本文共 52165 字，大约阅读时间需要 173 分钟。

jdk9做了一些限制，不过还是可以使用这个类。

这个类本身是单例的，需要通过静态方法获取唯一实例。不过编译器、运行时都会报错。编译器的降低下错误级别就行，运行时的，根据代码

public static Unsafe getUnsafe() {    Class
    caller = Reflection.getCallerClass();    if (!VM.isSystemDomainLoader(caller.getClassLoader())) // 判断调用Unsafe的类是否是BootstrapClassLoader加载的类        throw new SecurityException("Unsafe");    return theUnsafe;}

知道应该是通过类加载器限制。一般我们写的类都是由Application ClassLoader（sun.misc.Launcher$AppClassLoader）进行加载的，层级比较低，这里的SystemDomainLoader就是BootstarpClassLoader（C++写的），也就是加载rt.jar里面的类的加载器，所以java.*用就不会有事，我们用就会有事。

想要使用Unsafe有两种方式。一种是用反射，比较简单

Field unsafeField = Unsafe.class.getDeclaredFields()[0];unsafeField.setAccessible(true);Unsafe u = (Unsafe) unsafeField.get(null);

简单看几个例子，学习下如何使用Unsafe提供的底层读写功能。

// 属性读写，可以绕过访问限制public class TestUnsafe {    public static void main(String[] args) {        try {            Field unsafeField = Unsafe.class.getDeclaredFields()[0];            unsafeField.setAccessible(true);            Unsafe u = (Unsafe) unsafeField.get(null);            Class
    tk = A.class;            A a = new A();            // 下面这些偏移量实际都可以用常量来表示，在一次程序运行过程中它们是不变的            long offset = u.objectFieldOffset(tk.getDeclaredField("x"));            System.err.println(u.getInt(a, offset)); // 读取x            long finalOffset = u.objectFieldOffset(tk.getDeclaredField("finalX"));            System.err.println(u.getInt(a, finalOffset)); // 读取finalX 233            u.putInt(a, finalOffset, 100); // 可以用来更改final的值            System.err.println(u.getInt(a, finalOffset)); // 读取finalX 100            Object staticBase = u.staticFieldBase(tk.getDeclaredField("staticX")); // 这里就是返回 A.class，跟逻辑是一致            long staticOffset = u.staticFieldOffset(tk.getDeclaredField("staticX"));            System.err.println(u.getInt(staticBase, staticOffset));            long staticFinalOffset = u.staticFieldOffset(tk.getDeclaredField("staticFinalX"));            System.err.println(u.getInt(staticBase, staticFinalOffset));            System.err.println(A.class == staticBase); // true            System.err.println(u.getInt(A.class, staticOffset)); // 等价于u.getInt(staticBase, staticOffset)        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {            e.printStackTrace();        }    }}class A {    private int x = 123456;    private final int finalX = 233;    private static int staticX = 987654;    private static final int staticFinalX = 666;}

// Unsafe提供的强化数组读写的功能// 大多数情况都可以直接使用AtomicReferenceArray来代替public class TestArrayOffset {    static Unsafe U;    static {        try {            Field unsafeField = Unsafe.class.getDeclaredFields()[0];            unsafeField.setAccessible(true);            U = (Unsafe) unsafeField.get(null);        } catch (SecurityException | IllegalArgumentException | IllegalAccessException e) {            e.printStackTrace();        }    }    // 这个可以认为就是求a[i]，不过这种看不出有什么实际的价值    // getIntVolatile/putIntVolatile就很有价值了，提供了数组元素的volatile读写，如果写入的值不依赖当前值，可以做到原子读写    // AtomicReferenceArray就是这样实现数组中某个元素的原子读写    static int array(Object a, int i) {        return U.getInt(a, (long)(Unsafe.ARRAY_INT_BASE_OFFSET + Unsafe.ARRAY_INT_INDEX_SCALE * i));    }    public static void main(String[] args) {        int[] ints = {13, 17, 19, 23, 29, 31, 37};        for (int i = 0; i < ints.length; i++) {            System.err.println(array(ints, i));        }    }}

4、CAS相关

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;}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;}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;}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;}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;}

下面的是Unsafe的代码，没什么逻辑，价值不大，不过还是贴下

/* * 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.     *     * 
 The returned Unsafe 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.     *     * 
 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.     *     * 
 Here is a suggested idiom for using unsafe operations:     *     * 
   
    
     * class MyTrustedClass {     *   private static final Unsafe unsafe = Unsafe.getUnsafe();     *   ...     *   private long myCountAddress = ...;     *   public int getCount() { return unsafe.getByte(myCountAddress); }     * }     * 
   
     *     * (It may assist compilers to make the local variable be     * final.)     *     * @exception  SecurityException  if a security manager exists and its     *             checkPropertiesAccess 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 o at the given offset, or (if o is     * null) from the memory address whose numerical value is the given     * offset.     * 
     * The results are undefined unless one of the following cases is true:     * 
   
         * 
    
The offset was obtained from {@link #objectFieldOffset} on     * the {@link java.lang.reflect.Field} of some Java field and the object     * referred to by o is of a class compatible with that     * field's class.     *     * 
    
The offset and object reference o (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.     *     * 
    
The object referred to by o is an array, and the offset     * is an integer of the form B+N*S, where N is     * a valid index into the array, and B and S are     * the values obtained by {@link #arrayBaseOffset} and {@link     * #arrayIndexScale} (respectively) from the array's class.  The value     * referred to is the Nth element of the array.     *     * 
   
     * 
     * 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.     * 
     * This method refers to a variable by means of two parameters, and so     * it provides (in effect) a double-register 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     * single-register 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.     * 
     * 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.     * 
     * The variable must be of the same type as the method     * parameter x.     *     * @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.     * 
     * Unless the reference x being stored is either null     * or matches the field type, the results are undefined.     * If the reference o 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.     *     * 
 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.     *     * 
 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).     *     * 
This method determines a block's base address by means of two parameters,     * and so it provides (in effect) a double-register addressing mode,     * as discussed in {@link #getInt(Object,long)}.  When the object reference is null,     * the offset supplies an absolute base address.     *     * 
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 single-register addressing mode,     * as discussed in {@link #getInt(Object,long)}.     *     * 
Equivalent to setMemory(null, address, bytes, value).     */    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.     *     * 
This method determines each block's base address by means of two parameters,     * and so it provides (in effect) a double-register addressing mode,     * as discussed in {@link #getInt(Object,long)}.  When the object reference is null,     * the offset supplies an absolute base address.     *     * 
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 single-register addressing mode,     * as discussed in {@link #getInt(Object,long)}.     *     * Equivalent to copyMemory(null, srcAddress, null, destAddress, bytes).     */    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:     * 
   
    
     * public int fieldOffset(Field f) {     *     if (Modifier.isStatic(f.getModifiers()))     *         return (int) staticFieldOffset(f);     *     else     *         return (int) objectFieldOffset(f);     * }     * 
   
     * @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:     * 
   
    
     * 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;     * }     * 
   
     * @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.     *     * 
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.     *     * 
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}.     * 
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.     *     * 
Any given field will always have the same offset, and no two distinct     * fields of the same class will ever have the same offset.     *     * 
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}.     * 
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.     * 
     * For each CP entry, the corresponding CP patch must either be null or have     * the a format that matches its tag:     * 
   
         * 
    
Integer, Long, Float, Double: the corresponding wrapper object type from java.lang     * 
    
Utf8: a string (must have suitable syntax if used as signature or name)     * 
    
Class: any java.lang.Class object     * 
    
String: any object (not just a java.lang.String)     * 
    
InterfaceMethodRef: (NYI) a method handle to invoke on that call site's arguments     * 
   
     * @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 x if it is currently     * holding expected.     * @return true if successful     */    public final native boolean compareAndSwapObject(Object o, long offset,                                                     Object expected,                                                     Object x);    /**     * Atomically update Java variable to x if it is currently     * holding expected.     * @return true if successful     */    public final native boolean compareAndSwapInt(Object o, long offset,                                                  int expected,                                                  int x);    /**     * Atomically update Java variable to x if it is currently     * holding expected.     * @return true 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 park, or, if it is     * not blocked, cause the subsequent call to park 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     * unpark occurs, or a balancing unpark 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 unpark 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 nelem samples and     * assigns to the elements of the given loadavg 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 o     * at the given offset.     *     * @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 o     * at the given offset.     *     * @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 o     * at the given offset.     *     * @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 o     * at the given offset.     *     * @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 o at the given offset.     *     * @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();    }}

转载地址：http://ugcws.baihongyu.com/