what

CAS全称是compare and swap，将内存中的值和一个给定的值对比，如果相等该内存的值修改为给定的新值。注意这一步是属于原子操作。

how

通过简单的例子看一下本质，版本是jdk8：

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public class Test {

    public AtomicInteger mAtomicInteger;

    public void testAdd() {
        mAtomicInteger.getAndIncrement();
    }
}

直接看AtomicInteger.java代码

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/**
     * Atomically increments by one the current value.
     *
     * @return the previous value
     */
    public final int getAndIncrement() {
        return unsafe.getAndAddInt(this, valueOffset, 1);
    }

看下unsafe变量：

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// setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe(); //Unsafe是sun.misc.Unsafe类
    private static final long valueOffset; //表示AtomicInteger类value的地址

    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicInteger.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile int value;

跳转到Unsafe.java的getAndAddInt方法

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/**
     * Atomically adds the given value to the current value of a field
     * or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param delta the value to add
     * @return the previous value
     * @since 1.8
     */
    public final int getAndAddInt(Object o, long offset, int delta) {
        int v;
        do {
            v = getIntVolatile(o, offset); 
        } while (!compareAndSwapInt(o, offset, v, v + delta));
        return v;
    }

跳转到getIntVolatile方法

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/** Volatile version of {@link #getInt(Object, long)}  */
    public native int     getIntVolatile(Object o, long offset);

/** @see #getByte(long) */
    public native int     getInt(long address);

/**
     * Fetches a value from a given memory address.  If the address is zero, or
     * does not point into a block obtained from {@link #allocateMemory}, the
     * results are undefined.
     *
     * @see #allocateMemory
     */
    public native byte    getByte(long address);

很明显getIntVolatile的作用是获取给定地址的值。接下来看compareAndSwapInt方法

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/**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapInt(Object o, long offset,
                                                  int expected,
                                                  int x);

我们直接到UnSafe.cpp类查找compareAndSwapInt

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// These are the methods for 1.8.0
static JNINativeMethod methods_18[] = {
    {CC"getObject",        CC"("OBJ"J)"OBJ"",   FN_PTR(Unsafe_GetObject)},
    {CC"putObject",        CC"("OBJ"J"OBJ")V",  FN_PTR(Unsafe_SetObject)},
    {CC"getObjectVolatile",CC"("OBJ"J)"OBJ"",   FN_PTR(Unsafe_GetObjectVolatile)},
    {CC"putObjectVolatile",CC"("OBJ"J"OBJ")V",  FN_PTR(Unsafe_SetObjectVolatile)},

    DECLARE_GETSETOOP(Boolean, Z),
    DECLARE_GETSETOOP(Byte, B),
    DECLARE_GETSETOOP(Short, S),
    DECLARE_GETSETOOP(Char, C),
    DECLARE_GETSETOOP(Int, I),
    DECLARE_GETSETOOP(Long, J),
    DECLARE_GETSETOOP(Float, F),
    DECLARE_GETSETOOP(Double, D),

    DECLARE_GETSETNATIVE(Byte, B),
    DECLARE_GETSETNATIVE(Short, S),
    DECLARE_GETSETNATIVE(Char, C),
    DECLARE_GETSETNATIVE(Int, I),
    DECLARE_GETSETNATIVE(Long, J),
    DECLARE_GETSETNATIVE(Float, F),
    DECLARE_GETSETNATIVE(Double, D),

    {CC"getAddress",         CC"("ADR")"ADR,             FN_PTR(Unsafe_GetNativeAddress)},
    {CC"putAddress",         CC"("ADR""ADR")V",          FN_PTR(Unsafe_SetNativeAddress)},

    {CC"allocateMemory",     CC"(J)"ADR,                 FN_PTR(Unsafe_AllocateMemory)},
    {CC"reallocateMemory",   CC"("ADR"J)"ADR,            FN_PTR(Unsafe_ReallocateMemory)},
    {CC"freeMemory",         CC"("ADR")V",               FN_PTR(Unsafe_FreeMemory)},

    {CC"objectFieldOffset",  CC"("FLD")J",               FN_PTR(Unsafe_ObjectFieldOffset)},
    {CC"staticFieldOffset",  CC"("FLD")J",               FN_PTR(Unsafe_StaticFieldOffset)},
    {CC"staticFieldBase",    CC"("FLD")"OBJ,             FN_PTR(Unsafe_StaticFieldBaseFromField)},
    {CC"ensureClassInitialized",CC"("CLS")V",            FN_PTR(Unsafe_EnsureClassInitialized)},
    {CC"arrayBaseOffset",    CC"("CLS")I",               FN_PTR(Unsafe_ArrayBaseOffset)},
    {CC"arrayIndexScale",    CC"("CLS")I",               FN_PTR(Unsafe_ArrayIndexScale)},
    {CC"addressSize",        CC"()I",                    FN_PTR(Unsafe_AddressSize)},
    {CC"pageSize",           CC"()I",                    FN_PTR(Unsafe_PageSize)},

    {CC"defineClass",        CC"("DC_Args")"CLS,         FN_PTR(Unsafe_DefineClass)},
    {CC"allocateInstance",   CC"("CLS")"OBJ,             FN_PTR(Unsafe_AllocateInstance)},
    {CC"monitorEnter",       CC"("OBJ")V",               FN_PTR(Unsafe_MonitorEnter)},
    {CC"monitorExit",        CC"("OBJ")V",               FN_PTR(Unsafe_MonitorExit)},
    {CC"tryMonitorEnter",    CC"("OBJ")Z",               FN_PTR(Unsafe_TryMonitorEnter)},
    {CC"throwException",     CC"("THR")V",               FN_PTR(Unsafe_ThrowException)},
    {CC"compareAndSwapObject", CC"("OBJ"J"OBJ""OBJ")Z",  FN_PTR(Unsafe_CompareAndSwapObject)},
    {CC"compareAndSwapInt",  CC"("OBJ"J""I""I"")Z",      FN_PTR(Unsafe_CompareAndSwapInt)},
    {CC"compareAndSwapLong", CC"("OBJ"J""J""J"")Z",      FN_PTR(Unsafe_CompareAndSwapLong)},
    {CC"putOrderedObject",   CC"("OBJ"J"OBJ")V",         FN_PTR(Unsafe_SetOrderedObject)},
    {CC"putOrderedInt",      CC"("OBJ"JI)V",             FN_PTR(Unsafe_SetOrderedInt)},
    {CC"putOrderedLong",     CC"("OBJ"JJ)V",             FN_PTR(Unsafe_SetOrderedLong)},
    {CC"park",               CC"(ZJ)V",                  FN_PTR(Unsafe_Park)},
    {CC"unpark",             CC"("OBJ")V",               FN_PTR(Unsafe_Unpark)}
};

compareAndSwapInt会出现在多处，我们选择methods_18，因为是jdk8。然后搜索methods_18

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// This one function is exported, used by NativeLookup.
// The Unsafe_xxx functions above are called only from the interpreter.
// The optimizer looks at names and signatures to recognize
// individual functions.

JVM_ENTRY(void, JVM_RegisterUnsafeMethods(JNIEnv *env, jclass unsafecls))
  UnsafeWrapper("JVM_RegisterUnsafeMethods");
  {
    ThreadToNativeFromVM ttnfv(thread);

    // Unsafe methods
    {
      bool success = false;
      // We need to register the 1.6 methods first because the 1.8 methods would register fine on 1.7 and 1.6
      if (!success) {
        success = register_natives("1.6 methods",   env, unsafecls, methods_16,  sizeof(methods_16)/sizeof(JNINativeMethod));
      }
      if (!success) {
        success = register_natives("1.8 methods",   env, unsafecls, methods_18,  sizeof(methods_18)/sizeof(JNINativeMethod));
      }
      if (!success) {
        success = register_natives("1.5 methods",   env, unsafecls, methods_15,  sizeof(methods_15)/sizeof(JNINativeMethod));
      }
      if (!success) {
        success = register_natives("1.4.1 methods", env, unsafecls, methods_141, sizeof(methods_141)/sizeof(JNINativeMethod));
      }
      if (!success) {
        success = register_natives("1.4.0 methods", env, unsafecls, methods_140, sizeof(methods_140)/sizeof(JNINativeMethod));
      }
      guarantee(success, "register unsafe natives");
    }

    // Unsafe.getLoadAverage
    register_natives("1.6 loadavg method", env, unsafecls, loadavg_method, sizeof(loadavg_method)/sizeof(JNINativeMethod));

    // Prefetch methods
    register_natives("1.6 prefetch methods", env, unsafecls, prefetch_methods, sizeof(prefetch_methods)/sizeof(JNINativeMethod));

    // Memory copy methods
    {
      bool success = false;
      if (!success) {
        success = register_natives("1.7 memory copy methods", env, unsafecls, memcopy_methods_17, sizeof(memcopy_methods_17)/sizeof(JNINativeMethod));
      }
      if (!success) {
        success = register_natives("1.5 memory copy methods", env, unsafecls, memcopy_methods_15, sizeof(memcopy_methods_15)/sizeof(JNINativeMethod));
      }
    }

    // Unsafe.defineAnonymousClass
    if (EnableInvokeDynamic) {
      register_natives("1.7 define anonymous class method", env, unsafecls, anonk_methods, sizeof(anonk_methods)/sizeof(JNINativeMethod));
    }

    // Unsafe.shouldBeInitialized
    if (EnableInvokeDynamic) {
      register_natives("1.7 LambdaForm support", env, unsafecls, lform_methods, sizeof(lform_methods)/sizeof(JNINativeMethod));
    }

    // Fence methods
    register_natives("1.8 fence methods", env, unsafecls, fence_methods, sizeof(fence_methods)/sizeof(JNINativeMethod));
  }

根据注释直接查找NativeLookup.cpp，搜索JVM_RegisterUnsafeMethods

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#define CC (char*)  /* cast a literal from (const char*) */
#define FN_PTR(f) CAST_FROM_FN_PTR(void*, &f)

static JNINativeMethod lookup_special_native_methods[] = {
  // Next two functions only exist for compatibility with 1.3.1 and earlier.
  { CC"Java_java_io_ObjectOutputStream_getPrimitiveFieldValues",   NULL, FN_PTR(JVM_GetPrimitiveFieldValues)     },  // intercept ObjectOutputStream getPrimitiveFieldValues for faster serialization
  { CC"Java_java_io_ObjectInputStream_setPrimitiveFieldValues",    NULL, FN_PTR(JVM_SetPrimitiveFieldValues)     },  // intercept ObjectInputStream setPrimitiveFieldValues for faster serialization

  { CC"Java_sun_misc_Unsafe_registerNatives",                      NULL, FN_PTR(JVM_RegisterUnsafeMethods)       },
  { CC"Java_java_lang_invoke_MethodHandleNatives_registerNatives", NULL, FN_PTR(JVM_RegisterMethodHandleMethods) },
  { CC"Java_sun_misc_Perf_registerNatives",                        NULL, FN_PTR(JVM_RegisterPerfMethods)         },
  { CC"Java_sun_hotspot_WhiteBox_registerNatives",                 NULL, FN_PTR(JVM_RegisterWhiteBoxMethods)     },
};

这样我们就找到了Java_sun_misc_Unsafe_registerNatives，其实就是指Java_sun_misc_Unsafe_registerNatives对应JVM_RegisterPerfMethods，而Java_sun_misc_Unsafe_registerNatives根据命名知道在Unsafe.java中。

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private static native void registerNatives();
    static {
        registerNatives();
        sun.reflect.Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe");
    }

即在Unsafe加载之后就会调用registerNatives，即调用JVM_RegisterUnsafeMethods，即调用了Unsafe.cpp的register_natives

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/**
 * Helper method to register native methods.
 */
static bool register_natives(const char* message, JNIEnv* env, jclass clazz, const JNINativeMethod* methods, jint nMethods) {
  int status = env->RegisterNatives(clazz, methods, nMethods);
  if (status < 0 || env->ExceptionOccurred()) {
    if (PrintMiscellaneous && (Verbose || WizardMode)) {
      tty->print_cr("Unsafe:  failed registering %s", message);
    }
    env->ExceptionClear();
    return false;
  } else {
    if (PrintMiscellaneous && (Verbose || WizardMode)) {
      tty->print_cr("Unsafe:  successfully registered %s", message);
    }
    return true;
  }
}

即注册了对应的方法。即compareAndSwapInt对应Unsafe_CompareAndSwapInt方法

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UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
  UnsafeWrapper("Unsafe_CompareAndSwapInt");
  oop p = JNIHandles::resolve(obj); //此处p指Unsafe.java对象
  jint* addr = (jint *) index_oop_from_field_offset_long(p, offset); //Unsafe.java中value变量的地址
  return (jint)(Atomic::cmpxchg(x, addr, e)) == e; e是compare value
UNSAFE_END

直接看Atomic.cpp的cmpxchg方法

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unsigned Atomic::cmpxchg(unsigned int exchange_value,
                         volatile unsigned int* dest, unsigned int compare_value) {
  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
  return (unsigned int)Atomic::cmpxchg((jint)exchange_value, (volatile jint*)dest,
                                       (jint)compare_value);
}

在runtime/atomic.inline.hpp可以知道有很多不同版本的cmpxchg实现

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/*
 * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#ifndef SHARE_VM_RUNTIME_ATOMIC_INLINE_HPP
#define SHARE_VM_RUNTIME_ATOMIC_INLINE_HPP

#include "runtime/atomic.hpp"

// Linux
#ifdef TARGET_OS_ARCH_linux_x86
# include "atomic_linux_x86.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_linux_sparc
# include "atomic_linux_sparc.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_linux_zero
# include "atomic_linux_zero.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_linux_arm
# include "atomic_linux_arm.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_linux_ppc
# include "atomic_linux_ppc.inline.hpp"
#endif

// Solaris
#ifdef TARGET_OS_ARCH_solaris_x86
# include "atomic_solaris_x86.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_solaris_sparc
# include "atomic_solaris_sparc.inline.hpp"
#endif

// Windows
#ifdef TARGET_OS_ARCH_windows_x86
# include "atomic_windows_x86.inline.hpp"
#endif

// AIX
#ifdef TARGET_OS_ARCH_aix_ppc
# include "atomic_aix_ppc.inline.hpp"
#endif

// BSD
#ifdef TARGET_OS_ARCH_bsd_x86
# include "atomic_bsd_x86.inline.hpp"
#endif
#ifdef TARGET_OS_ARCH_bsd_zero
# include "atomic_bsd_zero.inline.hpp"
#endif

#endif // SHARE_VM_RUNTIME_ATOMIC_INLINE_HPP

本来想看arm平台的，没找到，最后看atomic_linux_x86.inline.hpp的，

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inline jint     Atomic::cmpxchg    (jint     exchange_value, volatile jint*     dest, jint     compare_value) {
  int mp = os::is_MP();
  __asm__ volatile (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)"
                    : "=a" (exchange_value)
                    : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                    : "cc", "memory");
  return exchange_value;
}

由于篇幅已经够长了，因此开了一篇新文章，用以解释上面的代码，推荐看下那篇文章，这里直接告诉结果：

%1表示exchange_value，%3表示dest，这里”a” (compare_value)将compare_value的值赋给eax寄存器，这里cmpxchgl的指令作用是比较compare_value和dest的值，如果相等，将exchange_value赋值给dest，如果不相等，将dest的值赋值给exchange_value。而Unsafe_CompareAndSwapInt方法，返回值是cmpxchgl的返回值和e比较，不相等时，一直跑循环，否则，退出。这样就完成了原子自增操作。