What are the roots?

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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.

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