Question

Is my class threadsafe? If not why?

class Foo {
 boolean b = false;

 void doSomething() throws Exception {
    while (b) Thread.sleep();
 }

 void setB(boolean b) {
     this.b = b;
 }
}

Answer 1

The code is not thread safe because a running thread may see changes until the code is compiled (which could be at a random point later) and you no longer see changes.

BTW: This makes testing it very hard to do. e.g. if you sleep for 1 second you might not see this behaviour for almost three hours.

i.e. it may or may not work and you cannot say just because it has worked it will continue to work.

---

As b is not volatile the JIT can and will optimise

while (b) Thread.sleep(N);

to be

boolean b = this.b;
if (b) while (true) Thread.sleep(N);

so the value of b is not read each time.