一、为什么要有Unsfae、我们为什么要了解这个类
1. java 通常的代码无法直接使用操作底层的硬件，为了使java具备该能力，增加了Unsafe类
2.java的并发包中底层大量的使用这个类的功能，了解这个类有助于了解java 并发包，理解为什么并发包中的类的并发性能为什么会比使用synchronized好
二、Unsafe的主要功能
1.读功能
读功能有两类：
1）读内存中某一块的数据，getXXX volatile 和getXXX。前者支持Volatile语义，后者不支持，可以看最后面的c代码了解不同
2）获取某一个类中某一个属性在非堆内存中的位置,因为在非堆中，这个属性需要满足下面两个属性之一：
a。这个属性是一个静态属性
b。这个属性是一个 volatile 修饰的属性
2.写功能
写功能主要是三方面：putXXX,putOrderXXX,putXXXVolatile。
从性能上讲，这三个方法由高到低；从数据的实时性讲，这三个方法是由低到高。可以看最后面的c代码了解不同
3. cas操作
这些方法类似compareAndSwapXXX，这些操作是一个原子操作，也就是说cpu在执行cas时对这一块内存是独占排他的。在并发包中很多操作真正执行的也是cas，并发包中的类并发性能比使用 synchronized 关键字好也在于此：锁的粒度小了许多并且少了 线程 上下文切换。
4.park、unpark
是并发包中锁的基础，将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。
三、Unsafe类源码
上面是对Unsafe类的小结，如果要对其有详细的了解，需要看其源码。
几点说明：1.这个源代码对应的jdk版本是有些老的 2.在jdk9中，这个类很有可能会去掉，尽管这个类被许多中间件用到了
//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 Object FieldOffset(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);
}
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natUnsafe.cc
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// natUnsafe.cc - Implementation of sun.misc.Unsafe native methods.
/** Copyright (C) 2006, 2007
Free Software Foundation
This file is part of libgcj.
This software is copyrighted work licensed under the terms of the
Libgcj License. Please consult the file "LIBGCJ_LICENSE" for
details. */
# 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);
} 
