What are the roots?

 

What are the roots in garbage collection?

I have read the definition of root as "any reference that you program can access to" and definition of live is that an object that is being used, which can be a local variable, static variable.

I m little confused with discriminating the difference between root and live objects.

What is path to root? How does root and live objects work?

Can someone elaborate ?

If you think of the objects in memory as a tree, the "roots" would be the root nodes - every object immediately accessible by your program. 

Person p = new Person();
p.car = new Car(RED);
p.car.engine = new Engine();
p.car.horn = new AnnoyingHorn();

There are four objects; a person, a red car, its engine and horn. Draw the reference graph: 

     Person [p]
        |
     Car (red)
   /           
Engine    AnnoyingHorn

And you'll end up with Person at the "root" of the tree. It's live because it's referenced by a local variable, p , which the program might use at any time to refer to the Person object. This also goes for the other objects, through p.car , p.car.engine , etc.

Since Person and all other objects recursively connected to it are live, there would be trouble if the GC collected them.

Consider, however, if the following is run after a while: 

p.car = new Car(BLUE);

And redraw the graph: 

     Person [p]
        |
     Car (blue)       Car (red)
                    /           
                Engine    AnnoyingHorn

Now the Person is accessible through p and the blue car through p.car , but there is no way the red car or its parts can ever be accessed again - they are not connected to a live root. They can be safely collected.

So it's really a matter of taking every starting point (every local variable, globals, statics, everything in other threads and stack frames) — every root — and recursively following all the references to make up a list of all the "live" objects: objects which are in use and unsuitable for deletion. Everything else is garbage, waiting to be collected.

The GC (Garbage Collector) roots are objects special for garbage collector. The Garbage Collector collects those objects that are not GC roots and are not accessible by references from GC roots.

There are several kinds of GC roots. One object can belong to more than one kind of root. The root kinds are:

  • Class - class loaded by system class loader. Such classes can never be unloaded. They can hold objects via static fields. Please note that classes loaded by custom class loaders are not roots, unless corresponding instances of java.lang.Class happen to be roots of other kind(s).
  • Thread - live thread
  • Stack Local - local variable or parameter of Java method
  • JNI Local - local variable or parameter of JNI method
  • JNI Global - global JNI reference
  • Monitor Used - objects used as a monitor for synchronization
  • Held by JVM - objects held from garbage collection by JVM for its purposes. Actually the list of such objects depends on JVM implementation. Possible known cases are: the system class loader, a few important exception classes which the JVM knows about, a few pre-allocated objects for exception handling, and custom class loaders when they are in the process of loading classes. Unfortunately, JVM provides absolutely no additional detail for such objects. Thus it is up to the analyst to decide to which case a certain "Held by JVM" belongs.

    • (credit to YourKit's website)

    Not mentioned by YourKit is the fact that objects awaiting finalization will be retained as roots until the GC runs the finalize() method. That can cause transient retention of large graphs somewhat unexpectedly. The general rule of thumb is not to use finalizers (but that's a different question).

    Roots or garbage collection roots are the objects that are always reachable. If an object is always reachable, then it is not eligible for garbage collection; roots therefore are always ineligible for collection. It is the initial set of objects from where reachability of all other objects on the heap are determined.

    Other objects on the heap reachable from the garbage collection roots are considered to be live objects, and ineligible for collection; the objects that are unreachable can be marked for reclamation.

    I know Java more than the .Net platform, so I'll speak only for one. On the Java platform, the GC roots are actually implementation dependent. In most runtime however, the GC roots tend to be the operands on the stack (for they are currently in use by threads) and class (static) members of classes. Reachability is calculated from these objects in most JVMs. There are other cases where local parameters and operands used by JNI calls will be considered part of the root set, and also used to calculate reachability.

    I hope this clears any lingering doubts over what is a root (set) and what is a live object.

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