Atomic原子类及AtomicReference及ABA问题

原子类

我们可以看到这些getAndAdd这种复杂类型，底层也是运行while true,cas方式来运行

AtomicReference

public class Test32 {
    public static void main(String[] args) {
        DecimalAccountCas decimalAccountCas = new DecimalAccountCas(new BigDecimal("10000"));
        DecimalAccount.demo(decimalAccountCas);
    }
}
class DecimalAccountCas implements DecimalAccount{
    private AtomicReference<BigDecimal> balance;
    public DecimalAccountCas(BigDecimal balance) {
        this.balance = new AtomicReference<>(balance);
    }
    @Override
    public BigDecimal getBalance() {
        return balance.get();
    }
    @Override
    public void withdraw(BigDecimal amount) {
        while (true){
            BigDecimal prev = balance.get();
            BigDecimal next = prev.subtract(amount);
            if (balance.compareAndSet(prev,next)) {
                break;
            }
        }
    }
}
interface DecimalAccount{
    BigDecimal getBalance();
    void withdraw(BigDecimal amount);
    static void demo(DecimalAccount account){
        List<Thread> ts = new ArrayList<>();
        long start = System.nanoTime();
        for (int i = 0;i<1000;i++){
            ts.add(new Thread(()->{
                account.withdraw(new BigDecimal("10.00"));
            }));
        }
        ts.forEach(Thread::start);
        ts.forEach(t->{
            try{
                t.join();
            }catch (Exception e){
                e.printStackTrace();
            }
        });
        long end = System.nanoTime();
        System.out.println(account.getBalance() + "cost : " + ((end-start)/1000_000) +" ms");
    }
}

ABA问题

这边修改时，并没有感知。所以需要加上版本号

ABA问题解决

static AtomicStampedReference<String> ref = new AtomicStampedReference<>("A",0);
    public static void main(String[] args) {
        String s = ref.getReference();
        int stamp = ref.getStamp();
        other();
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        boolean c = ref.compareAndSet(s, "C",stamp,stamp+1);
        System.out.println(c);
    }
    public static void other(){
        new Thread(()->{
            String reference = ref.getReference();
            int stamp = ref.getStamp();
            System.out.println(ref.compareAndSet(reference, "B",stamp,stamp+1));
        }).start();
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            String reference = ref.getReference();
            int stamp = ref.getStamp();
            System.out.println(ref.compareAndSet(reference, "A",stamp,stamp+1));
        }).start();
    }