Clean Code

Appendix A: Concurrency II
12: Runnable runnable = new Runnable() {
13: public void run() {
14: classWithThreadingProblem.takeNextId();
15: }
16: };
17: 
18: for (int i = 0; i < 50000; ++i) {
19: int startingId = classWithThreadingProblem.lastId;
20: int expectedResult = 2 + startingId;
21: 
22: Thread t1 = new Thread(runnable);
23: Thread t2 = new Thread(runnable);
24: t1.start();
25: t2.start();
26: t1.join();
27: t2.join();
28: 
29: int endingId = classWithThreadingProblem.lastId;
30: 
31: if (endingId != expectedResult)
32: return;
33: }
34: 
35: fail("Should have exposed a threading issue but it did not.");
36: }
37: }
Line
Description
10
Create a single instance of 
ClassWithThreadingProblem
. Note, we must use
the final keyword because we use it below in an anonymous inner class.
12–16
Create an anonymous inner class that uses the single instance of
ClassWithThreadingProblem
.
18
Run this code “enough” times to demonstrate that the code failed, but not
so much that the test “takes too long.” This is a balancing act; we don’t
want to wait too long to demonstrate failure. Picking this number is hard—
although later we’ll see that we can greatly reduce this number.
19
Remember the starting value. This test is trying to prove that the code in
ClassWithThreadingProblem
is broken. If this test passes, it proved that the
code was broken. If this test fails, the test was unable to prove that the code
is broken.
20
We expect the final value to be two more than the current value.
22–23
Create two threads, both of which use the object we created in lines 12–16.
This gives us the potential of two threads trying to use our single instance
of 
ClassWithThreadingProblem
and interfering with each other.