Races & Visibility

Race conditions and visibility

Two problems plague shared mutable state:

Interference — count++ is read-modify-write; two threads can read the same value and lose an increment:

class Counter { int count; void inc() { count++; } }
// 2 threads × 10_000 increments → often < 20_000

Visibility — without synchronization, one thread's writes may never become visible to another (the JIT may hoist the read out of a loop):

boolean stop = false;          // plain field — may never be seen!
// Thread A: while (!stop) work();
// Thread B: stop = true;

Fixes, weakest to strongest:

• volatile — visibility only; fine for flags, wrong for count++
• AtomicLong/AtomicInteger — atomic read-modify-write (incrementAndGet())
• synchronized blocks — mutual exclusion for compound actions

class SafeCounter {
    private final AtomicLong count = new AtomicLong();
    void inc() { count.incrementAndGet(); }
    long value() { return count.get(); }
}

Rule of thumb: make shared state immutable, or confine it to one thread, or protect every access consistently — pick one and document it.