CAS

CAS即CompareandSwap，其具体的意思是比较并交换。

它是JDK提供的非阻塞原子性操作，它通过硬件保证了“比较、更新”操作的原子性。

Unsafe

JDK 里的 Unsafe 类提供了一系列的 compareAndSwap*方法，代码文件路径如下： 

其定义了如下几个 compareAndSwap*方法：

剖析compareAndSwapInt

相关代码定义如下：

    /**
     * Atomically updates Java variable to {@code x} if it is currently
     * holding {@code expected}.
     *
     * 
This operation has memory semantics of a {@code volatile} read
     * and write.  Corresponds to C11 atomic_compare_exchange_strong.
     *
     * @return {@code true} if successful
     */
    @ForceInline
    public final boolean compareAndSwapInt(Object o, long offset,
                                           int expected,
                                           int x) {
        return theInternalUnsafe.compareAndSetInt(o, offset, expected, x);
    }

从上述代码可以看到CAS有四个操作数，分别是：对象内存位置、对象中的变量的偏移量、变量预期值和新值。

具体含义：

如果对象 obj 中内存偏移量为 valueOffset的变量的值为 expect，则使用新值 update替换旧值 expect；这是处理器提供的一个原子性指令。

Note：

从上述方法的定义可以发现其实际上是调用了变量 theInternalUnsafe 的 compareAndSetInt 方法，该变量在类中的定义如下：

/**
 * A collection of methods for performing low-level, unsafe operations.
 * Although the class and all methods are public, use of this class is
 * limited because only trusted code can obtain instances of it.
 *
 * Note: It is the resposibility of the caller to make sure
 * arguments are checked before methods of this class are
 * called. While some rudimentary checks are performed on the input,
 * the checks are best effort and when performance is an overriding
 * priority, as when methods of this class are optimized by the
 * runtime compiler, some or all checks (if any) may be elided. Hence,
 * the caller must not rely on the checks and corresponding
 * exceptions!
 *
 * @author John R. Rose
 * @see #getUnsafe
 */

public final class Unsafe {

    static {
        Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe");
    }

    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();
    private static final jdk.internal.misc.Unsafe theInternalUnsafe = jdk.internal.misc.Unsafe.getUnsafe();

    /**
     * Provides the caller with the capability of performing unsafe
     * operations.
     *
     * 
The returned {@code Unsafe} object should be carefully guarded
     * by the caller, since it can be used to read and write data at arbitrary
     * memory addresses.  It must never be passed to untrusted code.
     *
     * 
Most methods in this class are very low-level, and correspond to a
     * small number of hardware instructions (on typical machines).  Compilers
     * are encouraged to optimize these methods accordingly.
     *
     * 
Here is a suggested idiom for using unsafe operations:
     *
     * 
 {@code
     * class MyTrustedClass {
     *   private static final Unsafe unsafe = Unsafe.getUnsafe();
     *   ...
     *   private long myCountAddress = ...;
     *   public int getCount() { return unsafe.getByte(myCountAddress); }
     * }}
     *
     * (It may assist compilers to make the local variable {@code final}.)
     *
     * @throws  SecurityException if the class loader of the caller
     *          class is not in the system domain in which all permissions
     *          are granted.
     */
    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(caller.getClassLoader()))
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }

关于 theInternalUnsafe 将在后面详细讲述。

ABA 问题

关于CAS操作有个经典的ABA问题，具体案例如下：

线程I在获取变量X的值（变量X的初始值是A）后使用CAS操作将其修改为 B，假设操作成功，则程序运行一定是正确的吗?

其实未必，在线程I获取变量 X 的值后，在执行 CAS 前，线程II使用 CAS 修改变量X的值为B，随后又使用CAS修改变量X的值为A；则该场景中线程I在执行CAS时X的值虽然是A，但这个A己经不是线程I获取的A了。

ABA 问题的产生是因为变量的状态值产生了环形转换，即变量的值从 A 到 B, 然后再从 B 到 A。

如果变量的值只能朝着一个方向转换，比如 A 到 B，B 到 C，不构成环形，则不会存在问题。

JDK 中的 AtomicStampedReference 类给每个变量的状态值都配备了一个时间戳，从而避免了ABA问题的产生。

theInternalUnsafe

前述 compareAndSwapInt 方法中使用的 theInternalUnsafe 也是Unsafe类的实例，由此可知JDK里还存在另一个 Unsafe 类，代码文件路径如下：

这两个Unsafe类对应的包名如下图所示

由JDK11中AtomicInteger的定义，知其使用的是jdk.internal.misc.Unsafe。

（JDK8中使用的是sun.misc.Unsafe，地址）

AtomicInteger的代码片段如下：

/**
 * An {@code int} value that may be updated atomically.  See the
 * {@link VarHandle} specification for descriptions of the properties
 * of atomic accesses. 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;

    /*
     * This class intended to be implemented using VarHandles, but there
     * are unresolved cyclic startup dependencies.
     */
    private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
    private static final long VALUE = U.objectFieldOffset(AtomicInteger.class, "value");

    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() {
    }

jdk.internal.misc.Unsafe对应的代码片段如下：

/**
 * A collection of methods for performing low-level, unsafe operations.
 * Although the class and all methods are public, use of this class is
 * limited because only trusted code can obtain instances of it.
 *
 * Note: It is the resposibility of the caller to make sure
 * arguments are checked before methods of this class are
 * called. While some rudimentary checks are performed on the input,
 * the checks are best effort and when performance is an overriding
 * priority, as when methods of this class are optimized by the
 * runtime compiler, some or all checks (if any) may be elided. Hence,
 * the caller must not rely on the checks and corresponding
 * exceptions!
 *
 * @author John R. Rose
 * @see #getUnsafe
 */

public final class Unsafe {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();

    /**
     * Provides the caller with the capability of performing unsafe
     * operations.
     *
     * 
The returned {@code Unsafe} object should be carefully guarded
     * by the caller, since it can be used to read and write data at arbitrary
     * memory addresses.  It must never be passed to untrusted code.
     *
     * 
Most methods in this class are very low-level, and correspond to a
     * small number of hardware instructions (on typical machines).  Compilers
     * are encouraged to optimize these methods accordingly.
     *
     * 
Here is a suggested idiom for using unsafe operations:
     *
     * 
 {@code
     * class MyTrustedClass {
     *   private static final Unsafe unsafe = Unsafe.getUnsafe();
     *   ...
     *   private long myCountAddress = ...;
     *   public int getCount() { return unsafe.getByte(myCountAddress); }
     * }}
     *
     * (It may assist compilers to make the local variable {@code final}.)
     */
    public static Unsafe getUnsafe() {
        return theUnsafe;
    }

该类中有很多CompareAndSetXXX方法

剖析compareAndSetInt

    /**
     * Atomically updates Java variable to {@code x} if it is currently
     * holding {@code expected}.
     *
     * 
This operation has memory semantics of a {@code volatile} read
     * and write.  Corresponds to C11 atomic_compare_exchange_strong.
     *
     * @return {@code true} if successful
     */
    @HotSpotIntrinsicCandidate
    public final native boolean compareAndSetInt(Object o, long offset,
                                                 int expected,
                                                 int x);

    @HotSpotIntrinsicCandidate
    public final native int compareAndExchangeInt(Object o, long offset,
                                                  int expected,
                                                  int x);

    @HotSpotIntrinsicCandidate
    public final int compareAndExchangeIntAcquire(Object o, long offset,
                                                         int expected,
                                                         int x) {
        return compareAndExchangeInt(o, offset, expected, x);
    }

    @HotSpotIntrinsicCandidate
    public final int compareAndExchangeIntRelease(Object o, long offset,
                                                         int expected,
                                                         int x) {
        return compareAndExchangeInt(o, offset, expected, x);
    }

    @HotSpotIntrinsicCandidate
    public final boolean weakCompareAndSetIntPlain(Object o, long offset,
                                                   int expected,
                                                   int x) {
        return compareAndSetInt(o, offset, expected, x);
    }

    @HotSpotIntrinsicCandidate
    public final boolean weakCompareAndSetIntAcquire(Object o, long offset,
                                                     int expected,
                                                     int x) {
        return compareAndSetInt(o, offset, expected, x);
    }

    @HotSpotIntrinsicCandidate
    public final boolean weakCompareAndSetIntRelease(Object o, long offset,
                                                     int expected,
                                                     int x) {
        return compareAndSetInt(o, offset, expected, x);
    }

    @HotSpotIntrinsicCandidate
    public final boolean weakCompareAndSetInt(Object o, long offset,
                                              int expected,
                                              int x) {
        return compareAndSetInt(o, offset, expected, x);
    }

从上述代码可以看到该Unsafe类中的compareAndSetInt 和 compareAndExchangeInt方法都是 native方法，它们使用 JNI 的方式访问本地C++实现库。

参考资料：

https://howtodoinjava.com/java/multi-threading/compare-and-swap-cas-algorithm/