源码剖析sun.misc.Unsafe && Compare And Swap(CAS)操作

首先介绍一下什么是Compare And Swap(CAS)？简单的说就是比较并交换。

CAS 操作包含三个操作数 —— 内存位置（V）、预期原值（A）和新值(B)。如果内存位置的值与预期原值相匹配，那么处理器会自动将该位置值更新为新值。否则，处理器不做任何操作。无论哪种情况，它都会在 CAS 指令之前返回该位置的值。CAS 有效地说明了“我认为位置 V 应该包含值 A；如果包含该值，则将 B 放到这个位置；否则，不要更改该位置，只告诉我这个位置现在的值即可。” Java并发包(java.util.concurrent)中大量使用了CAS操作,涉及到并发的地方都调用了sun.misc.Unsafe类方法进行CAS操作。

在看一下volatile, Volatile修饰的成员变量在每次被线程访问时，都强迫从共享内存中重读该成员变量的值。而且，当成员变量发生变化时，强迫线程将变化值回写到共享内存。这样在任何时刻，两个不同的线程总是看到某个成员变量的值是相同的，更简单一点理解就是volatile修饰的变量值发生变化时对于另外的线程是可见的。

如何正确使用volatile可以参考下面这篇文章：

http://www.ibm.com/developerworks/cn/java/j-jtp06197.html Java 理论与实践: 正确使用 Volatile 变量

下面来看看java中具体的CAS操作类sun.misc.Unsafe。Unsafe类提供了硬件级别的原子操作，Java无法直接访问到操作系统底层（如系统硬件等)，为此Java使用native方法来扩展Java程序的功能。具体实现使用c++,详见文件sun.misc.natUnsafe.cc();sun.misc包的源代码可以在这里找到：

http://www.oschina.net/code/explore/gcc-4.5.2/libjava/sun/misc
//下面是sun.misc.Unsafe.java类源码

package sun.misc;

import java.lang.reflect.Field;

/***

* This class should provide access to low-level operations and its

* use should be limited to trusted code. Fields can be accessed using

* memory addresses, with undefined behaviour occurring if invalid memory

* addresses are given.

* 这个类提供了一个更底层的操作并且应该在受信任的代码中使用。可以通过内存地址

* 存取fields,如果给出的内存地址是无效的那么会有一个不确定的运行表现。

*

* @author Tom Tromey (tromey@redhat.com)

* @author Andrew John Hughes (gnu_andrew@member.fsf.org)

*/

public class Unsafe

{

// Singleton class.

private static Unsafe unsafe = new Unsafe();

/***

* Private default constructor to prevent creation of an arbitrary

* number of instances.

* 使用私有默认构造器防止创建多个实例

*/

private Unsafe()

{

}

/***

* Retrieve the singleton instance of <code>Unsafe</code>. The calling

* method should guard this instance from untrusted code, as it provides

* access to low-level operations such as direct memory access.

* 获取<code>Unsafe</code>的单例,这个方法调用应该防止在不可信的代码中实例，

* 因为unsafe类提供了一个低级别的操作，例如直接内存存取。

*

* @throws SecurityException if a security manager exists and prevents

* access to the system properties.

* 如果安全管理器不存在或者禁止访问系统属性

*/

public static Unsafe getUnsafe()

{

SecurityManager sm = System.getSecurityManager();

if (sm != null)

sm.checkPropertiesAccess();

return unsafe;

}

/***

* Returns the memory address offset of the given static field.

* The offset is merely used as a means to access a particular field

* in the other methods of this class. The value is unique to the given

* field and the same value should be returned on each subsequent call.

* 返回指定静态field的内存地址偏移量,在这个类的其他方法中这个值只是被用作一个访问

* 特定field的一个方式。这个值对于 给定的field是唯一的，并且后续对该方法的调用都应该

* 返回相同的值。

*

* @param field the field whose offset should be returned.

* 需要返回偏移量的field

* @return the offset of the given field.

* 指定field的偏移量

*/

public native long objectFieldOffset(Field field);

/***

* Compares the value of the integer field at the specified offset

* in the supplied object with the given expected value, and updates

* it if they match. The operation of this method should be atomic,

* thus providing an uninterruptible way of updating an integer field.

* 在obj的offset位置比较integer field和期望的值，如果相同则更新。这个方法

* 的操作应该是原子的，因此提供了一种不可中断的方式更新integer field。

*

* @param obj the object containing the field to modify.

* 包含要修改field的对象

* @param offset the offset of the integer field within <code>obj</code>.

* <code>obj</code>中整型field的偏移量

* @param expect the expected value of the field.

* 希望field中存在的值

* @param update the new value of the field if it equals <code>expect</code>.

* 如果期望值expect与field的当前值相同，设置filed的值为这个新值

* @return true if the field was changed.

* 如果field的值被更改

*/

public native boolean compareAndSwapInt(Object obj, long offset,

int expect, int update);

/***

* Compares the value of the long field at the specified offset

* in the supplied object with the given expected value, and updates

* it if they match. The operation of this method should be atomic,

* thus providing an uninterruptible way of updating a long field.

* 在obj的offset位置比较long field和期望的值，如果相同则更新。这个方法

* 的操作应该是原子的，因此提供了一种不可中断的方式更新long field。

*

* @param obj the object containing the field to modify.

* 包含要修改field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @param expect the expected value of the field.

* 希望field中存在的值

* @param update the new value of the field if it equals <code>expect</code>.

* 如果期望值expect与field的当前值相同，设置filed的值为这个新值

* @return true if the field was changed.

* 如果field的值被更改

*/

public native boolean compareAndSwapLong(Object obj, long offset,

long expect, long update);

/***

* Compares the value of the object field at the specified offset

* in the supplied object with the given expected value, and updates

* it if they match. The operation of this method should be atomic,

* thus providing an uninterruptible way of updating an object field.

* 在obj的offset位置比较object field和期望的值，如果相同则更新。这个方法

* 的操作应该是原子的，因此提供了一种不可中断的方式更新object field。

*

* @param obj the object containing the field to modify.

* 包含要修改field的对象

* @param offset the offset of the object field within <code>obj</code>.

* <code>obj</code>中object型field的偏移量

* @param expect the expected value of the field.

* 希望field中存在的值

* @param update the new value of the field if it equals <code>expect</code>.

* 如果期望值expect与field的当前值相同，设置filed的值为这个新值

* @return true if the field was changed.

* 如果field的值被更改

*/

public native boolean compareAndSwapObject(Object obj, long offset,

Object expect, Object update);

/***

* Sets the value of the integer field at the specified offset in the

* supplied object to the given value. This is an ordered or lazy

* version of <code>putIntVolatile(Object,long,int)</code>, which

* doesn't guarantee the immediate visibility of the change to other

* threads. It is only really useful where the integer field is

* <code>volatile</code>, and is thus expected to change unexpectedly.

* 设置obj对象中offset偏移地址对应的整型field的值为指定值。这是一个有序或者

* 有延迟的<code>putIntVolatile</cdoe>方法，并且不保证值的改变被其他线程立

* 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候

* 使用才有用。

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the integer field within <code>obj</code>.

* <code>obj</code>中整型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putIntVolatile(Object,long,int)

*/

public native void putOrderedInt(Object obj, long offset, int value);

/***

* Sets the value of the long field at the specified offset in the

* supplied object to the given value. This is an ordered or lazy

* version of <code>putLongVolatile(Object,long,long)</code>, which

* doesn't guarantee the immediate visibility of the change to other

* threads. It is only really useful where the long field is

* <code>volatile</code>, and is thus expected to change unexpectedly.

* 设置obj对象中offset偏移地址对应的long型field的值为指定值。这是一个有序或者

* 有延迟的<code>putLongVolatile</cdoe>方法，并且不保证值的改变被其他线程立

* 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候

* 使用才有用。

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putLongVolatile(Object,long,long)

*/

public native void putOrderedLong(Object obj, long offset, long value);

/***

* Sets the value of the object field at the specified offset in the

* supplied object to the given value. This is an ordered or lazy

* version of <code>putObjectVolatile(Object,long,Object)</code>, which

* doesn't guarantee the immediate visibility of the change to other

* threads. It is only really useful where the object field is

* <code>volatile</code>, and is thus expected to change unexpectedly.

* 设置obj对象中offset偏移地址对应的object型field的值为指定值。这是一个有序或者

* 有延迟的<code>putObjectVolatile</cdoe>方法，并且不保证值的改变被其他线程立

* 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候

* 使用才有用。

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the object field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

*/

public native void putOrderedObject(Object obj, long offset, Object value);

/***

* Sets the value of the integer field at the specified offset in the

* supplied object to the given value, with volatile store semantics.

* 设置obj对象中offset偏移地址对应的整型field的值为指定值。支持volatile store语义

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the integer field within <code>obj</code>.

* <code>obj</code>中整型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

*/

public native void putIntVolatile(Object obj, long offset, int value);

/***

* Retrieves the value of the integer field at the specified offset in the

* supplied object with volatile load semantics.

* 获取obj对象中offset偏移地址对应的整型field的值,支持volatile load语义。

*

* @param obj the object containing the field to read.

* 包含需要去读取的field的对象

* @param offset the offset of the integer field within <code>obj</code>.

* <code>obj</code>中整型field的偏移量

*/

public native int getIntVolatile(Object obj, long offset);

/***

* Sets the value of the long field at the specified offset in the

* supplied object to the given value, with volatile store semantics.

* 设置obj对象中offset偏移地址对应的long型field的值为指定值。支持volatile store语义

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putLong(Object,long,long)

*/

public native void putLongVolatile(Object obj, long offset, long value);

/***

* Sets the value of the long field at the specified offset in the

* supplied object to the given value.

* 设置obj对象中offset偏移地址对应的long型field的值为指定值。

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putLongVolatile(Object,long,long)

*/

public native void putLong(Object obj, long offset, long value);

/***

* Retrieves the value of the long field at the specified offset in the

* supplied object with volatile load semantics.

* 获取obj对象中offset偏移地址对应的long型field的值,支持volatile load语义。

*

* @param obj the object containing the field to read.

* 包含需要去读取的field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @see #getLong(Object,long)

*/

public native long getLongVolatile(Object obj, long offset);

/***

* Retrieves the value of the long field at the specified offset in the

* supplied object.

* 获取obj对象中offset偏移地址对应的long型field的值

*

* @param obj the object containing the field to read.

* 包含需要去读取的field的对象

* @param offset the offset of the long field within <code>obj</code>.

* <code>obj</code>中long型field的偏移量

* @see #getLongVolatile(Object,long)

*/

public native long getLong(Object obj, long offset);

/***

* Sets the value of the object field at the specified offset in the

* supplied object to the given value, with volatile store semantics.

* 设置obj对象中offset偏移地址对应的object型field的值为指定值。支持volatile store语义

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the object field within <code>obj</code>.

* <code>obj</code>中object型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putObject(Object,long,Object)

*/

public native void putObjectVolatile(Object obj, long offset, Object value);

/***

* Sets the value of the object field at the specified offset in the

* supplied object to the given value.

* 设置obj对象中offset偏移地址对应的object型field的值为指定值。

*

* @param obj the object containing the field to modify.

* 包含需要修改field的对象

* @param offset the offset of the object field within <code>obj</code>.

* <code>obj</code>中object型field的偏移量

* @param value the new value of the field.

* field将被设置的新值

* @see #putObjectVolatile(Object,long,Object)

*/

public native void putObject(Object obj, long offset, Object value);

/***

* Retrieves the value of the object field at the specified offset in the

* supplied object with volatile load semantics.

* 获取obj对象中offset偏移地址对应的object型field的值,支持volatile load语义。

*

* @param obj the object containing the field to read.

* 包含需要去读取的field的对象

* @param offset the offset of the object field within <code>obj</code>.

* <code>obj</code>中object型field的偏移量

*/

public native Object getObjectVolatile(Object obj, long offset);

/***

* Returns the offset of the first element for a given array class.

* To access elements of the array class, this value may be used along with

* with that returned by

* <a href="#arrayIndexScale"><code>arrayIndexScale</code></a>,

* if non-zero.

* 获取给定数组中第一个元素的偏移地址。

* 为了存取数组中的元素，这个偏移地址与<a href="#arrayIndexScale"><code>arrayIndexScale

* </code></a>方法的非0返回值一起被使用。

* @param arrayClass the class for which the first element's address should

* be obtained.

* 第一个元素地址被获取的class

* @return the offset of the first element of the array class.

* 数组第一个元素 的偏移地址

* @see arrayIndexScale(Class)

*/

public native int arrayBaseOffset(Class arrayClass);

/***

* Returns the scale factor used for addressing elements of the supplied

* array class. Where a suitable scale factor can not be returned (e.g.

* for primitive types), zero should be returned. The returned value

* can be used with

* <a href="#arrayBaseOffset"><code>arrayBaseOffset</code></a>

* to access elements of the class.

* 获取用户给定数组寻址的换算因子.一个合适的换算因子不能返回的时候(例如：基本类型),

* 返回0.这个返回值能够与<a href="#arrayBaseOffset"><code>arrayBaseOffset</code>

* </a>一起使用去存取这个数组class中的元素

*

* @param arrayClass the class whose scale factor should be returned.

* @return the scale factor, or zero if not supported for this array class.

*/

public native int arrayIndexScale(Class arrayClass);

/***

* Releases the block on a thread created by

* <a href="#park"><code>park</code></a>. This method can also be used

* to terminate a blockage caused by a prior call to <code>park</code>.

* This operation is unsafe, as the thread must be guaranteed to be

* live. This is true of Java, but not native code.

* 释放被<a href="#park"><code>park</code></a>创建的在一个线程上的阻塞.这个

* 方法也可以被使用来终止一个先前调用<code>park</code>导致的阻塞.

* 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。

* @param thread the thread to unblock.

* 要解除阻塞的线程

*/

public native void unpark(Thread thread);

/***

* Blocks the thread until a matching

* <a href="#unpark"><code>unpark</code></a> occurs, the thread is

* interrupted or the optional timeout expires. If an <code>unpark</code>

* call has already occurred, this also counts. A timeout value of zero

* is defined as no timeout. When <code>isAbsolute</code> is

* <code>true</code>, the timeout is in milliseconds relative to the

* epoch. Otherwise, the value is the number of nanoseconds which must

* occur before timeout. This call may also return spuriously (i.e.

* for no apparent reason).

* 阻塞一个线程直到<a href="#unpark"><code>unpark</code></a>出现、线程

* 被中断或者timeout时间到期。如果一个<code>unpark</code>调用已经出现了，

* 这里只计数。timeout为0表示永不过期.当<code>isAbsolute</code>为true时，

* timeout是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。这个方法执行时

* 也可能不合理地返回(没有具体原因)

*

* @param isAbsolute true if the timeout is specified in milliseconds from

* the epoch.

* 如果为true timeout的值是一个相对于新纪元之后的毫秒数

* @param time either the number of nanoseconds to wait, or a time in

* milliseconds from the epoch to wait for.

* 可以是一个要等待的纳秒数，或者是一个相对于新纪元之后的毫秒数直到

* 到达这个时间点

*/

public native void park(boolean isAbsolute, long time);

}

//下面是sun.misc.natUnsafe.cc源码

#include <gcj/cni.h>

#include <gcj/field.h>

#include <gcj/javaprims.h>

#include <jvm.h>

#include <sun/misc/Unsafe.h>

#include <java/lang/System.h>

#include <java/lang/InterruptedException.h>

#include <java/lang/Thread.h>

#include <java/lang/Long.h>

#include "sysdep/locks.h"

// Use a spinlock for multi-word accesses

class spinlock

{

static volatile obj_addr_t lock;

public:

spinlock ()

{

while (! compare_and_swap (&lock, 0, 1))

_Jv_ThreadYield ();

}

~spinlock ()

{

release_set (&lock, 0);

}

};

// This is a single lock that is used for all synchronized accesses if

// the compiler can't generate inline compare-and-swap operations. In

// most cases it'll never be used, but the i386 needs it for 64-bit

// locked accesses and so does PPC32. It's worth building libgcj with

// target=i486 (or above) to get the inlines.

volatile obj_addr_t spinlock::lock;

static inline bool

compareAndSwap (volatile jint *addr, jint old, jint new_val)

{

jboolean result = false;

spinlock lock;

if ((result = (*addr == old)))

*addr = new_val;

return result;

}

static inline bool

compareAndSwap (volatile jlong *addr, jlong old, jlong new_val)

{

jboolean result = false;

spinlock lock;

if ((result = (*addr == old)))

*addr = new_val;

return result;

}

static inline bool

compareAndSwap (volatile jobject *addr, jobject old, jobject new_val)

{

jboolean result = false;

spinlock lock;

if ((result = (*addr == old)))

*addr = new_val;

return result;

}

jlong

sun::misc::Unsafe::objectFieldOffset (::java::lang::reflect::Field *field)

{

_Jv_Field *fld = _Jv_FromReflectedField (field);

// FIXME: what if it is not an instance field?

return fld->getOffset();

}

jint

sun::misc::Unsafe::arrayBaseOffset (jclass arrayClass)

{

// FIXME: assert that arrayClass is array.

jclass eltClass = arrayClass->getComponentType();

return (jint)(jlong) _Jv_GetArrayElementFromElementType (NULL, eltClass);

}

jint

sun::misc::Unsafe::arrayIndexScale (jclass arrayClass)

{

// FIXME: assert that arrayClass is array.

jclass eltClass = arrayClass->getComponentType();

if (eltClass->isPrimitive())

return eltClass->size();

return sizeof (void *);

}

// These methods are used when the compiler fails to generate inline

// versions of the compare-and-swap primitives.

jboolean

sun::misc::Unsafe::compareAndSwapInt (jobject obj, jlong offset,

jint expect, jint update)

{

jint *addr = (jint *)((char *)obj + offset);

return compareAndSwap (addr, expect, update);

}

jboolean

sun::misc::Unsafe::compareAndSwapLong (jobject obj, jlong offset,

jlong expect, jlong update)

{

volatile jlong *addr = (jlong*)((char *) obj + offset);

return compareAndSwap (addr, expect, update);

}

jboolean

sun::misc::Unsafe::compareAndSwapObject (jobject obj, jlong offset,

jobject expect, jobject update)

{

jobject *addr = (jobject*)((char *) obj + offset);

return compareAndSwap (addr, expect, update);

}

void

sun::misc::Unsafe::putOrderedInt (jobject obj, jlong offset, jint value)

{

volatile jint *addr = (jint *) ((char *) obj + offset);

*addr = value;

}

void

sun::misc::Unsafe::putOrderedLong (jobject obj, jlong offset, jlong value)

{

volatile jlong *addr = (jlong *) ((char *) obj + offset);

spinlock lock;

*addr = value;

}

void

sun::misc::Unsafe::putOrderedObject (jobject obj, jlong offset, jobject value)

{

volatile jobject *addr = (jobject *) ((char *) obj + offset);

*addr = value;

}

void

sun::misc::Unsafe::putIntVolatile (jobject obj, jlong offset, jint value)

{

write_barrier ();

volatile jint *addr = (jint *) ((char *) obj + offset);

*addr = value;

}

void

sun::misc::Unsafe::putLongVolatile (jobject obj, jlong offset, jlong value)

{

volatile jlong *addr = (jlong *) ((char *) obj + offset);

spinlock lock;

*addr = value;

}

void

sun::misc::Unsafe::putObjectVolatile (jobject obj, jlong offset, jobject value)

{

write_barrier ();

volatile jobject *addr = (jobject *) ((char *) obj + offset);

*addr = value;

}

#if 0 // FIXME

void

sun::misc::Unsafe::putInt (jobject obj, jlong offset, jint value)

{

jint *addr = (jint *) ((char *) obj + offset);

*addr = value;

}

#endif

void

sun::misc::Unsafe::putLong (jobject obj, jlong offset, jlong value)

{

jlong *addr = (jlong *) ((char *) obj + offset);

spinlock lock;

*addr = value;

}

void

sun::misc::Unsafe::putObject (jobject obj, jlong offset, jobject value)

{

jobject *addr = (jobject *) ((char *) obj + offset);

*addr = value;

}

jint

sun::misc::Unsafe::getIntVolatile (jobject obj, jlong offset)

{

volatile jint *addr = (jint *) ((char *) obj + offset);

jint result = *addr;

read_barrier ();

return result;

}

jobject

sun::misc::Unsafe::getObjectVolatile (jobject obj, jlong offset)

{

volatile jobject *addr = (jobject *) ((char *) obj + offset);

jobject result = *addr;

read_barrier ();

return result;

}

jlong

sun::misc::Unsafe::getLong (jobject obj, jlong offset)

{

jlong *addr = (jlong *) ((char *) obj + offset);

spinlock lock;

return *addr;

}

jlong

sun::misc::Unsafe::getLongVolatile (jobject obj, jlong offset)

{

volatile jlong *addr = (jlong *) ((char *) obj + offset);

spinlock lock;

return *addr;

}

void

sun::misc::Unsafe::unpark (::java::lang::Thread *thread)

{

natThread *nt = (natThread *) thread->data;

nt->park_helper.unpark ();

}

void

sun::misc::Unsafe::park (jboolean isAbsolute, jlong time)

{

using namespace ::java::lang;

Thread *thread = Thread::currentThread();

natThread *nt = (natThread *) thread->data;

nt->park_helper.park (isAbsolute, time);

}

* 如果您有不同见解或发现错误，欢迎讨论或指出。qq：294016299。
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