Java并发中的ABA问题学习与解决方案

    public class Account {
        private AtomicInteger balance;
        private AtomicInteger transactionCount;
        private ThreadLocal<Integer> currentThreadCASFailureCount;
        public Account() {
            this.balance = new AtomicInteger(0);
            this.transactionCount = new AtomicInteger(0);
            this.currentThreadCASFailureCount = new ThreadLocal<>();
            this.currentThreadCASFailureCount.set(0);
        }
        public int getBalance() {
            return balance.get();
        }
        public int getTransactionCount() {
            return transactionCount.get();
        }
        public int getCurrentThreadCASFailureCount() {
            return Optional.ofNullable(currentThreadCASFailureCount.get()).orElse(0);
        }
        public boolean withdraw(int amount) {
            int current = getBalance();
            maybeWait();
            boolean result = balance.compareAndSet(current, current - amount);
            if (result) {
                transactionCount.incrementAndGet();
            } else {
                int currentCASFailureCount = currentThreadCASFailureCount.get();
                currentThreadCASFailureCount.set(currentCASFailureCount + 1);
            }
            return result;
        }
        private void maybeWait() {
            if ("thread1".equals(Thread.currentThread().getName())) {
                try {
                    TimeUnit.SECONDS.sleep(2);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }
        }
        public boolean deposit(int amount) {
            int current = balance.get();
            boolean result = balance.compareAndSet(current, current + amount);
            if (result) {
                transactionCount.incrementAndGet();
            } else {
                int currentCASFailureCount = currentThreadCASFailureCount.get();
                currentThreadCASFailureCount.set(currentCASFailureCount + 1);
            }
            return result;
        }
    }

    @Test
    public void abaProblemTest() throws InterruptedException {
        final int defaultBalance = 50;
        final int amountToWithdrawByThread1 = 20;
        final int amountToWithdrawByThread2 = 10;
        final int amountToDepositByThread2 = 10;
        Assert.assertEquals(0, account.getTransactionCount());
        Assert.assertEquals(0, account.getCurrentThreadCASFailureCount());
        account.deposit(defaultBalance);
        Assert.assertEquals(1, account.getTransactionCount());
        Thread thread1 = new Thread(() -> {
            // this will take longer due to the name of the thread
            Assert.assertTrue(account.withdraw(amountToWithdrawByThread1));
            // thread 1 fails to capture ABA problem
            Assert.assertNotEquals(1, account.getCurrentThreadCASFailureCount());
        }, "thread1");
        Thread thread2 = new Thread(() -> {
            Assert.assertTrue(account.deposit(amountToDepositByThread2));
            Assert.assertEquals(defaultBalance + amountToDepositByThread2, account.getBalance());
            // this will be fast due to the name of the thread
            Assert.assertTrue(account.withdraw(amountToWithdrawByThread2));
            // thread 1 didn't finish yet, so the original value will be in place for it
            Assert.assertEquals(defaultBalance, account.getBalance());
            Assert.assertEquals(0, account.getCurrentThreadCASFailureCount());
        }, "thread2");
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        // compareAndSet operation succeeds for thread 1
        Assert.assertEquals(defaultBalance - amountToWithdrawByThread1, account.getBalance());
        //but there are other transactions
        Assert.assertNotEquals(2, account.getTransactionCount());
        // thread 2 did two modifications as well
        Assert.assertEquals(4, account.getTransactionCount());
    }

    static class Stack {
        private AtomicReference<node> top = new AtomicReference<>();
        static class Node {
            String value;
            Node next;
            public Node (String value) {
                this.value = value;
            }
        }
        //出栈
        public Node pop(int time) {
            Node newTop;
            Node oldTop;
            do {
                oldTop = top.get();
                if (oldTop == null) {
                    return null;
                }
                newTop = oldTop.next;
                try {
                    //休眠一段时间，模拟ABA问题
                    TimeUnit.SECONDS.sleep(time);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } while (!top.compareAndSet(oldTop, newTop));
            return oldTop;
        }
        public void push (Node node) {
            Node oldTop;
            do {
                oldTop = top.get();
                node.next = oldTop;
            } while (!top.compareAndSet(oldTop, node));
        }
        public AtomicReference<Node> getTop() {
            return top;
        }
    }
    @Test
    public void testStack() throws Exception{
        Stack stack = new Stack();
        Stack.Node a = new Stack.Node("A");
        Stack.Node b = new Stack.Node("B");
        // 初始化栈结构
        stack.push(b);
        stack.push(a);
        // ABA 测试
        Thread t1 = new Thread(() -> {
            stack.pop(2);
        });
        
        Stack.Node c = new Stack.Node("C");
        Stack.Node d = new Stack.Node("D");
        Thread t2 = new Thread(() -> {
            stack.pop(0);
            stack.pop(0);
            stack.push(d);
            stack.push(c);
            stack.push(a);
        });
        //
        t1.start();
        t2.start();
        TimeUnit.SECONDS.sleep(5);
        Stack.Node top = stack.getTop().get();
        do {
            System.out.println(top.value);
            top = top.next;
        } while (top != null);
    }