First of all, the JVM is more specific about what constitutes reachability of an object. Instead of the vaguely defined green clouds that we saw on earlier chapters, we have a very specific and explicit set of objects that are called the Garbage Collection Roots:
- Local variables
- Active threads
- Static fields
- JNI references
The method used by JVM to track down all the reachable (live) objects and to make sure the memory claimed by non-reachable objects can be reused is called the Mark and Sweep algorithm. It consists of two steps:
- Marking is walking through all reachable objects, starting from GC roots and keeping a ledger in native memory about all such objects
- Sweeping is making sure the memory addresses occupied by non-reachable objects can be reused by the next allocations.
Different GC algorithms within the JVM, such as Parallel Scavenge, Parallel Mark+Copy or CMS, are implementing those phases slightly differently, but at the conceptual level the process remains similar to the two steps described above.
A crucially important thing about this approach is that the cycles are no longer leaked:
The not-so-good thing is that the application threads need to be stopped for the collection to happen, as you cannot really count references if they keep changing all the time. Such a situation when the application is temporarily stopped so that the JVM can indulge in housekeeping activities is called a Stop The World pause. They may happen for many reasons, but garbage collection is by far the most popular one.