A write barrier is usually a small piece of code generated by the compiler next to that of a field store. This is needed when the semantics of a field store affect other parts of the system. For example, in generational GCs, it is common to use some kind of write barrier to flag which parts of the entire heap have been written to, or "dirtied". This is because the GC not only needs to trace references pointing from a young generation to the old generation, but the other way around as well. Trivially, there would be no performance benefit from generational GC if the entire old space had to be traversed for each nursery collection. Better instead to pay the price of a few extra instructions of write barrier code for each field store.

See also Read barrier, Generational GC, Old space, and Young space.