This class provides thread-local variables. These variables differ from
their normal counterparts in that each thread that accesses one (via its
{@code get} or {@code set} method) has its own, independently initialized
copy of the variable. {@code ThreadLocal} instances are typically private
static fields in classes that wish to associate state with a thread (e.g.,
a user ID or Transaction ID).

For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes {@code ThreadId.get()} and remains unchanged on subsequent calls.

import java.util.concurrent.atomic.AtomicInteger;