多线程之ABA问题

ゞ浴缸里的玫瑰 2022-12-08 04:53 89阅读 0赞

### 1、ABA问题 ###

使用`AtomicReference`的**变量**在修改变量之前，感知不到其他线程对变量的修改。

import lombok.extern.slf4j.Slf4j;
    import java.util.concurrent.atomic.AtomicReference;
    
    @Slf4j
    public class ABA问题 {
        
    
        static AtomicReference<String> ref = new AtomicReference<>("A");
    
        public static void main(String[] args) throws InterruptedException {
        
            log.debug("mian start ......");
            //获取A的值
            //这个共享变量被其他线程修改过
            String prev = ref.get();
            other();
            Thread.sleep(1000);
            //尝试改为C
            log.debug("change A -> C {}",ref.compareAndSet(prev,"C"));
        }
    
        public static void other() throws InterruptedException {
        
            new Thread(() -> {
        
                log.debug("change A -> B {}",ref.compareAndSet(ref.get(),"B"));
            },"t1").start();
            Thread.sleep(500);
            new Thread(() -> {
        
                log.debug("change B -> A {}",ref.compareAndSet(ref.get(),"A"));
            },"t2").start();
        }
    }

输出：

19:17:45.888 [main] DEBUG ABA问题 - mian start ......
    19:17:45.968 [t1] DEBUG ABA问题 - change A -> B true
    19:17:46.477 [t2] DEBUG ABA问题 - change B -> A true
    19:17:47.485 [main] DEBUG ABA问题 - change A -> C true

--------------------

### 2、解决ABA问题（一） ###

使用`AtomicStampedReference`解决ABA问题，使用了版本号解决。

import lombok.extern.slf4j.Slf4j;
    import java.util.concurrent.atomic.AtomicStampedReference;
    
    @Slf4j
    public class 解决ABA问题 {
        
    
        static AtomicStampedReference<String> ref = new AtomicStampedReference<>("A",0);
    
        public static void main(String[] args) throws InterruptedException {
        
            log.debug("mian start ......");
            //获取A的值
            //这个共享变量被其他线程修改过
            String prev = ref.getReference();
            //获取版本号
            int stamp = ref.getStamp();
            other();
            Thread.sleep(1000);
            //尝试改为C
            log.debug("change A -> C {}",ref.compareAndSet(prev,"C",stamp,stamp + 1));
        }
    
        public static void other() throws InterruptedException {
        
            new Thread(() -> {
        
                int stamp = ref.getStamp();
                log.debug("change A -> B {}",ref.compareAndSet(ref.getReference(),"B",stamp,stamp+1));
            },"t1").start();
            Thread.sleep(500);
            new Thread(() -> {
        
                int stamp = ref.getStamp();
                log.debug("change B -> A {}",ref.compareAndSet(ref.getReference(),"A",stamp,stamp+1));
            },"t2").start();
        }
    }

解决：

20:51:38.275 [main] DEBUG ABA问题 - mian start ......
    20:51:38.347 [t1] DEBUG ABA问题 - change A -> B true
    20:51:38.847 [t2] DEBUG ABA问题 - change B -> A true
    20:51:39.847 [main] DEBUG ABA问题 - change A -> C false

### 解决ABA问题（二） ###

如果只是期望知道主线程在修改之前有没有被别的线程修改过，使用`AtomicMarkableReference`即可实现。

import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.atomic.AtomicMarkableReference;
    
    @Slf4j
    public class ABA问题2 {
        
        public static void main(String[] args) throws InterruptedException {
        
            GarbageBag bag = new GarbageBag("装满了垃圾");
            AtomicMarkableReference<GarbageBag> ref = new AtomicMarkableReference<>(bag,true);
    
            log.debug("start...");
            GarbageBag prev = ref.getReference();
            log.debug(prev.toString());
    
            new Thread(() -> {
        
                log.debug("start....");
                bag.setDesc("空垃圾袋");
                ref.compareAndSet(bag,bag,true,false);
                log.debug(bag.toString());
            }).start();
    
            Thread.sleep(1000);
            log.debug("想换一只新塑料袋？");
            boolean success = ref.compareAndSet(prev, new GarbageBag("空塑料袋"), true, false);
            log.debug("换了么？ {}",success);
            log.debug(ref.getReference().toString());
        }
    }
    
    class GarbageBag{
        
        private String desc;
    
        public GarbageBag(String desc){
        
            this.desc = desc;
        }
    
        public void setDesc(String desc) {
        
            this.desc = desc;
        }
    
        @Override
        public String toString() {
        
            return "GarbageBag{" +
                    "desc='" + desc + '\'' +
                    '}';
        }
    }

输出：

00:17:59.983 [main] DEBUG ABA问题2 - start...
    00:17:59.988 [main] DEBUG ABA问题2 - GarbageBag{
        desc='装满了垃圾'}
    00:18:00.062 [Thread-0] DEBUG ABA问题2 - start....
    00:18:00.063 [Thread-0] DEBUG ABA问题2 - GarbageBag{
        desc='空垃圾袋'}
    00:18:01.063 [main] DEBUG ABA问题2 - 想换一只新塑料袋？
    00:18:01.063 [main] DEBUG ABA问题2 - 换了么？ false
    00:18:01.065 [main] DEBUG ABA问题2 - GarbageBag{
        desc='空垃圾袋'}