JAVA中神奇的双刃剑--Unsafe

前提

参考资料:

Unsafe介绍

在Oracle的Jdk8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,包的路径是:

  • openjdk-8u40-src-b25-10_feb_2015\openjdk\jdk\src\share\classes\sun\misc。

当然,不同的openjdk版本的根目录(这里是openjdk-8u40-src-b25-10_feb_2015)不一定相同。sun.misc包含了低级(native硬件级别的原子操作)、不安全的操作集合。

Java无法直接访问到操作系统底层(如系统硬件等),为此Java使用native方法来扩展Java程序的功能。Unsafe类提供了硬件级别的原子操作,提供了一些绕开JVM的更底层功能,由此提高效率。本文的Unsafe类来源于openjdk-8u40-src-b25-10_feb_2015。

Unsafe的使用建议

建议先看这个知乎帖子第一楼R大的回答:为什么JUC中大量使用了sun.misc.Unsafe 这个类,但官方却不建议开发者使用

使用Unsafe要注意以下几个问题:

  • 1、Unsafe有可能在未来的Jdk版本移除或者不允许Java应用代码使用,这一点可能导致使用了Unsafe的应用无法运行在高版本的Jdk。
  • 2、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。
  • 3、Unsafe提供的直接内存访问的方法中使用的内存不受JVM管理(无法被GC),需要手动管理,一旦出现疏忽很有可能成为内存泄漏的源头。

暂时总结出以上三点问题。Unsafe在JUC(java.util.concurrent)包中大量使用(主要是CAS),在netty中方便使用直接内存,还有一些高并发的交易系统为了提高CAS的效率也有可能直接使用到Unsafe。总而言之,Unsafe类是一把双刃剑。

Unsafe详解

Unsafe中一共有82个public native修饰的方法,还有几十个基于这82个public native方法的其他方法。

初始化代码

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

    private Unsafe() {}

    private static final Unsafe theUnsafe = new Unsafe();

    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class<?> caller = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(caller.getClassLoader()))
            throw new SecurityException("Unsafe");
        return theUnsafe;
    }

初始化的代码主要包括调用JVM本地方法registerNatives()sun.reflect.Reflection#registerMethodsToFilter。然后新建一个Unsafe实例命名为theUnsafe,通过静态方法getUnsafe()获取,获取的时候需要做权限判断。由此可见,Unsafe使用了单例设计(可见构造私有化了)。Unsafe类做了限制,如果是普通的调用的话,它会抛出一个SecurityException异常;只有由主类加载器(BootStrap classLoader)加载的类才能调用这个类中的方法。最简单的使用方式是基于反射获取Unsafe实例。

Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);

类、对象和变量相关方法

主要包括类的非常规实例化、基于偏移地址获取或者设置变量的值、基于偏移地址获取或者设置数组元素的值等。

getObject

  • public native Object getObject(Object o, long offset);

通过给定的Java变量获取引用值。这里实际上是获取一个Java对象o中,获取偏移地址为offset的属性的值,此方法可以突破修饰符的抑制,也就是无视private、protected和default修饰符。类似的方法有getInt、getDouble等等。

putObject

  • public native void putObject(Object o, long offset, Object x);

将引用值存储到给定的Java变量中。这里实际上是设置一个Java对象o中偏移地址为offset的属性的值为x,此方法可以突破修饰符的抑制,也就是无视private、protected和default修饰符。类似的方法有putInt、putDouble等等。

getObjectVolatile

  • public native Object getObjectVolatile(Object o, long offset);

此方法和上面的getObject功能类似,不过附加了'volatile'加载语义,也就是强制从主存中获取属性值。类似的方法有getIntVolatile、getDoubleVolatile等等。这个方法要求被使用的属性被volatile修饰,否则功能和getObject方法相同。

putObjectVolatile

  • public native void putObjectVolatile(Object o, long offset, Object x);

此方法和上面的putObject功能类似,不过附加了'volatile'加载语义,也就是设置值的时候强制(JMM会保证获得锁到释放锁之间所有对象的状态更新都会在锁被释放之后)更新到主存,从而保证这些变更对其他线程是可见的。类似的方法有putIntVolatile、putDoubleVolatile等等。这个方法要求被使用的属性被volatile修饰,否则功能和putObject方法相同。

putOrderedObject

  • public native void putOrderedObject(Object o, long offset, Object x);

设置o对象中offset偏移地址offset对应的Object型field的值为指定值x。这是一个有序或者有延迟的putObjectVolatile方法,并且不保证值的改变被其他线程立即看到。只有在field被volatile修饰并且期望被修改的时候使用才会生效。类似的方法有putOrderedIntputOrderedLong

staticFieldOffset

  • public native long staticFieldOffset(Field f);

返回给定的静态属性在它的类的存储分配中的位置(偏移地址)。不要在这个偏移量上执行任何类型的算术运算,它只是一个被传递给不安全的堆内存访问器的cookie。注意:这个方法仅仅针对静态属性,使用在非静态属性上会抛异常。下面源码中的方法注释估计有误,staticFieldOffset和objectFieldOffset的注释估计是对调了,为什么会出现这个问题无法考究。

objectFieldOffset

  • public native long objectFieldOffset(Field f);

返回给定的非静态属性在它的类的存储分配中的位置(偏移地址)。不要在这个偏移量上执行任何类型的算术运算,它只是一个被传递给不安全的堆内存访问器的cookie。注意:这个方法仅仅针对非静态属性,使用在静态属性上会抛异常。

staticFieldBase

  • public native Object staticFieldBase(Field f);

返回给定的静态属性的位置,配合staticFieldOffset方法使用。实际上,这个方法返回值就是静态属性所在的Class对象的一个内存快照。注释中说到,此方法返回的Object有可能为null,它只是一个'cookie'而不是真实的对象,不要直接使用的它的实例中的获取属性和设置属性的方法,它的作用只是方便调用上面提到的像getInt(Object,long)等等的任意方法。

shouldBeInitialized

  • public native boolean shouldBeInitialized(Class<?> c);

检测给定的类是否需要初始化。通常需要使用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)。 此方法当且仅当ensureClassInitialized方法不生效的时候才返回false。

ensureClassInitialized

  • public native void ensureClassInitialized(Class<?> c);

检测给定的类是否已经初始化。通常需要使用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)。

arrayBaseOffset

  • public native int arrayBaseOffset(Class<?> arrayClass);

返回数组类型的第一个元素的偏移地址(基础偏移地址)。如果arrayIndexScale方法返回的比例因子不为0,你可以通过结合基础偏移地址和比例因子访问数组的所有元素。Unsafe中已经初始化了很多类似的常量如ARRAY_BOOLEAN_BASE_OFFSET等。

arrayIndexScale

  • public native int arrayIndexScale(Class<?> arrayClass);

返回数组类型的比例因子(其实就是数据中元素偏移地址的增量,因为数组中的元素的地址是连续的)。此方法不适用于数组类型为"narrow"类型的数组,"narrow"类型的数组类型使用此方法会返回0(这里narrow应该是狭义的意思,但是具体指哪些类型暂时不明确,笔者查了很多资料也没找到结果)。Unsafe中已经初始化了很多类似的常量如ARRAY_BOOLEAN_INDEX_SCALE等。

defineClass

  • public native Class<?> defineClass(String name, byte[] b, int off, int len,ClassLoader loader,ProtectionDomain protectionDomain);

告诉JVM定义一个类,返回类实例,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)实例应该来源于调用者。

defineAnonymousClass

  • public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);

这个方法的使用可以看R大的知乎回答:JVM crashes at libjvm.so,下面截取一点内容解释此方法。

  • 1、VM Anonymous Class可以看作一种模板机制,如果程序要动态生成很多结构相同、只是若干变量不同的类的话,可以先创建出一个包含占位符常量的正常类作为模板,然后利用sun.misc.Unsafe#defineAnonymousClass()方法,传入该类(host class,宿主类或者模板类)以及一个作为"constant pool path"的数组来替换指定的常量为任意值,结果得到的就是一个替换了常量的VM Anonymous Class。
  • 2、VM Anonymous Class从VM的角度看是真正的"没有名字"的,在构造出来之后只能通过Unsafe#defineAnonymousClass()返回出来一个Class实例来进行反射操作。

还有其他几点看以自行阅读。这个方法虽然翻译为"定义匿名类",但是它所定义的类和实际的匿名类有点不相同,因此一般情况下我们不会用到此方法。在Jdk中lambda表达式相关的东西用到它,可以看InnerClassLambdaMetafactory这个类。

allocateInstance

  • public native Object allocateInstance(Class<?> cls) throws InstantiationException;

通过Class对象创建一个类的实例,不需要调用其构造函数、初始化代码、JVM安全检查等等。同时,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。

内存管理

addressSize

  • public native int addressSize();

获取本地指针的大小(单位是byte),通常值为4或者8。常量ADDRESS_SIZE就是调用此方法。

pageSize

  • public native int pageSize();

获取本地内存的页数,此值为2的幂次方。

allocateMemory

  • public native long allocateMemory(long bytes);

分配一块新的本地内存,通过bytes指定内存块的大小(单位是byte),返回新开辟的内存的地址。如果内存块的内容不被初始化,那么它们一般会变成内存垃圾。生成的本机指针永远不会为零,并将对所有值类型进行对齐。可以通过freeMemory方法释放内存块,或者通过reallocateMemory方法调整内存块大小。bytes值为负数或者过大会抛出IllegalArgumentException异常,如果系统拒绝分配内存会抛出OutOfMemoryError异常。

reallocateMemory

  • public native long reallocateMemory(long address, long bytes);

通过指定的内存地址address重新调整本地内存块的大小,调整后的内存块大小通过bytes指定(单位为byte)。可以通过freeMemory方法释放内存块,或者通过reallocateMemory方法调整内存块大小。bytes值为负数或者过大会抛出IllegalArgumentException异常,如果系统拒绝分配内存会抛出OutOfMemoryError异常。

setMemory

  • public native void setMemory(Object o, long offset, long bytes, byte value);

将给定内存块中的所有字节设置为固定值(通常是0)。内存块的地址由对象引用o和偏移地址共同决定,如果对象引用o为null,offset就是绝对地址。第三个参数就是内存块的大小,如果使用allocateMemory进行内存开辟的话,这里的值应该和allocateMemory的参数一致。value就是设置的固定值,一般为0(这里可以参考netty的DirectByteBuffer)。一般而言,o为null,所有有个重载方法是public native void setMemory(long offset, long bytes, byte value);,等效于setMemory(null, long offset, long bytes, byte value);

多线程同步

主要包括监视器锁定、解锁以及CAS相关的方法。

monitorEnter

  • public native void monitorEnter(Object o);

锁定对象,必须通过monitorExit方法才能解锁。此方法经过实验是可以重入的,也就是可以多次调用,然后通过多次调用monitorExit进行解锁。

monitorExit

  • public native void monitorExit(Object o);

解锁对象,前提是对象必须已经调用monitorEnter进行加锁,否则抛出IllegalMonitorStateException异常。

tryMonitorEnter

  • public native boolean tryMonitorEnter(Object o);

尝试锁定对象,如果加锁成功返回true,否则返回false。必须通过monitorExit方法才能解锁。

compareAndSwapObject

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

针对Object对象进行CAS操作。即是对应Java变量引用o,原子性地更新o中偏移地址为offset的属性的值为x,当且仅的偏移地址为offset的属性的当前值为expected才会更新成功返回true,否则返回false。

  • o:目标Java变量引用。
  • offset:目标Java变量中的目标属性的偏移地址。
  • expected:目标Java变量中的目标属性的期望的当前值。
  • x:目标Java变量中的目标属性的目标更新值。

类似的方法有compareAndSwapIntcompareAndSwapLong,在Jdk8中基于CAS扩展出来的方法有getAndAddIntgetAndAddLonggetAndSetIntgetAndSetLonggetAndSetObject,它们的作用都是:通过CAS设置新的值,返回旧的值。

线程的挂起和恢复

unpark

  • public native void unpark(Object thread);

释放被park创建的在一个线程上的阻塞。这个方法也可以被使用来终止一个先前调用park导致的阻塞。这个操作是不安全的,因此必须保证线程是存活的(thread has not been destroyed)。从Java代码中判断一个线程是否存活的是显而易见的,但是从native代码中这机会是不可能自动完成的。

park

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

阻塞当前线程直到一个unpark方法出现(被调用)、一个用于unpark方法已经出现过(在此park方法调用之前已经调用过)、线程被中断或者time时间到期(也就是阻塞超时)。在time非零的情况下,如果isAbsolute为true,time是相对于新纪元之后的毫秒,否则time表示纳秒。这个方法执行时也可能不合理地返回(没有具体原因)。并发包java.util.concurrent中的框架对线程的挂起操作被封装在LockSupport类中,LockSupport类中有各种版本pack方法,但最终都调用了Unsafe#park()方法。

内存屏障

内存屏障相关的方法是在Jdk8添加的。内存屏障相关的知识可以先自行查阅。

loadFence

  • public native void loadFence();

在该方法之前的所有读操作,一定在load屏障之前执行完成。

storeFence

  • public native void storeFence();

在该方法之前的所有写操作,一定在store屏障之前执行完成

fullFence

  • public native void fullFence();

在该方法之前的所有读写操作,一定在full屏障之前执行完成,这个内存屏障相当于上面两个(load屏障和store屏障)的合体功能。

其他

getLoadAverage

  • public native int getLoadAverage(double[] loadavg, int nelems);

获取系统的平均负载值,loadavg这个double数组将会存放负载值的结果,nelems决定样本数量,nelems只能取值为1到3,分别代表最近1、5、15分钟内系统的平均负载。如果无法获取系统的负载,此方法返回-1,否则返回获取到的样本数量(loadavg中有效的元素个数)。实验中这个方法一直返回-1,其实完全可以使用JMX中的相关方法替代此方法。

throwException

  • public native void throwException(Throwable ee);

绕过检测机制直接抛出异常。

Unsafe使用例子

验证staticFieldOffset和objectFieldOffset

public class Main {

    public static void main(String[] args) throws Exception {
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) theUnsafe.get(null);
        Class<Person> personClass = Person.class;
        Field name = personClass.getField("NAME");
        Field age = personClass.getField("age");
        try {
            System.out.println("objectFieldOffset name -->" + unsafe.objectFieldOffset(name));
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
        try {
            System.out.println("objectFieldOffset age -->" + unsafe.objectFieldOffset(age));
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
        try {
            System.out.println("staticFieldOffset name -->" + unsafe.staticFieldOffset(name));
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
        try {
            System.out.println("staticFieldOffset age -->" + unsafe.staticFieldOffset(age));
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

@Data
public class Person {

    public static String NAME = "doge";
    public String age;
}

输出结果:

java.lang.IllegalArgumentException
    at sun.misc.Unsafe.objectFieldOffset(Native Method)
    at org.throwable.unsafe.Main.main(Main.java:23)
java.lang.IllegalArgumentException
    at sun.misc.Unsafe.staticFieldOffset(Native Method)
    at org.throwable.unsafe.Main.main(Main.java:38)
objectFieldOffset age -->12
staticFieldOffset name -->104

输出结果说明了staticFieldOffset只能使用在静态属性,objectFieldOffset只能使用在非静态属性。

不依赖Class直接获取静态属性的值

public class Main2 {

    public static void main(String[] args) throws Exception {
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) theUnsafe.get(null);
        //这里必须预先实例化Person,否则它的静态字段不会加载
        Person person = new Person();
        Class<?> personClass = person.getClass();
        Field name = personClass.getField("NAME");
        //注意,上面的Field实例是通过Class获取的,但是下面的获取静态属性的值没有依赖到Class
        System.out.println(unsafe.getObject(unsafe.staticFieldBase(name), unsafe.staticFieldOffset(name)));
    }
}
@Data
public class Person {

    public static String NAME = "doge";
    public String age;
}

输出结果:

doge

获取类中的静态属性值,只依赖到Field的实例,剩余工作交给Unsafe的API。

java.nio.DirectByteBuffer

这个是JDK中使用直接内存的Buffer。可以查看它的构造函数如下:

  DirectByteBuffer(int cap) {                   // package-private

        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 {
            base = unsafe.allocateMemory(size); //使用Unsafe分配内存
        } catch (OutOfMemoryError x) {
            Bits.unreserveMemory(size, cap);
            throw x;
        }
        //使用Unsafe设置内存固定值
        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;
        }
        cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
        att = null;
    }

Unsafe源码附录

如果你不想下载openjdk的源码,下面贴出Unsafe类的源码,来自openjdk-8u40-src-b25-10_feb_2015:

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}. */
    public native void monitorEnter(Object o);

    /**
     * Unlock the object.  It must have been locked via {@link
     * #monitorEnter}.
     */
    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}.
     */
    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();
    }

}
posted @ 2018-06-05 15:14 throwable 阅读(...) 评论(...) 编辑 收藏