# Reachability

Objects in a heap dump have references to other objects. These can either be via field references (for simple objects), array elements (for object arrays) or via various hidden references. For instance every object contains a reference to its type, and each class contain a reference to the class loader which loaded the class.

These objects and references form a directed graph. The objects are the nodes, the references are the directed links between the nodes. The garbage collection roots are the roots of this graph.

Reachable
  An object is reachable from another object if there is a path following the directed links from the source object to the destination object.
Unreachable object
  If there is no path from a garbage collection root to an object then it is unreachable. There is then no way a legitimate Java program can every get access to this object, so it is safe for the Java virtual machine to discard this object from the heap. This process of determining unreachable objects and discarding them, thus making room for more objects to be allocated as required, is called garbage collection.

Related concepts

- Garbage collection roots