原创

聊一聊Java中的Unsafe类


一、Unsafe类简介

如果看过JUC的源码,相信大家一定不会对Unsafe这个类陌生,整个JUC的底层核心就是这个Unsafe类Unsafe类是位于sun.misc包下的一个类,它是jdk提供的一个直接访问操作系统资源的工具类(底层c++实现)。正如它的名字一样,这个类是不安全的,但是功能特别强大。

那么它有什么样的强大功能呢?Unsafe类使Java拥有了像C语言的指针一样操作内存空间的能力。众所周知,相对于C和C++,Java语言使开发人员在开发时无需关注内存的管理,这些JVM全部自动处理了。但是如果Java开发人员想要直接操作内存,就可以使用Unsafe类。尤其在高并发的情况下,直接操作内存能够更好地提高效率

那为啥说它是不安全的呢?甚至直接起名为不安全?学过C和C++的小伙伴肯定知道,如果操作内存不当,会导致很多重大问题。所以一旦能够直接操作内存,这也就意味着不受JVM管理,也就意味着无法被GC,需要我们手动GC,稍有不慎就会出现内存泄漏;此外Unsafe的不少方法中必须提供原始地址(内存地址)和被替换对象的地址,偏移量要自己计算,一旦出现问题就是JVM崩溃级别的异常,会导致整个JVM实例崩溃,表现为应用程序直接crash掉。从而使Java这种安全性很高的语言变得不再安全。因此Java规定Unsafe类仅供Java核心类库使用,普通用户无法使用(当然硬是要使用也是可以取到的)。

二、如何获取Unsafe实例

查看Unsafe的源码(注:在OracleJdk8无法获取到sun.misc包的源码,想看此包的源码可以直接下载openjdk

//类被final修饰,无法被继承
public final class Unsafe {
    //构造方法被私有化
    private Unsafe() {}

    //表明Unsafe是单例的
    private static final Unsafe theUnsafe = new Unsafe();

   	//获取Unsafe实例
    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass();
        //非启动类加载器直接抛出异常
        if (!VM.isSystemDomainLoader(caller.getClassLoader()))
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }
}

从上面的代码我们可以得到如下结论:

  • Unsafe类是final类,无法被继承,因此无法通过子类继承Unsafe的功能
  • 构造方法被私有化,即无法直接通过new创建对象
  • Unsafe实例是一个单实例
  • 虽然提供了获取Unsafe实例的方法,但是我们自己开发的类中无法获取到,调用getUnsafe()方法会直接抛出SecurityException异常。这是因为在Unsafe类的getUnsafe()方法中,它做了一层校验,判断当前类的类加载器(ClassLoader)是不是启动类加载器(Bootstrap ClassLoader),如果不是,则会抛出SecurityException异常。在JVM的类加载机制中,自定义的类使用的类加载器是应用程序类加载器(Application ClassLoader),所以这个时候校验失败,从而抛出异常

所以我们自己写的类是无法直接通过常规方式获取到Unsafe的实例,但是我们可以提供其他特殊的方式获取。这里有两种方案

  • 第一种方案满足它的类加载器检测条件

    我们可以将自定义的类所在的jar包所在的路径通过-Xbootclasspath参数添加到Java命令中,这样当程序启动时,Bootstrap ClassLoader会加载Demo类,这样校验就通过了。但是这种方式比较麻烦,而且不太实用,因为在项目中,可能需要在很多地方都使用Unsafe类,如果通过Java命令行这种方式去指定,就会很麻烦,而且容易出现纰漏。

  • 第二种方案反射

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        Field field = Unsafe.class.getDeclaredField("theUnsafe");
        // 将字段的访问权限设置为true
        field.setAccessible(true);
        // 因为theUnsafe字段在Unsafe类中是一个静态字段,所以通过Field.get()获取字段值时,可以传null获取
        Unsafe unsafe = (Unsafe) field.get(null);
        //打印对象
        System.out.println(unsafe);
    }
    

三、Unsafe类功能介绍

Unsafe类中提供的API大致可分为8大类,下面针对每一类的相关方法和应用场景进行详细介绍。

1. CAS操作

Java锁中,我们经常会看到CAS操作,无一例外它们最终都是调用了Unsafe类中的CAS操作,方法如下

/**
 * @param o 内存中要操作的对象
 * @param offset 要操作的值的内存地址偏移量
 * @param expected 预期值
 * @param x 想要更新成的值
 * @return boolean true:更新成功 false:更新失败
 **/

//比较并交换Object类型
public final native boolean compareAndSwapObject(Object o, long offset,
                                                 Object expected,
                                                 Object x);
//比较并交换int类型
public final native boolean compareAndSwapInt(Object o, long offset,
                                              int expected,
                                              int x);
//比较并交换long类型
public final native boolean compareAndSwapLong(Object o, long offset,
                                               long expected,
                                               long x);

//int类型值加上指定数,返回原值
public final int getAndAddInt(Object o, long offset, int delta) {
    int v;
    do {
        //获取内存中的值
        v = getIntVolatile(o, offset);
    } while (!compareAndSwapInt(o, offset, v, v + delta));//采用自旋的方式更新数据
    return v;
}

//long类型值加上指定数,返回原值
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;
}

//int类型值设置为指定值,返回原值
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;
}

//long类型值设置为指定值,返回原值
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;
}

//Object类型值设置为指定值,返回原值
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;
}

上面compareAndSwapObjectcompareAndSwapIntcompareAndSwapLong是基础方法,使用native修饰,底层使用了C++来实现,其余方法如getAndAddInt都是调用这三个基础方法,通过不断的自旋和反复取内存中的值来实现数据更新的正确性。Unsafe在队列同步器AQS(AbstractQueuedSynchronizer)、原子类中都有应用,例如在原子类AtomicInteger中

public class AtomicInteger extends Number implements java.io.Serializable {
    //获取Unsafe类
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //value的内存地址偏移量
    private static final long valueOffset;

    static {
        try {
            //获取value的内存地址偏移量
            valueOffset = unsafe.objectFieldOffset
                (AtomicInteger.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    //使用volatile修饰,保证了可见性
    private volatile int value;
    
    //比较并更新
    public final boolean compareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }
}

2. 内存操作

主要包含堆外内存的分配、拷贝、释放、给定地址值操作等方法

//分配内存, 相当于C++的malloc函数
public native long allocateMemory(long bytes);
//扩充内存
public native long reallocateMemory(long address, long bytes);
//释放内存
public native void freeMemory(long address);
//在给定的内存块中设置值
public native void setMemory(Object o, long offset, long bytes, byte value);
//内存拷贝
public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes);
//获取给定地址值,忽略修饰限定符的访问限制。与此类似操作还有: getInt,getDouble,getLong,getChar等
public native Object getObject(Object o, long offset);
//为给定地址设置值,忽略修饰限定符的访问限制,与此类似操作还有: putInt,putDouble,putLong,putChar等
public native void putObject(Object o, long offset, Object x);
//获取给定地址的byte类型的值(当且仅当该内存地址为allocateMemory分配时,此方法结果为确定的)
public native byte getByte(long address);
//为给定地址设置byte类型的值(当且仅当该内存地址为allocateMemory分配时,此方法结果才是确定的)
public native void putByte(long address, byte x);

值得说一下的是,Unsafe直接申请的内存是堆外内存。这个堆外是相对于JVM的内存来说的,通常我们应用程序运行后,创建的对象均在JVM内存中的堆中,堆内存的管理是JVM来管理的,而堆外内存指的是计算机中的直接内存不受JVM管理。因此使用Unsafe类来申请对外内存时,要特别注意,否则容易出现内存泄漏等问题。

那么为什么要使用堆外内存呢?原因如下

  • 对垃圾回收停顿的改善。由于堆外内存是直接受操作系统管理而不是JVM,所以当我们使用堆外内存时,即可保持较小的堆内内存规模。从而在GC时减少回收停顿对于应用的影响。
  • 提升程序I/O操作的性能。通常在I/O通信过程中,会存在堆内内存到堆外内存的数据拷贝操作,对于需要频繁进行内存间数据拷贝且生命周期较短的暂存数据,都建议存储到堆外内存

DirectByteBuffer是Java用于实现堆外内存的一个重要类,通常用在通信过程中做缓冲池,如在NettyMINANIO框架中应用广泛。DirectByteBuffer对于堆外内存的创建、使用、销毁等逻辑均由Unsafe提供的堆外内存API来实现。下面是DirectByteBuffer类的部分源码

DirectByteBuffer(int cap) {                   

    super(-1, 0, cap, cap);
    boolean pa = VM.isDirectMemoryPageAligned();
    int ps = Bits.pageSize();
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
        // 调用unsafe申请内存
        base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {
        Bits.unreserveMemory(size, cap);
        throw x;
    }
    // 初始化内存
    unsafe.setMemory(base, size, (byte) 0);
    if (pa && (base % ps != 0)) {
        // Round up to page boundary
        address = base + ps - (base & (ps - 1));
    } else {
        address = base;
    }
    //使用Cleaner来释放内存
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;
}

Netty作为一个高性能框架,它有一个特点就是**“零拷贝**”,操作的是堆外内存。在操作堆外内存时,它最终使用的DirectByteBuffer来对堆外内存进行操作的,源码如下

public class UnpooledUnsafeDirectByteBuf extends AbstractReferenceCountedByteBuf {
    protected ByteBuffer allocateDirect(int initialCapacity) {
        // 调用ByteBuffer的allocateDirect来申请堆外内存,返回DirectByteBuffer类型
        return ByteBuffer.allocateDirect(initialCapacity);
    }
}

//ByteBuffer#allocateDirect
public static ByteBuffer allocateDirect(int capacity) {
    //直接new一个DirectByteBuffer对象返回
    return new DirectByteBuffer(capacity);
}

3. 线程调度相关操作

主要是线程挂起和恢复方法。

//取消阻塞线程,即唤醒指定线程
public native void unpark(Object thread);

//阻塞当前线程,isAbsolute表示是否是绝对时间(true表示会实现ms定时;false则会实现ns定时),time表示可以设置超时自动唤醒时间(0表示一直阻塞,等待唤醒)
public native void park(boolean isAbsolute, long time);

Java锁和AQS通过调用LockSupport.park()LockSupport.unpark()实现线程的阻塞和唤醒的,而LockSupport的park、unpark方法实际是调用Unsafe的park、unpark方式来实现的

public class LockSupport {
    // 阻塞线程
    public static void park() {
        //调用Unsafe的park方法
        UNSAFE.park(false, 0L);
    }
    
	// 唤醒线程
    public static void unpark(Thread thread) {
        if (thread != null)
            //调用Unsafe的unpark方法
            UNSAFE.unpark(thread);
    }
}

4. 数组相关操作

获取数组的基地址和每个元素占用内存大小,两者配合起来使用,即可定位数组中每个元素在内存中的位置

//返回数组中第一个元素的偏移地址
public native int arrayBaseOffset(Class<?> arrayClass);

//返回数组中每个元素占用的内存大小,单位是字节
public native int arrayIndexScale(Class<?> arrayClass);

这两个与数据操作相关的方法在AtomicIntegerArray类(可以实现对Integer数组中每个元素的原子性操作)中有典型的应用,如下AtomicIntegerArray的源码所示

public class AtomicIntegerArray implements java.io.Serializable {
    //获取Unsafe类
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //获取数组的基地址
    private static final int base = unsafe.arrayBaseOffset(int[].class);
    private static final int shift;
    private final int[] array;

    static {
        //获取数组中每个元素的占用的内存大小
        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);
    }

    //根据数组中第一个元素在内存中的偏移量和每个元素占用的大小,计算出数组中第i个元素在内存中的偏移量
    private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }
}

5. 对象相关操作

主要包含对象成员属性相关操作非常规的对象实例化方式等相关方法

//返回非静态成员属性在内存地址相对于此对象的内存地址的偏移量
public native long objectFieldOffset(Field f);
//获得给定对象的指定地址偏移量的值,与此类似操作还有:getInt,getDouble,getLong,getChar等
public native Object getObject(Object o, long offset);
//给定对象的指定地址偏移量设值,与此类似操作还有:putInt,putDouble,putLong,putChar等
public native void putObject(Object o, long offset, Object x);
//从对象的指定偏移量处获取变量的引用,使用volatile的加载语义
public native Object getObjectVolatile(Object o, long offset);
//存储变量的引用到对象的指定的偏移量处,使用volatile的存储语义
public native void putObjectVolatile(Object o, long offset, Object x);
//有序、延迟版本的putObjectVolatile方法,不保证值的改变被其他线程立即看到。只有在field被volatile修饰符修饰时有效
public native void putOrderedObject(Object o, long offset, Object x);
//绕过构造方法、初始化代码来创建对象
public native Object allocateInstance(Class<?> cls) throws InstantiationException;

对象操作的应用场景有如下两种

  • 1)常规的实例化方式

    我们通常所用到的创建对象的方式,从本质上来讲,都是通过new机制来实现对象的创建。但是,new机制有个特点就是当类只提供有参的构造函数且无显示声明无参构造函数时,则必须使用有参构造函数进行对象构造,而使用有参构造函数时,必须传递相应个数的参数才能完成对象实例化。而Unsafe中提供allocateInstance()方法,仅通过Class对象就可以创建此类的实例对象,而且不需要调用其构造函数、初始化代码、JVM安全检查等。它抑制修饰符检测,也就是即使构造器是private修饰的也能通过此方法实例化,只需提类对象即可创建相应的对象。由于这种特性,allocateInstance在java.lang.invoke、Objenesis(提供绕过类构造器的对象生成方式)、Gson(反序列化时用到)中都有相应的应用。

  • 2)获取对象及成员内存地址

    在Unsafe类中,大部分API方法都需要传入一个offset参数,这个参数表示的是偏移量,要想直接操作内存中某个地址的数据,就必须先找到这个数据在哪儿,而通过offset就能知道这个数据在哪儿。因此这个方法应用得十分广泛。比如在AtomicInteger类中,就调用objectFieldOffset方法获取属性所在的内存地址偏移量。

    public class AtomicInteger extends Number implements java.io.Serializable {
        //获取Unsafe类
        private static final Unsafe unsafe = Unsafe.getUnsafe();
        //value的内存地址偏移量
        private static final long valueOffset;
    
        static {
            try {
                //获取value的内存地址偏移量
                valueOffset = unsafe.objectFieldOffset
                    (AtomicInteger.class.getDeclaredField("value"));
            } catch (Exception ex) { throw new Error(ex); }
        }
    }
    

6. Class相关操作

主要提供Class和它的静态字段的操作相关方法,包含静态字段内存定位、定义类、定义匿名类、检验&确保初始化等。

//获取给定静态字段的内存地址偏移量,这个值对于给定的字段是唯一且固定不变的
public native long staticFieldOffset(Field f);
//获取一个静态类中给定字段的对象指针
public native Object staticFieldBase(Field f);
//判断是否需要初始化一个类,通常在获取一个类的静态属性的时候(因为一个类如果没初始化,它的静态属性也不会初始化)使用。 当且仅当ensureClassInitialized方法不生效时返回false。
public native boolean shouldBeInitialized(Class<?> c);
//检测给定的类是否已经初始化。通常在获取一个类的静态属性的时候(因为一个类如果没初始化,它的静态属性也不会初始化)使用。
public native void ensureClassInitialized(Class<?> c);
//定义一个类,此方法会跳过JVM的所有安全检查,默认情况下,ClassLoader(类加载器)和ProtectionDomain(保护域)实例来源于调用者
public native Class<?> defineClass(String name, byte[] b, int off, int len, ClassLoader loader, ProtectionDomain protectionDomain);
//定义一个匿名类
public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

我们可以获取静态字段在内存中的偏移量staticFieldOffset()获取静态字段的对象指针staticFieldBase()。另外一个重要的应用是实现jdk1.8的lambda表达式,lambda表达式的实现是由字节码指令invokedynimicVM Anonymous Class模板机制来实现的,VM Anonymous Class模板机制最终会使用到Unsafe类的defineAnonymousClass()方法来创建匿名类(细节部分大家可以自行搜索,这里不再赘述)。

7. 内存屏障相关操作

在Java 8中引入,用于定义内存屏障(也称内存栅栏,内存栅障,屏障指令等,是一类同步屏障指令,是CPU或编译器在对内存随机访问的操作中的一个同步点,使得此点之前的所有读写操作都执行后才可以开始执行此点之后的操作),避免代码重排序

//内存屏障,禁止load操作重排序。屏障前的load操作不能被重排序到屏障后,屏障后的load操作不能被重排序到屏障前
public native void loadFence();

//内存屏障,禁止store操作重排序。屏障前的store操作不能被重排序到屏障后,屏障后的store操作不能被重排序到屏障前
public native void storeFence();

//内存屏障,禁止load、store操作重排序
public native void fullFence();

jdk1.8引入的StampedLock类就是基于此实现的乐观读写锁

8. 系统相关操作

获取系统相关信息

// 获取指针的大小,单位是字节。
// 对于64位系统,返回8,表示指针大小是8字节
// 对于32位系统,返回4,表示指针大小是4字节
public native int addressSize();

// 返回内存页的大小,单位是字节。返回值一定是2的多少次幂
public native int pageSize();

java.nio下的工具类Bits中计算待申请内存所需内存页数量的静态方法,就是调用Unsafe中pageSize方法获取系统内存页大小实现后续计算逻辑。

class Bits {
    //获取Unsafe实例
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    
    private static int pageSize = -1;

    static Unsafe unsafe() {
        return unsafe;
    }
    
    static int pageSize() {
        if (pageSize == -1)
            //调用unsafe的pageSize方法获取内存页的大小
            pageSize = unsafe().pageSize();
        return pageSize;
    }

    //计算所需内存页的个数
    static int pageCount(long size) {
        return (int)(size + (long)pageSize() - 1L) / pageSize();
    }
}

附:Unsafe类源码

/*
 * 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();
    }

}
Java
Java并发
  • 作者:贤子磊
  • 发表时间:2020-09-25 01:02
  • 版权声明:自由转载-非商用-非衍生-保持署名
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