Chapter 2. Thread Safety
Perhaps surprisingly, concurrent programming isn't so much about threads or locks, any more than civil engineering is about rivets and I-beams. Of course, building bridges that don't fall down requires the correct use of a lot of rivets and I-beams, just as building concurrent programs require the correct use of threads and locks. But these are just mechanisms—means to an end. Writing thread-safe code is, at its core, about managing access to state, and in particular to shared, mutable state.
Informally, an object's state is its data, stored in state variables such as instance or static fields. An object's state may include fields from other, dependent objects; a
HashMap's state is partially stored in the
HashMap object ...