标签：vat ref ola ret stat 操作 替换 new obj
这三个都是自 JDK1.5 开始加入到 java.util.concurrent.atomic 下面的。他们都可以在 lock-free 的情况下以原子的方式更新对象引用。
以原子方式更新对象引用。
static class User { private int age; public int getAge() { return age; } public void setAge(int age) { this.age = age; } public User(int age) { this.age = age; } } public static void main(String[] args) { User user1 = new User(10); User user2 = new User(20); AtomicReference<User> atomicReference = new AtomicReference<>(user1); System.out.println(atomicReference.get().getAge()); atomicReference.compareAndSet(user1, user2); System.out.println(atomicReference.get().getAge()); }
解决了 AtomicReference 中 CAS 操作存在的 ABA 问题。
public static void main(String[] args) { User user1 = new User(10); User user2 = new User(20); AtomicStampedReference<User> stampedReference = new AtomicStampedReference<>(user1, 1); int[] stamp = new int[1]; // 获取引用对象和对应的版本号 System.out.println(stampedReference.get(stamp).getAge()); int oldStamp = stamp[0]; // 预期引用，新引用，预期版本号，新版本号 stampedReference.compareAndSet(user1, user2, oldStamp, 2); System.out.println(stampedReference.get(stamp).getAge()); }
內部定义了一个 Pair 对象，相当于给引用加了一个版本号
public class AtomicStampedReference<V> { private static class Pair<T> { final T reference; final int stamp; private Pair(T reference, int stamp) { this.reference = reference; this.stamp = stamp; } static <T> Pair<T> of(T reference, int stamp) { return new Pair<T>(reference, stamp); } } private volatile Pair<V> pair; public AtomicStampedReference(V initialRef, int initialStamp) { pair = Pair.of(initialRef, initialStamp); }
替换时的逻辑，当引用和版本号都相同时才使用 CAS 替换
public boolean compareAndSet(V expectedReference, V newReference, int expectedStamp, int newStamp) { Pair<V> current = pair; return expectedReference == current.reference && expectedStamp == current.stamp && ((newReference == current.reference && newStamp == current.stamp) || casPair(current, Pair.of(newReference, newStamp))); } private boolean casPair(Pair<V> cmp, Pair<V> val) { return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val); }
相对于 AtomicStampedReference，有时候，我们并不关心引用变量更改了几次，只是单纯的关心是否更改过
public static void main(String[] args) { User user1 = new User(10); User user2 = new User(20); AtomicMarkableReference<User> stampedReference = new AtomicMarkableReference<>(user1, false); boolean[] stamp = new boolean[1]; // 获取引用对象和对应的状态 System.out.println(stampedReference.get(stamp).getAge()); boolean oldStamp = stamp[0]; // 预期引用，新引用，预期状态，新状态 stampedReference.compareAndSet(user1, user2, oldStamp, false); System.out.println(stampedReference.get(stamp).getAge()); }
内部和 AtomicStampedReference 一样
public class AtomicMarkableReference<V> { private static class Pair<T> { final T reference; final boolean mark; private Pair(T reference, boolean mark) { this.reference = reference; this.mark = mark; } static <T> Pair<T> of(T reference, boolean mark) { return new Pair<T>(reference, mark); } } private volatile Pair<V> pair; public AtomicMarkableReference(V initialRef, boolean initialMark) { pair = Pair.of(initialRef, initialMark); } public boolean compareAndSet(V expectedReference, V newReference, boolean expectedMark, boolean newMark) { Pair<V> current = pair; return expectedReference == current.reference && expectedMark == current.mark && ((newReference == current.reference && newMark == current.mark) || casPair(current, Pair.of(newReference, newMark))); } private boolean casPair(Pair<V> cmp, Pair<V> val) { return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val); }
https://segmentfault.com/a/1190000015831791
AtomicReference、AtomicStampedReference 和 AtomicMarkableReference
标签：vat ref ola ret stat 操作 替换 new obj
原文地址：https://www.cnblogs.com/jhxxb/p/11552150.html
