JUC atomic reference and ABA problem

JUC Atomic reference

• AtomicReference
• AtomicMarkableReference
• AtomicStampedReference

class DecimalAccountSafeCas implements DecimalAccount {
    
 AtomicReference<BigDecimal> ref;
 public DecimalAccountSafeCas(BigDecimal balance) {
    
 ref = new AtomicReference<>(balance);
 }
 @Override
 public BigDecimal getBalance() {
    
 return ref.get();
 }
 @Override
 public void withdraw(BigDecimal amount) {
    
 while (true) {
    
 BigDecimal prev = ref.get();
 BigDecimal next = prev.subtract(amount);
 if (ref.compareAndSet(prev, next)) {
    
 break;
 }
 }
 }
}

ABA Problems and Solutions

ABA problem

static AtomicReference<String> ref = new AtomicReference<>("A");
public static void main(String[] args) throws InterruptedException {
    
 log.debug("main start...");
 //  Get value  A
 //  This shared variable has been modified by its thread ？
 String prev = ref.get();
 other();
 sleep(1);
 //  Try to change to  C
 log.debug("change A->C {}", ref.compareAndSet(prev, "C"));
}
private static void other() {
    
 new Thread(() -> {
    
 log.debug("change A->B {}", ref.compareAndSet(ref.get(), "B"));
 }, "t1").start();
 sleep(0.5);
 new Thread(() -> {
    
 log.debug("change B->A {}", ref.compareAndSet(ref.get(), "A"));
 }, "t2").start();
}

The main thread can only judge the value of the shared variable from the initial value A Are they the same? , Can't perceive this from A Change it to B also Change back A The situation of , If the main thread wants ：
As long as there are other threads 【 I've moved 】 Shared variables , So own cas Even if it fails , At this time , Just comparing values is not enough , You need to add another version number

AtomicStampedReference

static AtomicStampedReference<String> ref = new AtomicStampedReference<>("A", 0);
public static void main(String[] args) throws InterruptedException {
    
 log.debug("main start...");
 //  Get value  A
 String prev = ref.getReference();
 //  Get version number 
 int stamp = ref.getStamp();
 log.debug(" edition  {}", stamp);
 //  If there are other threads in the middle , It happened.  ABA  The phenomenon 
 other();
 sleep(1);
 //  Try to change to  C
 log.debug("change A->C {}", ref.compareAndSet(prev, "C", stamp, stamp + 1));
}
private static void other() {
    
 new Thread(() -> {
    
 log.debug("change A->B {}", ref.compareAndSet(ref.getReference(), "B", 
 ref.getStamp(), ref.getStamp() + 1));
 log.debug(" The updated version is  {}", ref.getStamp());
 }, "t1").start();
 sleep(0.5);
 new Thread(() -> {
    
 log.debug("change B->A {}", ref.compareAndSet(ref.getReference(), "A", 
 ref.getStamp(), ref.getStamp() + 1));
 log.debug(" The updated version is  {}", ref.getStamp());
 }, "t2").start();
}

AtomicStampedReference You can add version numbers to atomic references , Track the whole process of atomic reference , Such as ： A -> B -> A -> C , adopt AtomicStampedReference, We can know , The reference variable was changed several times in the middle of the way . But sometimes , I don't care how many times the reference variable has changed , Just care about whether it has been changed , So there it is AtomicMarkableReference

AtomicMarkableReference

class GarbageBag {
    
 String desc;
 public GarbageBag(String desc) {
    
 this.desc = desc;
 }
 public void setDesc(String desc) {
    
 this.desc = desc;
 }
 @Override
 public String toString() {
    
 return super.toString() + " " + desc;
 }
}

@Slf4j
public class TestABAAtomicMarkableReference {
    
 public static void main(String[] args) throws InterruptedException {
    
 GarbageBag bag = new GarbageBag(" Full of garbage ");
 //  Parameters 2 mark  It can be regarded as a mark , Indicates that the garbage bag is full 
 AtomicMarkableReference<GarbageBag> ref = new AtomicMarkableReference<>(bag, true);
 log.debug(" The main thread  start...");
 GarbageBag prev = ref.getReference();
 log.debug(prev.toString());
 new Thread(() -> {
    
 log.debug(" Cleaning thread  start...");
 bag.setDesc(" Empty garbage bags ");
 while (!ref.compareAndSet(bag, bag, true, false)) {
    }
 log.debug(bag.toString());
 }).start();
 Thread.sleep(1000);
 log.debug(" The main thread wants to change a new garbage bag ？");
 boolean success = ref.compareAndSet(prev, new GarbageBag(" Empty garbage bags "), true, false);
 log.debug(" Did you change it ？" + success);
 log.debug(ref.getReference().toString());
 }
}