java不能直接訪問操作系統底層，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類分配堆外內存
類中提供的3個本地方法allocateMemory、reallocateMemory、freeMemory分別用于分配內存，擴充內存和釋放內存，與C語言中的3個方法對應。
//分配var1字節大小的內存，返回起始地址偏移量
public native long allocateMemory(long var1);
//重新給var1起始地址的內存分配長度為var3字節大小的內存，返回新的內存起始地址偏移量
public native long reallocateMemory(long var1, long var3);
//釋放起始地址為var1的內存
public native void freeMemory(long var1);
分配內存方法還有重分配內存方法都是分配的堆外內存，返回的是一個long類型的地址偏移量。這個偏移量在你的Java程序中每塊內存都是唯一的。
操作類對象
可以定位對象某字段的內存位置，也可以修改對象的字段值，即使它是私有的；
首先獲取對象的基址（對象在內存的偏移量起始地址）。之后獲取某個filed在這個對象對應的類中的偏移地址，兩者相加修改。
/**
* 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，需要將目標對象設為輔助數組的第一個元素（也是唯一的元素）。
// 由于這是一個復雜類型元素（不是基本數據類型），它的地址存儲在數組的第一個元素。
// 然后，獲取輔助數組的基本偏移量。
// 數組的基本偏移量是指數組對象的起始地址與數組第一個元素之間的偏移量。
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方法實現的，調用 park后，線程將一直阻塞直到超時或者中斷等條件出現。unpark可以終止一個掛起的線程，使其恢復正常。整個并發框架中對線程的掛起操作被封裝在 LockSupport類中，LockSupport類中有各種版本pack方法，但最終都調用了Unsafe.park()方法。
CAS操作 是通過compareAndSwapXXX方法實現的
/**
* 比較obj的offset處內存位置中的值和期望的值，如果相同則更新。此更新是不可中斷的。
*
* @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，設置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
如何實現淺克隆？在clone(){…}方法中調用super.clone()，對嗎？這里存在的問題是首先你必須繼續Cloneable接口，并且在所有你需要做淺克隆的對象中實現clone()方法，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，我們可以僅使用幾行代碼就實現淺克隆，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么會有一個不確定的運行表現。
*
* @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，
* 因為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。這個值對于 給定的field是唯一的，并且后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，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。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.
* 設置obj對象中offset偏移地址對應的整型field的值為指定值。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。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 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的值為指定值。這是一個有序或者
* 有延遲的<code>putLongVolatile</cdoe>方法，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 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的值為指定值。這是一個有序或者
* 有延遲的<code>putObjectVolatile</cdoe>方法，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 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的值為指定值。
*
* @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的值為指定值。
*
* @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。
* 為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，
* 這里只計數。timeout為0表示永不過期.當<code>isAbsolute</code>為true時，
* 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。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.
* 如果為true timeout的值是一個相對于新紀元之后的毫秒數
* @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，或者是一個相對于新紀元之后的毫秒數直到
* 到達這個時間點
*/
public native void park(boolean isAbsolute, long time);
}
Ref:
