java不能直接訪問操作系統(tǒng)底層，而是通過本地方法來訪問。Unsafe類提供了硬件級別的原子操作，主要提供了以下功能：
通過Unsafe類分配堆外內(nèi)存
類中提供的3個本地方法allocateMemory砌烁、reallocateMemory、freeMemory分別用于分配內(nèi)存，擴充內(nèi)存和釋放內(nèi)存函喉，與C語言中的3個方法對應(yīng)避归。
//分配var1字節(jié)大小的內(nèi)存，返回起始地址偏移量
public native long allocateMemory(long var1);
//重新給var1起始地址的內(nèi)存分配長度為var3字節(jié)大小的內(nèi)存管呵，返回新的內(nèi)存起始地址偏移量
public native long reallocateMemory(long var1, long var3);
//釋放起始地址為var1的內(nèi)存
public native void freeMemory(long var1);
分配內(nèi)存方法還有重分配內(nèi)存方法都是分配的堆外內(nèi)存梳毙，返回的是一個long類型的地址偏移量。這個偏移量在你的Java程序中每塊內(nèi)存都是唯一的捐下。
操作類對象
可以定位對象某字段的內(nèi)存位置账锹，也可以修改對象的字段值，即使它是私有的坷襟；
首先獲取對象的基址（對象在內(nèi)存的偏移量起始地址）奸柬。之后獲取某個filed在這個對象對應(yīng)的類中的偏移地址，兩者相加修改啤握。
/**
* 獲取類的某個對象的某個field偏移地址
*/
Field f = null;
try {
f = TUnsafe.class.getDeclaredField("i");
} catch (NoSuchFieldException e) {
e.printStackTrace();
}
long iFiledAddressShift = UNSAFE.objectFieldOffset(f);
TUnsafe sampleClass = new TUnsafe();
// 獲取對象的偏移地址鸟缕，需要將目標對象設(shè)為輔助數(shù)組的第一個元素（也是唯一的元素）。
// 由于這是一個復雜類型元素（不是基本數(shù)據(jù)類型）排抬，它的地址存儲在數(shù)組的第一個元素懂从。
// 然后，獲取輔助數(shù)組的基本偏移量蹲蒲。
// 數(shù)組的基本偏移量是指數(shù)組對象的起始地址與數(shù)組第一個元素之間的偏移量番甩。
Object helperArray[] = new Object[1];
helperArray[0] = sampleClass;
long baseOffset = UNSAFE.arrayBaseOffset(Object[].class);
long addressOfSampleClass = UNSAFE.getLong(helperArray, baseOffset);
int i = UNSAFE.getInt(addressOfSampleClass + iFiledAddressShift);
System.out.println(new StringBuilder().append(" Field I Address:")
.append(addressOfSampleClass).append("+")
.append(iFiledAddressShift).append(" Value:").append(i));
UNSAFE.putInt(sampleClass, iFiledAddressShift, 1024);
System.out.println(new StringBuilder().append(" Field I Address:")
.append(addressOfSampleClass).append("+")
.append(iFiledAddressShift).append(" Value:")
.append(sampleClass.i));
線程掛起與恢復
將一個線程進行掛起是通過park方法實現(xiàn)的，調(diào)用 park后届搁，線程將一直阻塞直到超時或者中斷等條件出現(xiàn)缘薛。unpark可以終止一個掛起的線程，使其恢復正常卡睦。整個并發(fā)框架中對線程的掛起操作被封裝在 LockSupport類中宴胧，LockSupport類中有各種版本pack方法，但最終都調(diào)用了Unsafe.park()方法表锻。
CAS操作 是通過compareAndSwapXXX方法實現(xiàn)的
/**
* 比較obj的offset處內(nèi)存位置中的值和期望的值恕齐，如果相同則更新。此更新是不可中斷的瞬逊。
*
* @param obj 需要更新的對象
* @param offset obj中整型field的偏移量
* @param expect 希望field中存在的值
* @param update 如果期望值expect與field的當前值相同显歧，設(shè)置filed的值為這個新值
* @return 如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);
Clone
如何實現(xiàn)淺克隆确镊？在clone(){…}方法中調(diào)用super.clone()士骤，對嗎？這里存在的問題是首先你必須繼續(xù)Cloneable接口蕾域，并且在所有你需要做淺克隆的對象中實現(xiàn)clone()方法拷肌，對于一個懶懶的程序員來說，這個工作量太大了。
我不推薦上面的做法而是直接使用Unsafe巨缘，我們可以僅使用幾行代碼就實現(xiàn)淺克隆厢绝，并且它可以像某些工具類一樣用于任意類的克隆。
Ref:
http://blog.csdn.net/zhxdick/article/details/52003123
https://tech.meituan.com/2019/02/14/talk-about-java-magic-class-unsafe.html
Unsafe類
//下面是sun.misc.Unsafe.java類源碼
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.
* 這個類提供了一個更底層的操作并且應(yīng)該在受信任的代碼中使用带猴。可以通過內(nèi)存地址
* 存取fields,如果給出的內(nèi)存地址是無效的那么會有一個不確定的運行表現(xiàn)懈万。
*
* @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.
* 使用私有默認構(gòu)造器防止創(chuàng)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(diào)用應(yīng)該防止在不可信的代碼中實例拴清，
* 因為unsafe類提供了一個低級別的操作，例如直接內(nèi)存存取会通。
*
* @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(tǒng)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(tài)field的內(nèi)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的一個方式口予。這個值對于 給定的field是唯一的，并且后續(xù)對該方法的調(diào)用都應(yīng)該
* 返回相同的值涕侈。
*
* @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和期望的值沪停，如果相同則更新。這個方法
* 的操作應(yīng)該是原子的裳涛，因此提供了一種不可中斷的方式更新integer field木张。
*
* @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(shè)置filed的值為這個新值
* @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和期望的值端三，如果相同則更新舷礼。這個方法
* 的操作應(yīng)該是原子的，因此提供了一種不可中斷的方式更新long field郊闯。
*
* @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l(wèi)ong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的當前值相同妻献，設(shè)置filed的值為這個新值
* @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，如果相同則更新团赁。這個方法
* 的操作應(yīng)該是原子的育拨，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的當前值相同欢摄，設(shè)置filed的值為這個新值
* @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(shè)置obj對象中offset偏移地址對應(yīng)的整型field的值為指定值熬丧。這是一個有序或者
* 有延遲的<code>putIntVolatile</cdoe>方法，并且不保證值的改變被其他線程立
* 即看到剧浸。只有在field被<code>volatile</code>修飾并且期望被意外修改的時候
* 使用才有用锹引。
*
* @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(shè)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(shè)置obj對象中offset偏移地址對應(yīng)的long型field的值為指定值。這是一個有序或者
* 有延遲的<code>putLongVolatile</cdoe>方法唆香，并且不保證值的改變被其他線程立
* 即看到嫌变。只有在field被<code>volatile</code>修飾并且期望被意外修改的時候
* 使用才有用。
*
* @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l(wèi)ong型field的偏移量
* @param value the new value of the field.
* field將被設(shè)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(shè)置obj對象中offset偏移地址對應(yīng)的object型field的值為指定值躬它。這是一個有序或者
* 有延遲的<code>putObjectVolatile</cdoe>方法腾啥，并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>修飾并且期望被意外修改的時候
* 使用才有用。
*
* @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l(wèi)ong型field的偏移量
* @param value the new value of the field.
* field將被設(shè)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(shè)置obj對象中offset偏移地址對應(yīng)的整型field的值為指定值倘待。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(shè)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(yīng)的整型field的值,支持volatile load語義疮跑。
*
* @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(shè)置obj對象中offset偏移地址對應(yīng)的long型field的值為指定值。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l(wèi)ong型field的偏移量
* @param value the new value of the field.
* field將被設(shè)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(shè)置obj對象中offset偏移地址對應(yīng)的long型field的值為指定值凸舵。
*
* @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l(wèi)ong型field的偏移量
* @param value the new value of the field.
* field將被設(shè)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(yīng)的long型field的值,支持volatile load語義祖娘。
*
* @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l(wèi)ong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(yīng)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l(wèi)ong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(shè)置obj對象中offset偏移地址對應(yīng)的object型field的值為指定值。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(shè)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(shè)置obj對象中offset偏移地址對應(yīng)的object型field的值為指定值啊奄。
*
* @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(shè)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(yīng)的object型field的值,支持volatile load語義渐苏。
*
* @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(shù)組中第一個元素的偏移地址。
* 為了存取數(shù)組中的元素菇夸，這個偏移地址與<a href="#arrayIndexScale"><code>arrayIndexScale
* </code></a>方法的非0返回值一起被使用琼富。
* @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(shù)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(shù)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(shù)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(chuàng)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(diào)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(xiàn)庄新、線程
* 被中斷或者timeout時間到期鞠眉。如果一個<code>unpark</code>調(diào)用已經(jīng)出現(xiàn)了，
* 這里只計數(shù)择诈。timeout為0表示永不過期.當<code>isAbsolute</code>為true時械蹋，
* timeout是相對于新紀元之后的毫秒。否則這個值就是超時前的納秒數(shù)羞芍。這個方法執(zhí)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.
* 如果為true timeout的值是一個相對于新紀元之后的毫秒數(shù)
* @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(shù)朝蜘，或者是一個相對于新紀元之后的毫秒數(shù)直到
* 到達這個時間點
*/
public native void park(boolean isAbsolute, long time);
}
Ref:
