Java Garbage Collection Algorithm – Mark and Sweep Algorithm

The mark-and-sweep algorithm is called a tracing garbage collector because is traces out the entire collection of objects that are directly or indirectly accessible by the program.

Root: –

The objects that a program can access directly are those objects which are referenced by local variables on the processor stack as well as by any static variables that refer to objects. In the context of garbage collection, these variables are called the roots.

Live Objects: –

An object is indirectly accessible if it is referenced by a field in some other (directly or indirectly) accessible object. An accessible object is said to be live . Conversely, an object which is not live is garbage.

The mark-and-sweep algorithm is divided into two phases:

Mark phase: –

The garbage collector traverses the graph of references from the root nodes and marks each heap object it encounters. Each object has an extra bit: the mark bit – initially the mark bit is 0. It is set to 1 for the reachable objects in the mark phase.

Sweep phase:

The GC scans the heap looking for objects with mark bit 0 – these objects have not been visited in the mark phase – they are garbage. Any such object is added to the free list of objects that can be reallocated. The objects with a mark bit 1 have their mark bit reset to 0.