Java底層魔術類Unsafe用法簡述

1 引子

Java中沒有指針，不能直接對內存地址的變量進行控制，但Java提供了一個特殊的類Unsafe工具類來間接實現。Unsafe主要提供一些用於執行低級別、不安全操作的方法，如直接訪問系統內存資源、自主管理內存資源等，這些方法在提升Java運行效率、增強Java語言底層資源操作能力方面起到了很大的作用 。正如其名字unsafe，直接去使用這個工具類是不安全的，它能直接在硬件層（內存上）修改訪問變量，而無視各種訪問修飾符的限制。它幾乎所有的公共方法API都是本地方法，這些方法是使用C/C++方法實現的，它越過了虛擬機層面，直接在操作系統本地執行。因為這是一個底層類，如果在不了解其內部原理、未掌握其使用技巧的情況下，我們直接使用Unsafe類可能會造成一些意想不到或未知的錯誤，所以它被限制開發者直接使用，只能由JDK類庫的維護者使用。如果您喜歡閱讀JDK的源碼，那么你會發現在各種並發工具類的內部常常見到這個類的蹤影，它們經常通過這個類的一些方法根據相應內存地址在內存上直接CAS修改訪問共享變量的值。

Unsafe類在Oracle的官方JDK中沒有提供源碼，我們只能通過IDEA的反編譯工具看到反編譯后的源代碼，因此我們看不到方法注釋。而只OpenJDK中帶有所有JDK的源代碼，這里使用OpenJDK作參考講解材料。以下是OpenJDK中Unsafe的類注釋

 A collection of methods for performing low-level, unsafe operations. Although the class and all methods are public, use of this class is limited because only trusted code can obtain instances of it.

直譯過來大致意思是：此類擁有一組用於執行低級，不安全操作的方法。 盡管此類和所有方法都是公共的，但是由於只有可信代碼才能獲取該類的實例，因此此類的使用受到限制。

可以看出構造方法被私有化，只能通過靜態方法getUnsafe（）才能獲取此Unsafe單例對象，而此靜態方法的使用也是受到限制的，只能由JDK中的其它類來調用，普通開發者使用此方法將拋出異常。

    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();
    
    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass(); //調用者Class對象
        if (!VM.isSystemDomainLoader(caller.getClassLoader())) //判斷調用者的類加載器是否為系統類加載器
            //不是JAVA_HOME/jre/lib目錄下jar包中的類來調用此方法getUnsafe（）就會拋出異常
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }

 

 

此方法getUnsafe()上的注釋也說：

為調用提供執行不安全操作的能力。返回的Unsafe對象應由調用方小心保護，因為它可用於在任意內存地址處讀取和寫入數據。 絕不能將其傳遞給不受信任的代碼。此類中的大多數方法都是非常底層的，並且對應於少量的硬件指令（在典型的機器上）。 應鼓勵編譯器相應地優化這些方法，而不是使用Unsafe類來控制。

getUnsafe()要求JDK類庫自身調用，當然將開發者可以將自己定義的類放在JDK系統類庫中，但這種方式明顯是不安全、不方便的，其可行性太低。倘若開發者的確需要使用Unsafe類，我們可以使用反射的方式獲取Unsafe實例。

    private static Unsafe getUnsafeByReflect() {
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            return (Unsafe) f.get(null);
        } catch (Exception e) {
            throw new Error(e);
        }
    }

 使用反射方式，在開發者的classpath中獲取到Unsafe實例

package com.aaxis;

import java.lang.reflect.Field;

import sun.misc.Unsafe;

public class Student {
    private int stuId;
    private String name;
    private int age;
    private static final long STUID_OFFSET;
    private static final Unsafe UNSAFE = getUnsafeByReflect();
    static {
        try {
            STUID_OFFSET = UNSAFE.objectFieldOffset(Student.class.getDeclaredField("stuId"));
        } catch (NoSuchFieldException | SecurityException e) {
            throw new Error(e);
        }

    }
    private static Unsafe getUnsafeByReflect() {
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            return (Unsafe) f.get(null);
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    public static void unsafedPrintStuId() {
        Student student = new Student(34124, "小黃");
        int stuId = UNSAFE.getInt(student, STUID_OFFSET);

        System.out.println(student.getName() + "學號：" + stuId);
    }
    public static void main(String[] args) {
        unsafedPrintStuId();
    }
     //.....        
}

 

Unsafe類的主要功能如圖：

 

  

2 Java對象相關操作 

注意：因為反射中使用Field描述實例變量和靜態變量，現在將實例變量和靜態變量統稱為字段。

獲取字段相對偏移量

/**
 *  根據反射的字段f,獲取相應實例變量的偏移量
 *  此偏移量是實例變量的起始地址與對象的起始地址之差，對於一確定的java類，某字段與對象之間的起始地址之差是常數，
 *  靜態變量的偏移量與此類似
 */
public native long staticFieldOffset(Field f);
//根據反射的字段f,獲取相應靜態變量的偏移量（靜態變量的起始地址與相應靜態區Klass對象起始地址之差）
public native long objectFieldOffset(Field f);

 這里提到了字段的偏移量，這與Java對象的內存布局有密切關系。Java對象由對象頭和實際數據兩部分組成。

下圖中MarkWord包含對象的hashCode、鎖信息、垃圾回收的分代信息等，占32/64位；Class Metadata Pointer表示一個此對象數據類型的Class對象（虛擬機中的Klass對象）的指針，占32/64位；ArrayLength是數組對象特有的內容，表示數組的長度，占32位。數組對象的實際數據是各個元素的值或引用，普通對象的實際數據是各實例字段的值或引用。另外為了快速內存分配、快速內存尋址、提高性能，Java語言規范要求Java對象要做內存對齊處理，每個對象占用的內存字節數必須是8的倍數，若不是則要填零補足對齊。

從下圖可以看出，字段與對象頭之間的偏移量是固定的，只要知道字段的相對偏移量和對象起始地址，我們就能獲取此字段的絕對內存地址（fieldAddress=objAddress+fieldOffset），根據此絕對內存地址，我們就能忽略訪問修飾符的限制而可直接讀取/修改此字段的值或引用。

數組對象的元素內存定址，相對對於普通對象的字段定址有些不一樣，它要先計算出對象頭的長度，作為基礎偏移量；由於數組元素的數據類型是相同的，每個元素的值或引用所占內存空間是相同的，因此將元素值或引用或占內存作為每兩相鄰元素的相對偏移量。根據對象起始位置、基礎偏移量、相鄰元素相對偏移量及數組下標，就可以獲取到某個元素值或引用的絕對內存地址(itemAddress=arrayAddress+baseOffset+index*indexOffset),進而通過絕對內存地址讀取或修改此元素的值或引用。

 

根據字段偏移量設置/獲取字段值

    //根據反射的字段f,獲取相應的靜態變量的值
    public native Object staticFieldBase(Field f);
    
    /**
     *參數o是字段所屬的對象，offset表示相對偏移量，參數x是此字段要設置的新值
     */
    
    /*字段是引用數據類型*/
    public native Object getObject(Object o, long offset);//獲取字段值
    public native void putObject(Object o, long offset, Object x);//設置字段值

    /*字段為基本數據類型*/
    public native void putInt(Object o, long offset, int x);
    public native int getInt(Object o, long offset);
    public native boolean getBoolean(Object o, long offset);
    public native void    putBoolean(Object o, long offset, boolean x);
    public native byte    getByte(Object o, long offset);
    public native void    putByte(Object o, long offset, byte x);
    public native short   getShort(Object o, long offset);
    public native void    putShort(Object o, long offset, short x);
    public native char    getChar(Object o, long offset);
    public native void    putChar(Object o, long offset, char x);
    public native long    getLong(Object o, long offset);
    public native void    putLong(Object o, long offset, long x);
    public native float   getFloat(Object o, long offset);
    public native void    putFloat(Object o, long offset, float x);
    public native double  getDouble(Object o, long offset);
    public native void    putDouble(Object o, long offset, double x);

 使用示例：

我將一個自定義的普通（編譯后的）Java類放在JDK類庫的charset.jar包中，這個Student類使用了Unsafe類。

 Student.class的部分反編譯源碼

 

 測試Unsafe能否忽略訪問限制，讀取私有變量

package other;

import sun.awt.Student;

public class UnsafeTest {
    public static void main(String[] args) {
        Student.unsafedPrintStuId();
    }
}

 

 控制台輸出結果正確

volatile版本根據字段偏移量設置/獲取字段值(加上volatile語義)

保證對其他線程的可見性（只有字段被volatile修飾時有效）

    //volatile形式地獲取字段值，即使在多線條件下，從主內存中獲取值，使當前線程的工作內存的緩存值失效
    public native Object getObjectVolatile(Object o, long offset);
    //volatile形式地設置字段值，即使在多線條件下，設置的值將只保存到主內存中，不加載到線程本地緩存，保證可見性
    public native void    putObjectVolatile(Object o, long offset, Object x);
    public native int     getIntVolatile(Object o, long offset);
    public native void    putIntVolatile(Object o, long offset, int x);
    public native boolean getBooleanVolatile(Object o, long offset);
    public native void    putBooleanVolatile(Object o, long offset, boolean x);
    public native byte    getByteVolatile(Object o, long offset);
    public native void    putByteVolatile(Object o, long offset, byte x);
    public native short   getShortVolatile(Object o, long offset);
    public native void    putShortVolatile(Object o, long offset, short x);
    public native char    getCharVolatile(Object o, long offset);
    public native void    putCharVolatile(Object o, long offset, char x);
    public native long    getLongVolatile(Object o, long offset);
    public native void    putLongVolatile(Object o, long offset, long x);
    public native float   getFloatVolatile(Object o, long offset);
    public native void    putFloatVolatile(Object o, long offset, float x);
    public native double  getDoubleVolatile(Object o, long offset);
    public native void    putDoubleVolatile(Object o, long offset, double x);

 

有序延遲化地設置字段值

有序延遲化設值，對其他線程不保證可見性

    //有序延遲化地設置字段值，
    public native void    putOrderedObject(Object o, long offset, Object x);
    /** Ordered/Lazy version of {@link #putIntVolatile(Object, long, int)}  */
    public native void    putOrderedInt(Object o, long offset, int x);
    /** Ordered/Lazy version of {@link #putLongVolatile(Object, long, long)} */
    public native void    putOrderedLong(Object o, long offset, long x);

 

 數組相關的偏移量

    //第一個元素與數組對象兩者間起始地址之差(首元素與對象頭的相對偏移量）
    public native int arrayBaseOffset(Class<?> arrayClass);

    //相鄰元素間相對偏移量的位移表示(返回值的二進制形式的有效位數是x，那么相鄰元素的偏移量就是2的x次方)
    public native int arrayIndexScale(Class<?> arrayClass);
 

 

 使用示例：

java.util.concurrent.atomic.AtomicIntegerArray包下的AtomicIntegerArray結合以上兩個方法，進行數組元素地址定位。

class AtomicIntegerArray implements java.io.Serializable {
    private static final long serialVersionUID = 2862133569453604235L;

    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final int   base   = unsafe.arrayBaseOffset(int[].class);
    private static final int   shift;
    private final        int[] array;
    static {
        int scale = unsafe.arrayIndexScale(int[].class);
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        shift = 31 - Integer.numberOfLeadingZeros(scale);
    }
    private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }
    private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }
    public final void set(int i, int newValue) {
        unsafe.putIntVolatile(array, checkedByteOffset(i), newValue);
    }
}

 

3 Class相關操作

 創建Java類

    /**
     * 讓虛擬機知道我們定義一個類，但不進行安全檢查。 
     * 默認情況下，類加載器和保護域來自調用者的類。
     */
    public native Class<?> defineClass(String name, byte[] b, int off, int len,
                                       ClassLoader loader,
                                       ProtectionDomain protectionDomain);

    /*
    * 在類加載器和系統字典（system dictionary)不知道的情況下根據字節碼數據定義一個匿名的Class對象，相當於創建了一個Java類
     * @params hostClass context for linkage, access control, protection domain, and class loader
     * @params data     字節碼文件對應的字節數組
     * @params cpPatches where non-null entries exist, they replace corresponding CP entries in data
     */
    public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

 

 Java類初始化

shouldBeInitialized（Class）方法檢測Class對應的Java類是否被初始化
ensureClassInitialized（Class)方法強制Java類初始化，若沒初始化則進行初始化。
這兩個方法常與staticFieldBase(Field)一起使用，因為如果Java類沒有被初始化，靜態變量便沒有初始化，就不能直接獲取靜態變量的引用。

  /**
     * Detect if the given class may need to be initialized. This is often
     * needed in conjunction with obtaining the static field base of a
     * class.
     * @return false only if a call to {@code ensureClassInitialized} would have no effect
     */
    public native boolean shouldBeInitialized(Class<?> c);

    /**
     * Ensure the given class has been initialized. This is often
     * needed in conjunction with obtaining the static field base of a
     * class.
     */
    public native void ensureClassInitialized(Class<?> c);

 使用示例：

java.lang.invoke.DirectMethodHandle中的checkInitialized(MemberName)方法調用了以上兩個與類初始化相關的方法

 

 根據Class創建對象

僅通過Class對象就可以創建此類的實例對象，而且不需要調用其構造函數、初始化代碼、JVM安全檢查，等,。它抑制修飾符檢測，也就是即使構造器是private修飾的也能通過此方法實例化，只需提類對象即可創建相應的對象 .

    /** Allocate an instance but do not run any constructor.
        Initializes the class if it has not yet been. */
    public native Object allocateInstance(Class<?> cls)
        throws InstantiationException;

 使用示例：

Employe類的唯一構造方法被私有化，外界不能直接創建此類的對象。但通過"Constructor.setAccessible(true)"將私有構造器設為外部可訪問，使用反射機制也能創建一個Employee對象。

package other;

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class Employee {
    private static int count;
    private static long countL=1000;
    private long id;
    private String name;
    private int sex;// 1代表男性，0代表女性
    private long mgrId=11111;
    static {
        count = 1000;//目前員工人數的基數
    }
    private Employee() {
        sex = 1;//默認為男性
        name = "";
        count++;
        countL++;
    }
    @Override
    public String toString() {
        return "{Employee [id=" + id + ", name=" + name + ", sex=" + sex + ", mgrId=" + mgrId
                + "]}"+" ,{count="+count+", countLong="+countL+"}";
    }
}
class EmployeeTest {
    private static final Unsafe UNSAFE;
    static {
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            UNSAFE = (Unsafe) f.get(null);
        } catch (Exception e) {
            throw new Error(e);
        }
    }
    public static void main(String[] args) throws Exception {
        Employee employee = (Employee) UNSAFE.allocateInstance(Employee.class);
        System.out.println(employee);

/*        Class<Employee> clazz = Employee.class;
        Constructor<Employee> constructor = clazz.getDeclaredConstructor();
        constructor.setAccessible(true);
        Employee emp = constructor.newInstance();
        System.out.println(emp);*/
    }
}

 

兩種方式創建的對象toString()信息

Unsafe創建的對象
反射創建的對象

 

 從上面的控制台輸出信息可以看出，反射與Unsafe能均創建一個構造方法被私有化的對象。不同之處在於allocateInstance(Class)方法創建對象過程中不會進行對象初始化，但會進行類初始化；即不會執行實例變量初始化賦值、不執行構造代碼塊、不調用構造方法，但會執行靜態變量的初始化賦值、執行靜態代碼塊。

 

4 CAS更新操作

CAS是Java並發編程的最底層依據，它實現了非阻塞式地更新共享變量，自旋鎖與樂觀鎖的實現均依賴它。

   /**
     * CAS更新共享變量
     *
     * @param o        字段所屬對象
     * @param offset   字段的相對偏移量
     * @param expected 預期值
     * @param x        更新值
     * @return 更新成功則返回true
     */
    public final native boolean compareAndSwapObject(Object o, long offset, Object expected, Object x);

    public final native boolean compareAndSwapInt(Object o, long offset, int expected, int x);

    public final native boolean compareAndSwapLong(Object o, long offset, long expected, long x);

  使用示例：

 同步器AQS的compareAndSetXxx()方法都直接委托上面的CAS方法實現的

 

5 內存操作

 根據內存地址，設置/獲取對應的值

   /**
     *  參數address是絕對內存地址，參數x是設定的值
     *  如果address是零或不是通過allocMemery()方法分配的地址，那么結果未定義
     */
    public native byte    getByte(long address);
    public native void    putByte(long address, byte x);
    /** @see #getByte(long) */
    public native short   getShort(long address);
    /** @see #putByte(long, byte) */
    public native void    putShort(long address, short x);
    /** @see #getByte(long) */
    public native char    getChar(long address);
    /** @see #putByte(long, byte) */
    public native void    putChar(long address, char x);
    /** @see #getByte(long) */
    public native int     getInt(long address);
    /** @see #putByte(long, byte) */
    public native void    putInt(long address, int x);
    /** @see #getByte(long) */
    public native long    getLong(long address);
    /** @see #putByte(long, byte) */
    public native void    putLong(long address, long x);
    /** @see #getByte(long) */
    public native float   getFloat(long address);
    /** @see #putByte(long, byte) */
    public native void    putFloat(long address, float x);
    /** @see #getByte(long) */
    public native double  getDouble(long address);
    /** @see #putByte(long, byte) */
    public native void    putDouble(long address, double x);

 

 根據內存地址設置/獲取指針

    //根據內存地址獲取一個指針
    public native long getAddress(long address);

    //根據內存地址設置一個指針，adress是內存地址，x是指定的指針值
    public native void putAddress(long address, long x);

 

 分配、擴展、釋放內存

   //分配一塊指定的內存空間，返回一個指向此內存起始位置的指針
    public native long allocateMemory(long bytes);

    //擴展內存
    public native long reallocateMemory(long address, long bytes);

    //在指定的內存塊填充值
    public native void setMemory(Object o, long offset, long bytes, byte value);

    public void setMemory(long address, long bytes, byte value) {
        setMemory(null, address, bytes, value);
    }
    //將一處內存的數據復制另一處內存
    public native void copyMemory(Object srcBase, long srcOffset,
                                  Object destBase, long destOffset,
                                  long bytes);
    public void copyMemory(long srcAddress, long destAddress, long bytes) {
        copyMemory(null, srcAddress, null, destAddress, bytes);
    }
    //釋放內存
    public native void freeMemory(long address);

 

  使用示例：

 java.nio包下的DirectByteBuffer類的構造方法調用Unsafe.allocateMemory(int)分配初始條件下的的內存緩沖區

 

 DirectByteBuffer的靜態內部類Deallocator的run()調用Unsafe.freeMemory(long)釋放相應地址的內存空間

 

 

6 系統信息

獲取指定寬度、內存頁大小等系統軟硬件信息，這些信息對於本地內存的分配、使用、尋址很重要。

    //本地指針寬度，通常是4或8
    public native int addressSize();

    /**
     *內存頁的大小，它總是2的冪次方
     */
    public native int pageSize();

  使用示例：

sun.nio.ch包下NativeObject類的addressSize（）方法直接委托Unsafe.addressSize()實現

 

java.nio包下Bit類pageSize()方法:當pageSize非法時，將Unsafe.pageSize()作為返回值

 

 可以看出addressSize()、 pageSize()方法的調用者都是nio相關類，這是因為nio是直接使用JVM堆外的本地內存。

7 線程管理

 喚醒/休眠線程

    public native void unpark(Object thread);//喚醒

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

 

以上兩個方法是"等待/通知模型"的關鍵，它們的並發編程中常使用到的底層方法。以上兩個方法主要被LockSupport類直接引用，LockSupport.parkUtil(long) 、 LockSupport.upark(Thread)方法中沒有其他邏輯，就是直接委托以上兩個方法實現的。

  使用示例：

 

 搶鎖與釋放鎖（已經被棄用）

   //獲取鎖對象
    @Deprecated
    public native void monitorEnter(Object o);
    //釋放鎖對象
    @Deprecated
    public native void monitorExit(Object o);
    //嘗試獲取鎖對象
    @Deprecated
    public native boolean tryMonitorEnter(Object o);

 

 8 內存屏障

在Java 8中引入，用於定義內存屏障（也稱內存柵欄，內存柵障，屏障指令等，是一類同步屏障指令，是CPU或編譯器在對內存隨機訪問的操作中的一個同步點，使得此點之前的所有讀寫操作都執行后才可以開始執行此點之后的操作），避免代碼重排序

    /**
     * 內存屏障，禁止load重排序。屏障前不能重排序load,且只能在屏障后load或store
     */
    public native void loadFence();

    /**
     * 內存屏障，禁止store重排序。 屏障前不能重排序store操作,且只能在屏障后load或store
     */
    public native void storeFence();

    /**
     * 內存屏障，禁止store load重排序。
     */
    public native void fullFence();

   使用示例：

 loadFence()方法在StampedLock的validate方法有使用到，StampedLock是為了防止CAS更新時出現ABA問題而在JDK1.8新引入的並發工具。

 

 

 

OpenJDK1.8中含注釋的Unsafe類源代碼

/*
 * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package sun.misc;

import java.security.*;
import java.lang.reflect.*;

import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;


/**
 * A collection of methods for performing low-level, unsafe operations.
 * Although the class and all methods are public, use of this class is
 * limited because only trusted code can obtain instances of it.
 *
 * @author John R. Rose
 * @see #getUnsafe
 */

public final class Unsafe {

    private static native void registerNatives();
    static {
        registerNatives();
        sun.reflect.Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe");
    }

    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();

    /**
     * Provides the caller with the capability of performing unsafe
     * operations.
     *
     * <p> The returned <code>Unsafe</code> object should be carefully guarded
     * by the caller, since it can be used to read and write data at arbitrary
     * memory addresses.  It must never be passed to untrusted code.
     *
     * <p> Most methods in this class are very low-level, and correspond to a
     * small number of hardware instructions (on typical machines).  Compilers
     * are encouraged to optimize these methods accordingly.
     *
     * <p> Here is a suggested idiom for using unsafe operations:
     *
     * <blockquote><pre>
     * class MyTrustedClass {
     *   private static final Unsafe unsafe = Unsafe.getUnsafe();
     *   ...
     *   private long myCountAddress = ...;
     *   public int getCount() { return unsafe.getByte(myCountAddress); }
     * }
     * </pre></blockquote>
     *
     * (It may assist compilers to make the local variable be
     * <code>final</code>.)
     *
     * @exception  SecurityException  if a security manager exists and its
     *             <code>checkPropertiesAccess</code> method doesn't allow
     *             access to the system properties.
     */
    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(caller.getClassLoader()))
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }

    /// peek and poke operations
    /// (compilers should optimize these to memory ops)

    // These work on object fields in the Java heap.
    // They will not work on elements of packed arrays.

    /**
     * Fetches a value from a given Java variable.
     * More specifically, fetches a field or array element within the given
     * object <code>o</code> at the given offset, or (if <code>o</code> is
     * null) from the memory address whose numerical value is the given
     * offset.
     * <p>
     * The results are undefined unless one of the following cases is true:
     * <ul>
     * <li>The offset was obtained from {@link #objectFieldOffset} on
     * the {@link java.lang.reflect.Field} of some Java field and the object
     * referred to by <code>o</code> is of a class compatible with that
     * field's class.
     *
     * <li>The offset and object reference <code>o</code> (either null or
     * non-null) were both obtained via {@link #staticFieldOffset}
     * and {@link #staticFieldBase} (respectively) from the
     * reflective {@link Field} representation of some Java field.
     *
     * <li>The object referred to by <code>o</code> is an array, and the offset
     * is an integer of the form <code>B+N*S</code>, where <code>N</code> is
     * a valid index into the array, and <code>B</code> and <code>S</code> are
     * the values obtained by {@link #arrayBaseOffset} and {@link
     * #arrayIndexScale} (respectively) from the array's class.  The value
     * referred to is the <code>N</code><em>th</em> element of the array.
     *
     * </ul>
     * <p>
     * If one of the above cases is true, the call references a specific Java
     * variable (field or array element).  However, the results are undefined
     * if that variable is not in fact of the type returned by this method.
     * <p>
     * This method refers to a variable by means of two parameters, and so
     * it provides (in effect) a <em>double-register</em> addressing mode
     * for Java variables.  When the object reference is null, this method
     * uses its offset as an absolute address.  This is similar in operation
     * to methods such as {@link #getInt(long)}, which provide (in effect) a
     * <em>single-register</em> addressing mode for non-Java variables.
     * However, because Java variables may have a different layout in memory
     * from non-Java variables, programmers should not assume that these
     * two addressing modes are ever equivalent.  Also, programmers should
     * remember that offsets from the double-register addressing mode cannot
     * be portably confused with longs used in the single-register addressing
     * mode.
     *
     * @param o Java heap object in which the variable resides, if any, else
     *        null
     * @param offset indication of where the variable resides in a Java heap
     *        object, if any, else a memory address locating the variable
     *        statically
     * @return the value fetched from the indicated Java variable
     * @throws RuntimeException No defined exceptions are thrown, not even
     *         {@link NullPointerException}
     */
    public native int getInt(Object o, long offset);

    /**
     * Stores a value into a given Java variable.
     * <p>
     * The first two parameters are interpreted exactly as with
     * {@link #getInt(Object, long)} to refer to a specific
     * Java variable (field or array element).  The given value
     * is stored into that variable.
     * <p>
     * The variable must be of the same type as the method
     * parameter <code>x</code>.
     *
     * @param o Java heap object in which the variable resides, if any, else
     *        null
     * @param offset indication of where the variable resides in a Java heap
     *        object, if any, else a memory address locating the variable
     *        statically
     * @param x the value to store into the indicated Java variable
     * @throws RuntimeException No defined exceptions are thrown, not even
     *         {@link NullPointerException}
     */
    public native void putInt(Object o, long offset, int x);

    /**
     * Fetches a reference value from a given Java variable.
     * @see #getInt(Object, long)
     */
    public native Object getObject(Object o, long offset);

    /**
     * Stores a reference value into a given Java variable.
     * <p>
     * Unless the reference <code>x</code> being stored is either null
     * or matches the field type, the results are undefined.
     * If the reference <code>o</code> is non-null, car marks or
     * other store barriers for that object (if the VM requires them)
     * are updated.
     * @see #putInt(Object, int, int)
     */
    public native void putObject(Object o, long offset, Object x);

    /** @see #getInt(Object, long) */
    public native boolean getBoolean(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putBoolean(Object o, long offset, boolean x);
    /** @see #getInt(Object, long) */
    public native byte    getByte(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putByte(Object o, long offset, byte x);
    /** @see #getInt(Object, long) */
    public native short   getShort(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putShort(Object o, long offset, short x);
    /** @see #getInt(Object, long) */
    public native char    getChar(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putChar(Object o, long offset, char x);
    /** @see #getInt(Object, long) */
    public native long    getLong(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putLong(Object o, long offset, long x);
    /** @see #getInt(Object, long) */
    public native float   getFloat(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putFloat(Object o, long offset, float x);
    /** @see #getInt(Object, long) */
    public native double  getDouble(Object o, long offset);
    /** @see #putInt(Object, int, int) */
    public native void    putDouble(Object o, long offset, double x);

    /**
     * This method, like all others with 32-bit offsets, was native
     * in a previous release but is now a wrapper which simply casts
     * the offset to a long value.  It provides backward compatibility
     * with bytecodes compiled against 1.4.
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public int getInt(Object o, int offset) {
        return getInt(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putInt(Object o, int offset, int x) {
        putInt(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public Object getObject(Object o, int offset) {
        return getObject(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putObject(Object o, int offset, Object x) {
        putObject(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public boolean getBoolean(Object o, int offset) {
        return getBoolean(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putBoolean(Object o, int offset, boolean x) {
        putBoolean(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public byte getByte(Object o, int offset) {
        return getByte(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putByte(Object o, int offset, byte x) {
        putByte(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public short getShort(Object o, int offset) {
        return getShort(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putShort(Object o, int offset, short x) {
        putShort(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public char getChar(Object o, int offset) {
        return getChar(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putChar(Object o, int offset, char x) {
        putChar(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public long getLong(Object o, int offset) {
        return getLong(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putLong(Object o, int offset, long x) {
        putLong(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public float getFloat(Object o, int offset) {
        return getFloat(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putFloat(Object o, int offset, float x) {
        putFloat(o, (long)offset, x);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public double getDouble(Object o, int offset) {
        return getDouble(o, (long)offset);
    }

    /**
     * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long.
     * See {@link #staticFieldOffset}.
     */
    @Deprecated
    public void putDouble(Object o, int offset, double x) {
        putDouble(o, (long)offset, x);
    }

    // These work on values in the C heap.

    /**
     * Fetches a value from a given memory address.  If the address is zero, or
     * does not point into a block obtained from {@link #allocateMemory}, the
     * results are undefined.
     *
     * @see #allocateMemory
     */
    public native byte    getByte(long address);

    /**
     * Stores a value into a given memory address.  If the address is zero, or
     * does not point into a block obtained from {@link #allocateMemory}, the
     * results are undefined.
     *
     * @see #getByte(long)
     */
    public native void    putByte(long address, byte x);

    /** @see #getByte(long) */
    public native short   getShort(long address);
    /** @see #putByte(long, byte) */
    public native void    putShort(long address, short x);
    /** @see #getByte(long) */
    public native char    getChar(long address);
    /** @see #putByte(long, byte) */
    public native void    putChar(long address, char x);
    /** @see #getByte(long) */
    public native int     getInt(long address);
    /** @see #putByte(long, byte) */
    public native void    putInt(long address, int x);
    /** @see #getByte(long) */
    public native long    getLong(long address);
    /** @see #putByte(long, byte) */
    public native void    putLong(long address, long x);
    /** @see #getByte(long) */
    public native float   getFloat(long address);
    /** @see #putByte(long, byte) */
    public native void    putFloat(long address, float x);
    /** @see #getByte(long) */
    public native double  getDouble(long address);
    /** @see #putByte(long, byte) */
    public native void    putDouble(long address, double x);

    /**
     * Fetches a native pointer from a given memory address.  If the address is
     * zero, or does not point into a block obtained from {@link
     * #allocateMemory}, the results are undefined.
     *
     * <p> If the native pointer is less than 64 bits wide, it is extended as
     * an unsigned number to a Java long.  The pointer may be indexed by any
     * given byte offset, simply by adding that offset (as a simple integer) to
     * the long representing the pointer.  The number of bytes actually read
     * from the target address maybe determined by consulting {@link
     * #addressSize}.
     *
     * @see #allocateMemory
     */
    public native long getAddress(long address);

    /**
     * Stores a native pointer into a given memory address.  If the address is
     * zero, or does not point into a block obtained from {@link
     * #allocateMemory}, the results are undefined.
     *
     * <p> The number of bytes actually written at the target address maybe
     * determined by consulting {@link #addressSize}.
     *
     * @see #getAddress(long)
     */
    public native void putAddress(long address, long x);

    /// wrappers for malloc, realloc, free:

    /**
     * Allocates a new block of native memory, of the given size in bytes.  The
     * contents of the memory are uninitialized; they will generally be
     * garbage.  The resulting native pointer will never be zero, and will be
     * aligned for all value types.  Dispose of this memory by calling {@link
     * #freeMemory}, or resize it with {@link #reallocateMemory}.
     *
     * @throws IllegalArgumentException if the size is negative or too large
     *         for the native size_t type
     *
     * @throws OutOfMemoryError if the allocation is refused by the system
     *
     * @see #getByte(long)
     * @see #putByte(long, byte)
     */
    public native long allocateMemory(long bytes);

    /**
     * Resizes a new block of native memory, to the given size in bytes.  The
     * contents of the new block past the size of the old block are
     * uninitialized; they will generally be garbage.  The resulting native
     * pointer will be zero if and only if the requested size is zero.  The
     * resulting native pointer will be aligned for all value types.  Dispose
     * of this memory by calling {@link #freeMemory}, or resize it with {@link
     * #reallocateMemory}.  The address passed to this method may be null, in
     * which case an allocation will be performed.
     *
     * @throws IllegalArgumentException if the size is negative or too large
     *         for the native size_t type
     *
     * @throws OutOfMemoryError if the allocation is refused by the system
     *
     * @see #allocateMemory
     */
    public native long reallocateMemory(long address, long bytes);

    /**
     * Sets all bytes in a given block of memory to a fixed value
     * (usually zero).
     *
     * <p>This method determines a block's base address by means of two parameters,
     * and so it provides (in effect) a <em>double-register</em> addressing mode,
     * as discussed in {@link #getInt(Object,long)}.  When the object reference is null,
     * the offset supplies an absolute base address.
     *
     * <p>The stores are in coherent (atomic) units of a size determined
     * by the address and length parameters.  If the effective address and
     * length are all even modulo 8, the stores take place in 'long' units.
     * If the effective address and length are (resp.) even modulo 4 or 2,
     * the stores take place in units of 'int' or 'short'.
     *
     * @since 1.7
     */
    public native void setMemory(Object o, long offset, long bytes, byte value);

    /**
     * Sets all bytes in a given block of memory to a fixed value
     * (usually zero).  This provides a <em>single-register</em> addressing mode,
     * as discussed in {@link #getInt(Object,long)}.
     *
     * <p>Equivalent to <code>setMemory(null, address, bytes, value)</code>.
     */
    public void setMemory(long address, long bytes, byte value) {
        setMemory(null, address, bytes, value);
    }

    /**
     * Sets all bytes in a given block of memory to a copy of another
     * block.
     *
     * <p>This method determines each block's base address by means of two parameters,
     * and so it provides (in effect) a <em>double-register</em> addressing mode,
     * as discussed in {@link #getInt(Object,long)}.  When the object reference is null,
     * the offset supplies an absolute base address.
     *
     * <p>The transfers are in coherent (atomic) units of a size determined
     * by the address and length parameters.  If the effective addresses and
     * length are all even modulo 8, the transfer takes place in 'long' units.
     * If the effective addresses and length are (resp.) even modulo 4 or 2,
     * the transfer takes place in units of 'int' or 'short'.
     *
     * @since 1.7
     */
    public native void copyMemory(Object srcBase, long srcOffset,
                                  Object destBase, long destOffset,
                                  long bytes);
    /**
     * Sets all bytes in a given block of memory to a copy of another
     * block.  This provides a <em>single-register</em> addressing mode,
     * as discussed in {@link #getInt(Object,long)}.
     *
     * Equivalent to <code>copyMemory(null, srcAddress, null, destAddress, bytes)</code>.
     */
    public void copyMemory(long srcAddress, long destAddress, long bytes) {
        copyMemory(null, srcAddress, null, destAddress, bytes);
    }

    /**
     * Disposes of a block of native memory, as obtained from {@link
     * #allocateMemory} or {@link #reallocateMemory}.  The address passed to
     * this method may be null, in which case no action is taken.
     *
     * @see #allocateMemory
     */
    public native void freeMemory(long address);

    /// random queries

    /**
     * This constant differs from all results that will ever be returned from
     * {@link #staticFieldOffset}, {@link #objectFieldOffset},
     * or {@link #arrayBaseOffset}.
     */
    public static final int INVALID_FIELD_OFFSET   = -1;

    /**
     * Returns the offset of a field, truncated to 32 bits.
     * This method is implemented as follows:
     * <blockquote><pre>
     * public int fieldOffset(Field f) {
     *     if (Modifier.isStatic(f.getModifiers()))
     *         return (int) staticFieldOffset(f);
     *     else
     *         return (int) objectFieldOffset(f);
     * }
     * </pre></blockquote>
     * @deprecated As of 1.4.1, use {@link #staticFieldOffset} for static
     * fields and {@link #objectFieldOffset} for non-static fields.
     */
    @Deprecated
    public int fieldOffset(Field f) {
        if (Modifier.isStatic(f.getModifiers()))
            return (int) staticFieldOffset(f);
        else
            return (int) objectFieldOffset(f);
    }

    /**
     * Returns the base address for accessing some static field
     * in the given class.  This method is implemented as follows:
     * <blockquote><pre>
     * public Object staticFieldBase(Class c) {
     *     Field[] fields = c.getDeclaredFields();
     *     for (int i = 0; i < fields.length; i++) {
     *         if (Modifier.isStatic(fields[i].getModifiers())) {
     *             return staticFieldBase(fields[i]);
     *         }
     *     }
     *     return null;
     * }
     * </pre></blockquote>
     * @deprecated As of 1.4.1, use {@link #staticFieldBase(Field)}
     * to obtain the base pertaining to a specific {@link Field}.
     * This method works only for JVMs which store all statics
     * for a given class in one place.
     */
    @Deprecated
    public Object staticFieldBase(Class<?> c) {
        Field[] fields = c.getDeclaredFields();
        for (int i = 0; i < fields.length; i++) {
            if (Modifier.isStatic(fields[i].getModifiers())) {
                return staticFieldBase(fields[i]);
            }
        }
        return null;
    }

    /**
     * Report the location of a given field in the storage allocation of its
     * class.  Do not expect to perform any sort of arithmetic on this offset;
     * it is just a cookie which is passed to the unsafe heap memory accessors.
     *
     * <p>Any given field will always have the same offset and base, and no
     * two distinct fields of the same class will ever have the same offset
     * and base.
     *
     * <p>As of 1.4.1, offsets for fields are represented as long values,
     * although the Sun JVM does not use the most significant 32 bits.
     * However, JVM implementations which store static fields at absolute
     * addresses can use long offsets and null base pointers to express
     * the field locations in a form usable by {@link #getInt(Object,long)}.
     * Therefore, code which will be ported to such JVMs on 64-bit platforms
     * must preserve all bits of static field offsets.
     * @see #getInt(Object, long)
     */
    public native long staticFieldOffset(Field f);

    /**
     * Report the location of a given static field, in conjunction with {@link
     * #staticFieldBase}.
     * <p>Do not expect to perform any sort of arithmetic on this offset;
     * it is just a cookie which is passed to the unsafe heap memory accessors.
     *
     * <p>Any given field will always have the same offset, and no two distinct
     * fields of the same class will ever have the same offset.
     *
     * <p>As of 1.4.1, offsets for fields are represented as long values,
     * although the Sun JVM does not use the most significant 32 bits.
     * It is hard to imagine a JVM technology which needs more than
     * a few bits to encode an offset within a non-array object,
     * However, for consistency with other methods in this class,
     * this method reports its result as a long value.
     * @see #getInt(Object, long)
     */
    public native long objectFieldOffset(Field f);

    /**
     * Report the location of a given static field, in conjunction with {@link
     * #staticFieldOffset}.
     * <p>Fetch the base "Object", if any, with which static fields of the
     * given class can be accessed via methods like {@link #getInt(Object,
     * long)}.  This value may be null.  This value may refer to an object
     * which is a "cookie", not guaranteed to be a real Object, and it should
     * not be used in any way except as argument to the get and put routines in
     * this class.
     */
    public native Object staticFieldBase(Field f);

    /**
     * Detect if the given class may need to be initialized. This is often
     * needed in conjunction with obtaining the static field base of a
     * class.
     * @return false only if a call to {@code ensureClassInitialized} would have no effect
     */
    public native boolean shouldBeInitialized(Class<?> c);

    /**
     * Ensure the given class has been initialized. This is often
     * needed in conjunction with obtaining the static field base of a
     * class.
     */
    public native void ensureClassInitialized(Class<?> c);

    /**
     * Report the offset of the first element in the storage allocation of a
     * given array class.  If {@link #arrayIndexScale} returns a non-zero value
     * for the same class, you may use that scale factor, together with this
     * base offset, to form new offsets to access elements of arrays of the
     * given class.
     *
     * @see #getInt(Object, long)
     * @see #putInt(Object, long, int)
     */
    public native int arrayBaseOffset(Class<?> arrayClass);

    /** The value of {@code arrayBaseOffset(boolean[].class)} */
    public static final int ARRAY_BOOLEAN_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(boolean[].class);

    /** The value of {@code arrayBaseOffset(byte[].class)} */
    public static final int ARRAY_BYTE_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(byte[].class);

    /** The value of {@code arrayBaseOffset(short[].class)} */
    public static final int ARRAY_SHORT_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(short[].class);

    /** The value of {@code arrayBaseOffset(char[].class)} */
    public static final int ARRAY_CHAR_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(char[].class);

    /** The value of {@code arrayBaseOffset(int[].class)} */
    public static final int ARRAY_INT_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(int[].class);

    /** The value of {@code arrayBaseOffset(long[].class)} */
    public static final int ARRAY_LONG_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(long[].class);

    /** The value of {@code arrayBaseOffset(float[].class)} */
    public static final int ARRAY_FLOAT_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(float[].class);

    /** The value of {@code arrayBaseOffset(double[].class)} */
    public static final int ARRAY_DOUBLE_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(double[].class);

    /** The value of {@code arrayBaseOffset(Object[].class)} */
    public static final int ARRAY_OBJECT_BASE_OFFSET
            = theUnsafe.arrayBaseOffset(Object[].class);

    /**
     * Report the scale factor for addressing elements in the storage
     * allocation of a given array class.  However, arrays of "narrow" types
     * will generally not work properly with accessors like {@link
     * #getByte(Object, int)}, so the scale factor for such classes is reported
     * as zero.
     *
     * @see #arrayBaseOffset
     * @see #getInt(Object, long)
     * @see #putInt(Object, long, int)
     */
    public native int arrayIndexScale(Class<?> arrayClass);

    /** The value of {@code arrayIndexScale(boolean[].class)} */
    public static final int ARRAY_BOOLEAN_INDEX_SCALE
            = theUnsafe.arrayIndexScale(boolean[].class);

    /** The value of {@code arrayIndexScale(byte[].class)} */
    public static final int ARRAY_BYTE_INDEX_SCALE
            = theUnsafe.arrayIndexScale(byte[].class);

    /** The value of {@code arrayIndexScale(short[].class)} */
    public static final int ARRAY_SHORT_INDEX_SCALE
            = theUnsafe.arrayIndexScale(short[].class);

    /** The value of {@code arrayIndexScale(char[].class)} */
    public static final int ARRAY_CHAR_INDEX_SCALE
            = theUnsafe.arrayIndexScale(char[].class);

    /** The value of {@code arrayIndexScale(int[].class)} */
    public static final int ARRAY_INT_INDEX_SCALE
            = theUnsafe.arrayIndexScale(int[].class);

    /** The value of {@code arrayIndexScale(long[].class)} */
    public static final int ARRAY_LONG_INDEX_SCALE
            = theUnsafe.arrayIndexScale(long[].class);

    /** The value of {@code arrayIndexScale(float[].class)} */
    public static final int ARRAY_FLOAT_INDEX_SCALE
            = theUnsafe.arrayIndexScale(float[].class);

    /** The value of {@code arrayIndexScale(double[].class)} */
    public static final int ARRAY_DOUBLE_INDEX_SCALE
            = theUnsafe.arrayIndexScale(double[].class);

    /** The value of {@code arrayIndexScale(Object[].class)} */
    public static final int ARRAY_OBJECT_INDEX_SCALE
            = theUnsafe.arrayIndexScale(Object[].class);

    /**
     * Report the size in bytes of a native pointer, as stored via {@link
     * #putAddress}.  This value will be either 4 or 8.  Note that the sizes of
     * other primitive types (as stored in native memory blocks) is determined
     * fully by their information content.
     */
    public native int addressSize();

    /** The value of {@code addressSize()} */
    public static final int ADDRESS_SIZE = theUnsafe.addressSize();

    /**
     * Report the size in bytes of a native memory page (whatever that is).
     * This value will always be a power of two.
     */
    public native int pageSize();


    /// random trusted operations from JNI:

    /**
     * Tell the VM to define a class, without security checks.  By default, the
     * class loader and protection domain come from the caller's class.
     */
    public native Class<?> defineClass(String name, byte[] b, int off, int len,
                                       ClassLoader loader,
                                       ProtectionDomain protectionDomain);

    /**
     * Define a class but do not make it known to the class loader or system dictionary.
     * <p>
     * For each CP entry, the corresponding CP patch must either be null or have
     * the a format that matches its tag:
     * <ul>
     * <li>Integer, Long, Float, Double: the corresponding wrapper object type from java.lang
     * <li>Utf8: a string (must have suitable syntax if used as signature or name)
     * <li>Class: any java.lang.Class object
     * <li>String: any object (not just a java.lang.String)
     * <li>InterfaceMethodRef: (NYI) a method handle to invoke on that call site's arguments
     * </ul>
     * @params hostClass context for linkage, access control, protection domain, and class loader
     * @params data      bytes of a class file
     * @params cpPatches where non-null entries exist, they replace corresponding CP entries in data
     */
    public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);


    /** Allocate an instance but do not run any constructor.
        Initializes the class if it has not yet been. */
    public native Object allocateInstance(Class<?> cls)
        throws InstantiationException;

    /** Lock the object.  It must get unlocked via {@link #monitorExit}. */
    @Deprecated
    public native void monitorEnter(Object o);

    /**
     * Unlock the object.  It must have been locked via {@link
     * #monitorEnter}.
     */
    @Deprecated
    public native void monitorExit(Object o);

    /**
     * Tries to lock the object.  Returns true or false to indicate
     * whether the lock succeeded.  If it did, the object must be
     * unlocked via {@link #monitorExit}.
     */
    @Deprecated
    public native boolean tryMonitorEnter(Object o);

    /** Throw the exception without telling the verifier. */
    public native void throwException(Throwable ee);


    /**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapObject(Object o, long offset,
                                                     Object expected,
                                                     Object x);

    /**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapInt(Object o, long offset,
                                                  int expected,
                                                  int x);

    /**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapLong(Object o, long offset,
                                                   long expected,
                                                   long x);

    /**
     * Fetches a reference value from a given Java variable, with volatile
     * load semantics. Otherwise identical to {@link #getObject(Object, long)}
     */
    public native Object getObjectVolatile(Object o, long offset);

    /**
     * Stores a reference value into a given Java variable, with
     * volatile store semantics. Otherwise identical to {@link #putObject(Object, long, Object)}
     */
    public native void    putObjectVolatile(Object o, long offset, Object x);

    /** Volatile version of {@link #getInt(Object, long)}  */
    public native int     getIntVolatile(Object o, long offset);

    /** Volatile version of {@link #putInt(Object, long, int)}  */
    public native void    putIntVolatile(Object o, long offset, int x);

    /** Volatile version of {@link #getBoolean(Object, long)}  */
    public native boolean getBooleanVolatile(Object o, long offset);

    /** Volatile version of {@link #putBoolean(Object, long, boolean)}  */
    public native void    putBooleanVolatile(Object o, long offset, boolean x);

    /** Volatile version of {@link #getByte(Object, long)}  */
    public native byte    getByteVolatile(Object o, long offset);

    /** Volatile version of {@link #putByte(Object, long, byte)}  */
    public native void    putByteVolatile(Object o, long offset, byte x);

    /** Volatile version of {@link #getShort(Object, long)}  */
    public native short   getShortVolatile(Object o, long offset);

    /** Volatile version of {@link #putShort(Object, long, short)}  */
    public native void    putShortVolatile(Object o, long offset, short x);

    /** Volatile version of {@link #getChar(Object, long)}  */
    public native char    getCharVolatile(Object o, long offset);

    /** Volatile version of {@link #putChar(Object, long, char)}  */
    public native void    putCharVolatile(Object o, long offset, char x);

    /** Volatile version of {@link #getLong(Object, long)}  */
    public native long    getLongVolatile(Object o, long offset);

    /** Volatile version of {@link #putLong(Object, long, long)}  */
    public native void    putLongVolatile(Object o, long offset, long x);

    /** Volatile version of {@link #getFloat(Object, long)}  */
    public native float   getFloatVolatile(Object o, long offset);

    /** Volatile version of {@link #putFloat(Object, long, float)}  */
    public native void    putFloatVolatile(Object o, long offset, float x);

    /** Volatile version of {@link #getDouble(Object, long)}  */
    public native double  getDoubleVolatile(Object o, long offset);

    /** Volatile version of {@link #putDouble(Object, long, double)}  */
    public native void    putDoubleVolatile(Object o, long offset, double x);

    /**
     * Version of {@link #putObjectVolatile(Object, long, Object)}
     * that does not guarantee immediate visibility of the store to
     * other threads. This method is generally only useful if the
     * underlying field is a Java volatile (or if an array cell, one
     * that is otherwise only accessed using volatile accesses).
     */
    public native void    putOrderedObject(Object o, long offset, Object x);

    /** Ordered/Lazy version of {@link #putIntVolatile(Object, long, int)}  */
    public native void    putOrderedInt(Object o, long offset, int x);

    /** Ordered/Lazy version of {@link #putLongVolatile(Object, long, long)} */
    public native void    putOrderedLong(Object o, long offset, long x);

    /**
     * Unblock the given thread blocked on <tt>park</tt>, or, if it is
     * not blocked, cause the subsequent call to <tt>park</tt> not to
     * block.  Note: this operation is "unsafe" solely because the
     * caller must somehow ensure that the thread has not been
     * destroyed. Nothing special is usually required to ensure this
     * when called from Java (in which there will ordinarily be a live
     * reference to the thread) but this is not nearly-automatically
     * so when calling from native code.
     * @param thread the thread to unpark.
     *
     */
    public native void unpark(Object thread);

    /**
     * Block current thread, returning when a balancing
     * <tt>unpark</tt> occurs, or a balancing <tt>unpark</tt> has
     * already occurred, or the thread is interrupted, or, if not
     * absolute and time is not zero, the given time nanoseconds have
     * elapsed, or if absolute, the given deadline in milliseconds
     * since Epoch has passed, or spuriously (i.e., returning for no
     * "reason"). Note: This operation is in the Unsafe class only
     * because <tt>unpark</tt> is, so it would be strange to place it
     * elsewhere.
     */
    public native void park(boolean isAbsolute, long time);

    /**
     * Gets the load average in the system run queue assigned
     * to the available processors averaged over various periods of time.
     * This method retrieves the given <tt>nelem</tt> samples and
     * assigns to the elements of the given <tt>loadavg</tt> array.
     * The system imposes a maximum of 3 samples, representing
     * averages over the last 1,  5,  and  15 minutes, respectively.
     *
     * @params loadavg an array of double of size nelems
     * @params nelems the number of samples to be retrieved and
     *         must be 1 to 3.
     *
     * @return the number of samples actually retrieved; or -1
     *         if the load average is unobtainable.
     */
    public native int getLoadAverage(double[] loadavg, int nelems);

    // The following contain CAS-based Java implementations used on
    // platforms not supporting native instructions

    /**
     * Atomically adds the given value to the current value of a field
     * or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param delta the value to add
     * @return the previous value
     * @since 1.8
     */
    public final int getAndAddInt(Object o, long offset, int delta) {
        int v;
        do {
            v = getIntVolatile(o, offset);
        } while (!compareAndSwapInt(o, offset, v, v + delta));
        return v;
    }

    /**
     * Atomically adds the given value to the current value of a field
     * or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param delta the value to add
     * @return the previous value
     * @since 1.8
     */
    public final long getAndAddLong(Object o, long offset, long delta) {
        long v;
        do {
            v = getLongVolatile(o, offset);
        } while (!compareAndSwapLong(o, offset, v, v + delta));
        return v;
    }

    /**
     * Atomically exchanges the given value with the current value of
     * a field or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param newValue new value
     * @return the previous value
     * @since 1.8
     */
    public final int getAndSetInt(Object o, long offset, int newValue) {
        int v;
        do {
            v = getIntVolatile(o, offset);
        } while (!compareAndSwapInt(o, offset, v, newValue));
        return v;
    }

    /**
     * Atomically exchanges the given value with the current value of
     * a field or array element within the given object <code>o</code>
     * at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param newValue new value
     * @return the previous value
     * @since 1.8
     */
    public final long getAndSetLong(Object o, long offset, long newValue) {
        long v;
        do {
            v = getLongVolatile(o, offset);
        } while (!compareAndSwapLong(o, offset, v, newValue));
        return v;
    }

    /**
     * Atomically exchanges the given reference value with the current
     * reference value of a field or array element within the given
     * object <code>o</code> at the given <code>offset</code>.
     *
     * @param o object/array to update the field/element in
     * @param offset field/element offset
     * @param newValue new value
     * @return the previous value
     * @since 1.8
     */
    public final Object getAndSetObject(Object o, long offset, Object newValue) {
        Object v;
        do {
            v = getObjectVolatile(o, offset);
        } while (!compareAndSwapObject(o, offset, v, newValue));
        return v;
    }


    /**
     * Ensures lack of reordering of loads before the fence
     * with loads or stores after the fence.
     * @since 1.8
     */
    public native void loadFence();

    /**
     * Ensures lack of reordering of stores before the fence
     * with loads or stores after the fence.
     * @since 1.8
     */
    public native void storeFence();

    /**
     * Ensures lack of reordering of loads or stores before the fence
     * with loads or stores after the fence.
     * @since 1.8
     */
    public native void fullFence();

    /**
     * Throws IllegalAccessError; for use by the VM.
     * @since 1.8
     */
    private static void throwIllegalAccessError() {
       throw new IllegalAccessError();
    }

}

 

 

參考：《Java魔法類：Unsafe應用解析》