轉載：http://www.cnblogs.com/mickole/articles/3757278.html
sun.misc.Unsafe
/**
* 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.
* 這個類提供了一個更底層的操作並且應該在受信任的代碼中使用。可以通過內存地址
* 存取fields,如果給出的內存地址是無效的那麼會有一個不確定的運行表現。
*
* @author Tom Tromey ([email protected])
* @author Andrew John Hughes ([email protected])
*/
public class Unsafe {
// Singleton class.
private static Unsafe unsafe = new Unsafe();
/**
* Private default constructor to prevent creation of an arbitrary
* number of instances.
* 使用私有默認構造器防止創建多個實例
*/
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.
* 獲取<code>Unsafe</code>的單例,這個方法調用應該防止在不可信的代碼中實例，
* 因爲unsafe類提供了一個低級別的操作，例如直接內存存取。
*
* @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訪問系統屬性
*/
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.
* 返回指定靜態field的內存地址偏移量,在這個類的其他方法中這個值只是被用作一個訪問
* 特定field的一個方式。這個值對於 給定的field是唯一的，並且後續對該方法的調用都應該
* 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位置比較integer field和期望的值，如果相同則更新。這個方法
* 的操作應該是原子的，因此提供了一種不可中斷的方式更新integer field。
*
* @param obj the object containing the field to modify.
* 包含要修改field的對象
* @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與field的當前值相同，設置filed的值爲這個新值
* @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位置比較long field和期望的值，如果相同則更新。這個方法
* 的操作應該是原子的，因此提供了一種不可中斷的方式更新long field。
*
* @param obj the object containing the field to modify.
* 包含要修改field的對象
* @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與field的當前值相同，設置filed的值爲這個新值
* @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位置比較object field和期望的值，如果相同則更新。這個方法
* 的操作應該是原子的，因此提供了一種不可中斷的方式更新object field。
*
* @param obj the object containing the field to modify.
* 包含要修改field的對象
* @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與field的當前值相同，設置filed的值爲這個新值
* @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.
* 設置obj對象中offset偏移地址對應的整型field的值爲指定值。這是一個有序或者
* 有延遲的<code>putIntVolatile</cdoe>方法，並且不保證值的改變被其他線程立
* 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候
* 使用纔有用。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 設置obj對象中offset偏移地址對應的long型field的值爲指定值。這是一個有序或者
* 有延遲的<code>putLongVolatile</cdoe>方法，並且不保證值的改變被其他線程立
* 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候
* 使用纔有用。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 設置obj對象中offset偏移地址對應的object型field的值爲指定值。這是一個有序或者
* 有延遲的<code>putObjectVolatile</cdoe>方法，並且不保證值的改變被其他線程立
* 即看到。只有在field被<code>volatile</code>修飾並且期望被意外修改的時候
* 使用纔有用。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
*/
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.
* 設置obj對象中offset偏移地址對應的整型field的值爲指定值。支持volatile store語義
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
*/
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.
* 獲取obj對象中offset偏移地址對應的整型field的值,支持volatile load語義。
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對象
* @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.
* 設置obj對象中offset偏移地址對應的long型field的值爲指定值。支持volatile store語義
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 設置obj對象中offset偏移地址對應的long型field的值爲指定值。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 獲取obj對象中offset偏移地址對應的long型field的值,支持volatile load語義。
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對象
* @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.
* 獲取obj對象中offset偏移地址對應的long型field的值
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對象
* @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.
* 設置obj對象中offset偏移地址對應的object型field的值爲指定值。支持volatile store語義
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 設置obj對象中offset偏移地址對應的object型field的值爲指定值。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的對象
* @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將被設置的新值
* @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.
* 獲取obj對象中offset偏移地址對應的object型field的值,支持volatile load語義。
*
* @param obj the object containing the field to read.
* 包含需要去讀取的field的對象
* @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.
* 獲取給定數組中第一個元素的偏移地址。
* 爲了存取數組中的元素，這個偏移地址與<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.
* 第一個元素地址被獲取的class
* @return the offset of the first element of the array class.
* 數組第一個元素 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.
* 獲取用戶給定數組尋址的換算因子.一個合適的換算因子不能返回的時候(例如：基本類型),
* 返回0.這個返回值能夠與<a href="#arrayBaseOffset"><code>arrayBaseOffset</code>
* </a>一起使用去存取這個數組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.
* 釋放被<a href="#park"><code>park</code></a>創建的在一個線程上的阻塞.這個
* 方法也可以被使用來終止一個先前調用<code>park</code>導致的阻塞.
* 這個操作操作時不安全的,因此線程必須保證是活的.這是java代碼不是native代碼。
*
* @param thread the thread to unblock.
* 要解除阻塞的線程
*/
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).
* 阻塞一個線程直到<a href="#unpark"><code>unpark</code></a>出現、線程
* 被中斷或者timeout時間到期。如果一個<code>unpark</code>調用已經出現了，
* 這裏只計數。timeout爲0表示永不過期.當<code>isAbsolute</code>爲true時，
* timeout是相對於新紀元之後的毫秒。否則這個值就是超時前的納秒數。這個方法執行時
* 也可能不合理地返回(沒有具體原因)
*
* @param isAbsolute true if the timeout is specified in milliseconds from
* the epoch.
* 如果爲true timeout的值是一個相對於新紀元之後的毫秒數
* @param time either the number of nanoseconds to wait, or a time in
* milliseconds from the epoch to wait for.
* 可以是一個要等待的納秒數，或者是一個相對於新紀元之後的毫秒數直到
* 到達這個時間點
*/
public native void park(boolean isAbsolute, long time);
}sun.misc.natUnsafe.cc源碼
#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);
}