在计算机科学中，比较和交换（Conmpare And Swap）是用于实现多线程同步的原子指令。它将内存位置的内容与给定值进行比较，只有在相同的情况下，将该内存位置的内容修改为新的给定值。这是作为单个原子操作完成的。 原子性保证新值基于最新信息计算；如果该值在同一时间被另一个线程更新，则写入将失败。操作结果必须说明是否进行替换；这可以通过一个简单的布尔响应（这个变体通常称为比较和设置），或通过返回从内存位置读取的值来完成。 查看JUC（java.util.concurrent）下的atomic包：
以AtomicInteger为例：
package java.util.concurrent.atomic; import java.util.function.IntUnaryOperator; import java.util.function.IntBinaryOperator; import sun.misc.Unsafe; /** * An {@code int} value that may be updated atomically. See the * {@link java.util.concurrent.atomic} package specification for * description of the properties of atomic variables. An * {@code AtomicInteger} is used in applications such as atomically * incremented counters, and cannot be used as a replacement for an * {@link java.lang.Integer}. However, this class does extend * {@code Number} to allow uniform access by tools and utilities that * deal with numerically-based classes. * * @since 1.5 * @author Doug Lea */ public class AtomicInteger extends Number implements java.io.Serializable { private static final long serialVersionUID = 6214790243416807050L; // setup to use Unsafe.compareAndSwapInt for updates private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final long valueOffset; static { try { valueOffset = unsafe.objectFieldOffset (AtomicInteger.class.getDeclaredField("value")); } catch (Exception ex) { throw new Error(ex); } } private volatile int value; /** * Creates a new AtomicInteger with the given initial value. * * @param initialValue the initial value */ public AtomicInteger(int initialValue) { value = initialValue; } /** * Creates a new AtomicInteger with initial value {@code 0}. */ public AtomicInteger() { } /** * Gets the current value. * * @return the current value */ public final int get() { return value; } /** * Sets to the given value. * * @param newValue the new value */ public final void set(int newValue) { value = newValue; } /** * Eventually sets to the given value. * * @param newValue the new value * @since 1.6 */ public final void lazySet(int newValue) { unsafe.putOrderedInt(this, valueOffset, newValue); } /** * Atomically sets to the given value and returns the old value. * * @param newValue the new value * @return the previous value */ public final int getAndSet(int newValue) { return unsafe.getAndSetInt(this, valueOffset, newValue); } /** * Atomically sets the value to the given updated value * if the current value {@code ==} the expected value. * * @param expect the expected value * @param update the new value * @return {@code true} if successful. False return indicates that * the actual value was not equal to the expected value. */ public final boolean compareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } /** * Atomically sets the value to the given updated value * if the current value {@code ==} the expected value. * * <p><a href="package-summary.html#weakCompareAndSet">May fail * spuriously and does not provide ordering guarantees</a>, so is * only rarely an appropriate alternative to {@code compareAndSet}. * * @param expect the expected value * @param update the new value * @return {@code true} if successful */ public final boolean weakCompareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } /** * Atomically increments by one the current value. * * @return the previous value */ public final int getAndIncrement() { return unsafe.getAndAddInt(this, valueOffset, 1); } /** * Atomically decrements by one the current value. * * @return the previous value */ public final int getAndDecrement() { return unsafe.getAndAddInt(this, valueOffset, -1); } /** * Atomically adds the given value to the current value. * * @param delta the value to add * @return the previous value */ public final int getAndAdd(int delta) { return unsafe.getAndAddInt(this, valueOffset, delta); } /** * Atomically increments by one the current value. * * @return the updated value */ public final int incrementAndGet() { return unsafe.getAndAddInt(this, valueOffset, 1) + 1; } /** * Atomically decrements by one the current value. * * @return the updated value */ public final int decrementAndGet() { return unsafe.getAndAddInt(this, valueOffset, -1) - 1; } /** * Atomically adds the given value to the current value. * * @param delta the value to add * @return the updated value */ public final int addAndGet(int delta) { return unsafe.getAndAddInt(this, valueOffset, delta) + delta; } /** * Atomically updates the current value with the results of * applying the given function, returning the previous value. The * function should be side-effect-free, since it may be re-applied * when attempted updates fail due to contention among threads. * * @param updateFunction a side-effect-free function * @return the previous value * @since 1.8 */ public final int getAndUpdate(IntUnaryOperator updateFunction) { int prev, next; do { prev = get(); next = updateFunction.applyAsInt(prev); } while (!compareAndSet(prev, next)); return prev; } /** * Atomically updates the current value with the results of * applying the given function, returning the updated value. The * function should be side-effect-free, since it may be re-applied * when attempted updates fail due to contention among threads. * * @param updateFunction a side-effect-free function * @return the updated value * @since 1.8 */ public final int updateAndGet(IntUnaryOperator updateFunction) { int prev, next; do { prev = get(); next = updateFunction.applyAsInt(prev); } while (!compareAndSet(prev, next)); return next; } /** * Atomically updates the current value with the results of * applying the given function to the current and given values, * returning the previous value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. The function * is applied with the current value as its first argument, * and the given update as the second argument. * * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the previous value * @since 1.8 */ public final int getAndAccumulate(int x, IntBinaryOperator accumulatorFunction) { int prev, next; do { prev = get(); next = accumulatorFunction.applyAsInt(prev, x); } while (!compareAndSet(prev, next)); return prev; } /** * Atomically updates the current value with the results of * applying the given function to the current and given values, * returning the updated value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. The function * is applied with the current value as its first argument, * and the given update as the second argument. * * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the updated value * @since 1.8 */ public final int accumulateAndGet(int x, IntBinaryOperator accumulatorFunction) { int prev, next; do { prev = get(); next = accumulatorFunction.applyAsInt(prev, x); } while (!compareAndSet(prev, next)); return next; } //...... }可以看出自JDK1.5就开始引入CAS来解决多线程中的并发问题。 查看方法源码，可以看出所有的CAS操作都是通过sun.misc包下Unsafe类实现的。而sun.misc包存在于JDK的rt.jar包，是由JVM本地实现。 Unsafe是CAS的核心类。由于Java无法直接访问底层系统，则需要通过本地（native）来访问。Unsafe可以直接操作特定内存的数，其内部方法可以像C语言的指针一样直接操作内存。 注意：Unsafe类的所有方法都是native修饰的，即Unsafe类的所有方法都可以直接调用底层操作系统资源。
以重入锁ReentrantLock为例。通过查看部分源码：
public class ReentrantLock implements Lock, java.io.Serializable { private static final long serialVersionUID = 7373984872572414699L; /** Synchronizer providing all implementation mechanics */ private final Sync sync; /** * Base of synchronization control for this lock. Subclassed * into fair and nonfair versions below. Uses AQS state to * represent the number of holds on the lock. */ abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = -5179523762034025860L; /** * Performs {@link Lock#lock}. The main reason for subclassing * is to allow fast path for nonfair version. */ abstract void lock(); /** * Performs non-fair tryLock. tryAcquire is implemented in * subclasses, but both need nonfair try for trylock method. */ final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) // overflow throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; } //...... } //...... }可以看出，内部抽象类Sync继承自AbstractQueuedSynchronizer类。AbstractQueuedSynchronizer作为Java多种锁的父类，有很多地方通过CAS操作来提高并发效率。查看AbstractQueuedSynchronizer部分源码：
/** * Inserts node into queue, initializing if necessary. See picture above. * @param node the node to insert * @return node's predecessor */ private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head; } else { node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t; } } } }可以看出在上述的同步队列的入队操作时，在多线程环境下，对其头尾节点的操作都有可能失败，失败后通过自旋操作再次尝试，直到成功，这也是一种乐观锁的实现。
比如说一个线程1从内存位置V中取出A，另一个线程2也从内存中取出A，线程2将A变成了B，然后将V位置的数据变成A，这时候线程1进行CAS操作发现内存中仍然是A，那么线程1操作成功。尽管线程1的CAS操作成功，但是不代表这个过程就是没有问题的。 如果链表的头在变化了两次后恢复了原值，但是不代表链表就没有变化。 所以JAVA中提供了AtomicStampedReference或AtomicMarkableReference来处理ABA问题，主要是在对象中额外再增加一个标记来标识对象是否有过变更。
