They are unreachable and there is no possible way for the application to be using these objects. The objects that are not ready for removal are marked; the unmarked objects will be removed.
How exactly this is implemented differs depending on the implementation of Java and the specific garbage collector you are using. But at a high level, this process starts from the stack. All the object references on the stack are followed and the objects are marked.
If we look at our previous example, this is how they would be marked. We...
