Java中的底层魔法类Unsafe基本用法概述
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1 引子
Java中没有指针，不能直接对内存地址的变量进行控制，但Java提供了一个特殊的类Unsafe工具类来间接实现。Unsafe主要提供一些用于执行低级别、不安全操作的方法，如直接访问系统内存资源、自主管理内存资源等，这些方法在提升Java运行效率、增强Java语言底层资源操作能力方面起到了很大的作用 。正如其名字unsafe，直接去使用这个工具类是不安全的，它能直接在硬件层（内存上）修改访问变量，而无视各种访问修饰符的限制。它几乎所有的公共方法API都是本地方法，这些方法是使用C/C++方法实现的，它越过了虚拟机层面，直接在操作系统本地执行。因为这是一个底层类，如果不理解不了解它的内部原理，直接使用Unsafe可能会造成一个意外的结果,所以它被限制开发者直接使用，只能由JDK类库的维护者使用。如果您喜欢阅读JDK的源码，那么你会发现在各种并发工具类的内部常常见到这个类的踪影，它们经常通过这个类的一些方法根据相应内存地址在内存上直接CAS修改访问共享变量的值。
Unsafe类在Oracle的官方JDK中没有提供源码，我们只能通过IDEA的反编译工具看到反编译后的源代码，因此我们看不到方法注释。而只OpenJDK中带有所有JDK的源代码，这里使用OpenJDK作参考讲解材料。以下是OpenJDK中Unsafe的类注释
A collection of methods for performing low-level, unsafe operations. Although the class and all methods are public, use of this class is limited because only trusted code can obtain instances of it.
直译过来大致意思是：此类拥有一组用于执行低级，不安全操作的方法。 尽管此类和所有方法都是公共的，但是由于只有可信代码才能获取该类的实例，因此此类的使用受到限制。
获取Unsafe的方式
可以看出构造方法被私有化，只能通过静态方法getUnsafe（）才能获取此Unsafe对象，而此静态方法的使用也是受到限制的，只能由JDK中的其它类来调用，普通开发者使用此方法将抛出异常。
private Unsafe() {} private static final Unsafe theUnsafe = new Unsafe(); @CallerSensitive public static Unsafe getUnsafe() { Class<?> caller = Reflection.getCallerClass(); //调用者Class对象 if (!VM.isSystemDomainLoader(caller.getClassLoader())) //判断调用者的类加载器是否为系统类加载器 //不是JAVA_HOME/jre/lib目录下jar包中的类来调用此方法getUnsafe（）就会抛出异常 throw new SecurityException("Unsafe"); return theUnsafe; }
此方法注释也说：
为调用提供执行不安全操作的能力。返回的Unsafe对象应由调用方小心保护，因为它可用于在任意内存地址处读取和写入数据。 绝不能将其传递给不受信任的代码。此类中的大多数方法都是非常底层的，并且对应于少量的硬件指令（在典型的机器上）。 应鼓励编译器相应地优化这些方法，而不是使用Unsafe类来控制。
因为反射中使用Field描述实例变量和静态变量，现在将实例变量和静态变量统称为字段。
2 Java对象相关操作
获取字段相对偏移量
/** * 根据反射的字段f,获取相应实例变量的偏移量 * 此偏移量是实例变量的起始地址与对象的起始地址之差，对于一确定的java类，某字段与对象之间的起始地址之差是常数， * 静态变量的偏移量与此类似 */ public native long staticFieldOffset(Field f); //根据反射的字段f,获取相应静态变量的偏移量（静态变量的起始地址与相应静态区Klass对象起始地址之差） public native long objectFieldOffset(Field f);
根据字段偏移量设置/获取字段值
//根据反射的字段f,获取相应的静态变量的值 public native Object staticFieldBase(Field f); /** *参数o是字段所属的对象，offset表示相对偏移量，参数x是此字段要设置的新值 */ /*字段是引用数据类型*/ public native Object getObject(Object o, long offset);//获取字段值 public native void putObject(Object o, long offset, Object x);//设置字段值 /*字段为基本数据类型*/ public native void putInt(Object o, long offset, int x); public native int getInt(Object o, long offset); public native boolean getBoolean(Object o, long offset); public native void putBoolean(Object o, long offset, boolean x); public native byte getByte(Object o, long offset); public native void putByte(Object o, long offset, byte x); public native short getShort(Object o, long offset); public native void putShort(Object o, long offset, short x); public native char getChar(Object o, long offset); public native void putChar(Object o, long offset, char x); public native long getLong(Object o, long offset); public native void putLong(Object o, long offset, long x); public native float getFloat(Object o, long offset); public native void putFloat(Object o, long offset, float x); public native double getDouble(Object o, long offset); public native void putDouble(Object o, long offset, double x);
Volatile版本的根据字段偏移量设置/获取字段值(加上volatile语义)
//volatile形式地获取字段值，即使在多线条件下，其值与工作内存（主）中的值一致，非缓存中的值 public native Object getObjectVolatile(Object o, long offset); //volatile形式地设置字段值，即使在多线条件下，设置的值将立即同步到工作（主）内存中，而非久驻缓存 public native void putObjectVolatile(Object o, long offset, Object x); public native int getIntVolatile(Object o, long offset); public native void putIntVolatile(Object o, long offset, int x); public native boolean getBooleanVolatile(Object o, long offset); public native void putBooleanVolatile(Object o, long offset, boolean x); public native byte getByteVolatile(Object o, long offset); public native void putByteVolatile(Object o, long offset, byte x); public native short getShortVolatile(Object o, long offset); public native void putShortVolatile(Object o, long offset, short x); public native char getCharVolatile(Object o, long offset); public native void putCharVolatile(Object o, long offset, char x); public native long getLongVolatile(Object o, long offset); public native void putLongVolatile(Object o, long offset, long x); public native float getFloatVolatile(Object o, long offset); public native void putFloatVolatile(Object o, long offset, float x); public native double getDoubleVolatile(Object o, long offset); public native void putDoubleVolatile(Object o, long offset, double x);
有序延迟化地设置字段值
上面的putXxxVolatile（）方法不能保证其他线程立即可见,下面的三个方法能保证其他线程立即可见。但这里有个前提，这些字段必须被Volatile修饰，否则仍然不能保证其他线程立即可见。
//有序延迟化地设置字段值，上面的putXxxVolatile（）方法不能保证其他线程立即可见 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);
3 Class相关操作
创建Java类
/** * 让虚拟机知道我们定义一个类，但不进行安全检查。 * 默认情况下，类加载器和保护域来自调用者的类。 */ public native Class<?> defineClass(String name, byte[] b, int off, int len, ClassLoader loader, ProtectionDomain protectionDomain); /* * 在类加载器和系统字典（system dictionary)不知道的情况下根据字节码数据定义一个匿名的Class对象，相当于创建了一个Java类 * @params hostClass context for linkage, access control, protection domain, and class loader * @params data 字节码文件对应的字节数组 * @params cpPatches where non-null entries exist, they replace corresponding CP entries in data */ public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);
Java类初始化
shouldBeInitialized（Class）方法检测Class对应的Java类是否被初始化
ensureClassInitialized（Class)方法强制Java类初始化，若没初始化则进行初始化。
这两个方法常与staticFieldBase(Field)一起使用，因为如果Java类没有被初始化，静态变量便没有初始化，就不能直接获取静态变量的引用。
/** * Detect if the given class may need to be initialized. This is often * needed in conjunction with obtaining the static field base of a * class. * @return false only if a call to {@code ensureClassInitialized} would have no effect */ public native boolean shouldBeInitialized(Class<?> c); /** * Ensure the given class has been initialized. This is often * needed in conjunction with obtaining the static field base of a * class. */ public native void ensureClassInitialized(Class<?> c);
根据Class创建对象
仅通过Class对象就可以创建此类的实例对象，而且不需要调用其构造函数、初始化代码、JVM安全检查，等,。它抑制修饰符检测，也就是即使构造器是private修饰的也能通过此方法实例化，只需提类对象即可创建相应的对象 .
/** 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;
4 CAS更新操作
/** * CAS更新共享变量 * * @param o 字段所属对象 * @param offset 字段的相对偏移量 * @param expected 预期值 * @param x 更新值 * @return 更新成功则返回true */ public final native boolean compareAndSwapObject(Object o, long offset, Object expected, Object x); public final native boolean compareAndSwapInt(Object o, long offset, int expected, int x); public final native boolean compareAndSwapLong(Object o, long offset, long expected, long x);
5 内存操作
根据内存地址，设置/获取对应的值
/** * 参数address是绝对内存地址，参数x是设定的值 * 如果address是零或不是通过allocMemery()方法分配的地址，那么结果未定义 */ public native byte getByte(long address); public native void putByte(long address, byte x); /** @see #getByte(long) */ public native short getShort(long address); /** @see #putByte(long, byte) */ public native void putShort(long address, short x); /** @see #getByte(long) */ public native char getChar(long address); /** @see #putByte(long, byte) */ public native void putChar(long address, char x); /** @see #getByte(long) */ public native int getInt(long address); /** @see #putByte(long, byte) */ public native void putInt(long address, int x); /** @see #getByte(long) */ public native long getLong(long address); /** @see #putByte(long, byte) */ public native void putLong(long address, long x); /** @see #getByte(long) */ public native float getFloat(long address); /** @see #putByte(long, byte) */ public native void putFloat(long address, float x); /** @see #getByte(long) */ public native double getDouble(long address); /** @see #putByte(long, byte) */ public native void putDouble(long address, double x);
根据内存地址设置/获取指针
//根据内存地址获取一个指针 public native long getAddress(long address); //根据内存地址设置一个指针，adress是内存地址，x是指定的指针值 public native void putAddress(long address, long x);
分配、扩展、释放内存
//分配一块指定的内存空间，返回一个指向此内存起始位置的指针 public native long allocateMemory(long bytes); //扩展内存 public native long reallocateMemory(long address, long bytes); //在指定的内存块填充值 public native void setMemory(Object o, long offset, long bytes, byte value); public void setMemory(long address, long bytes, byte value) { setMemory(null, address, bytes, value); } //将一处内存的数据复制另一处内存 public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes); public void copyMemory(long srcAddress, long destAddress, long bytes) { copyMemory(null, srcAddress, null, destAddress, bytes); } //释放内存 public native void freeMemory(long address);
6 系统信息
//本地指针宽度，通常是4或8 public native int addressSize(); /** *内存页的大小，它总是2的幂次方 */ public native int pageSize();
7 线程管理
唤醒/休眠线程
public native void unpark(Object thread);//唤醒 public native void park(boolean isAbsolute, long time);//休眠
抢锁与释放锁（已经被弃用）
//锁住对象 @Deprecated public native void monitorEnter(Object o); //释放锁对象 @Deprecated public native void monitorExit(Object o); //尝试锁信对象 @Deprecated public native boolean tryMonitorEnter(Object o);
8 数组相关的偏移量
//第一个元素与数组对象两者间起始地址之差(首元素与对象头的相对偏移量） public native int arrayBaseOffset(Class<?> arrayClass); //相邻元素起始地址差 public native int arrayIndexScale(Class<?> arrayClass);
9 内存屏障
/** * 内存屏障，禁止load重排序。屏障前不能重排序load,且只能在屏障后load或store */ public native void loadFence(); /** * 内存屏障，禁止store重排序。 屏障前不能重排序store操作,且只能在屏障后load或store */ public native void storeFence(); /** * 内存屏障，禁止store load重排序。 */ public native void fullFence();
OpenJDK1.8中含注释的Unsafe类源代码
/* * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package sun.misc; import java.security.*; import java.lang.reflect.*; import sun.reflect.CallerSensitive; import sun.reflect.Reflection; /** * A collection of methods for performing low-level, unsafe operations. * Although the class and all methods are public, use of this class is * limited because only trusted code can obtain instances of it. * * @author John R. Rose * @see #getUnsafe */ public final class Unsafe { private static native void registerNatives(); static { registerNatives(); sun.reflect.Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe"); } private Unsafe() {} private static final Unsafe theUnsafe = new Unsafe(); /** * Provides the caller with the capability of performing unsafe * operations. * * <p> The returned <code>Unsafe</code> object should be carefully guarded * by the caller, since it can be used to read and write data at arbitrary * memory addresses. It must never be passed to untrusted code. * * <p> Most methods in this class are very low-level, and correspond to a * small number of hardware instructions (on typical machines). Compilers * are encouraged to optimize these methods accordingly. * * <p> Here is a suggested idiom for using unsafe operations: * * <blockquote><pre> * class MyTrustedClass { * private static final Unsafe unsafe = Unsafe.getUnsafe(); * ... * private long myCountAddress = ...; * public int getCount() { return unsafe.getByte(myCountAddress); } * } * </pre></blockquote> * * (It may assist compilers to make the local variable be * <code>final</code>.) * * @exception SecurityException if a security manager exists and its * <code>checkPropertiesAccess</code> method doesn‘t allow * access to the system properties. */ @CallerSensitive public static Unsafe getUnsafe() { Class<?> caller = Reflection.getCallerClass(); if (!VM.isSystemDomainLoader(caller.getClassLoader())) throw new SecurityException("Unsafe"); return theUnsafe; } /// peek and poke operations /// (compilers should optimize these to memory ops) // These work on object fields in the Java heap. // They will not work on elements of packed arrays. /** * Fetches a value from a given Java variable. * More specifically, fetches a field or array element within the given * object <code>o</code> at the given offset, or (if <code>o</code> is * null) from the memory address whose numerical value is the given * offset. * <p> * The results are undefined unless one of the following cases is true: * <ul> * <li>The offset was obtained from {@link #objectFieldOffset} on * the {@link java.lang.reflect.Field} of some Java field and the object * referred to by <code>o</code> is of a class compatible with that * field‘s class. * * <li>The offset and object reference <code>o</code> (either null or * non-null) were both obtained via {@link #staticFieldOffset} * and {@link #staticFieldBase} (respectively) from the * reflective {@link Field} representation of some Java field. * * <li>The object referred to by <code>o</code> is an array, and the offset * is an integer of the form <code>B+N*S</code>, where <code>N</code> is * a valid index into the array, and <code>B</code> and <code>S</code> are * the values obtained by {@link #arrayBaseOffset} and {@link * #arrayIndexScale} (respectively) from the array‘s class. The value * referred to is the <code>N</code><em>th</em> element of the array. * * </ul> * <p> * If one of the above cases is true, the call references a specific Java * variable (field or array element). However, the results are undefined * if that variable is not in fact of the type returned by this method. * <p> * This method refers to a variable by means of two parameters, and so * it provides (in effect) a <em>double-register</em> addressing mode * for Java variables. When the object reference is null, this method * uses its offset as an absolute address. This is similar in operation * to methods such as {@link #getInt(long)}, which provide (in effect) a * <em>single-register</em> addressing mode for non-Java variables. * However, because Java variables may have a different layout in memory * from non-Java variables, programmers should not assume that these * two addressing modes are ever equivalent. Also, programmers should * remember that offsets from the double-register addressing mode cannot * be portably confused with longs used in the single-register addressing * mode. * * @param o Java heap object in which the variable resides, if any, else * null * @param offset indication of where the variable resides in a Java heap * object, if any, else a memory address locating the variable * statically * @return the value fetched from the indicated Java variable * @throws RuntimeException No defined exceptions are thrown, not even * {@link NullPointerException} */ public native int getInt(Object o, long offset); /** * Stores a value into a given Java variable. * <p> * The first two parameters are interpreted exactly as with * {@link #getInt(Object, long)} to refer to a specific * Java variable (field or array element). The given value * is stored into that variable. * <p> * The variable must be of the same type as the method * parameter <code>x</code>. * * @param o Java heap object in which the variable resides, if any, else * null * @param offset indication of where the variable resides in a Java heap * object, if any, else a memory address locating the variable * statically * @param x the value to store into the indicated Java variable * @throws RuntimeException No defined exceptions are thrown, not even * {@link NullPointerException} */ public native void putInt(Object o, long offset, int x); /** * Fetches a reference value from a given Java variable. * @see #getInt(Object, long) */ public native Object getObject(Object o, long offset); /** * Stores a reference value into a given Java variable. * <p> * Unless the reference <code>x</code> being stored is either null * or matches the field type, the results are undefined. * If the reference <code>o</code> is non-null, car marks or * other store barriers for that object (if the VM requires them) * are updated. * @see #putInt(Object, int, int) */ public native void putObject(Object o, long offset, Object x); /** @see #getInt(Object, long) */ public native boolean getBoolean(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putBoolean(Object o, long offset, boolean x); /** @see #getInt(Object, long) */ public native byte getByte(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putByte(Object o, long offset, byte x); /** @see #getInt(Object, long) */ public native short getShort(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putShort(Object o, long offset, short x); /** @see #getInt(Object, long) */ public native char getChar(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putChar(Object o, long offset, char x); /** @see #getInt(Object, long) */ public native long getLong(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putLong(Object o, long offset, long x); /** @see #getInt(Object, long) */ public native float getFloat(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putFloat(Object o, long offset, float x); /** @see #getInt(Object, long) */ public native double getDouble(Object o, long offset); /** @see #putInt(Object, int, int) */ public native void putDouble(Object o, long offset, double x); /** * This method, like all others with 32-bit offsets, was native * in a previous release but is now a wrapper which simply casts * the offset to a long value. It provides backward compatibility * with bytecodes compiled against 1.4. * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public int getInt(Object o, int offset) { return getInt(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putInt(Object o, int offset, int x) { putInt(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public Object getObject(Object o, int offset) { return getObject(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putObject(Object o, int offset, Object x) { putObject(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public boolean getBoolean(Object o, int offset) { return getBoolean(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putBoolean(Object o, int offset, boolean x) { putBoolean(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public byte getByte(Object o, int offset) { return getByte(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putByte(Object o, int offset, byte x) { putByte(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public short getShort(Object o, int offset) { return getShort(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putShort(Object o, int offset, short x) { putShort(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public char getChar(Object o, int offset) { return getChar(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putChar(Object o, int offset, char x) { putChar(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public long getLong(Object o, int offset) { return getLong(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putLong(Object o, int offset, long x) { putLong(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public float getFloat(Object o, int offset) { return getFloat(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putFloat(Object o, int offset, float x) { putFloat(o, (long)offset, x); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public double getDouble(Object o, int offset) { return getDouble(o, (long)offset); } /** * @deprecated As of 1.4.1, cast the 32-bit offset argument to a long. * See {@link #staticFieldOffset}. */ @Deprecated public void putDouble(Object o, int offset, double x) { putDouble(o, (long)offset, x); } // These work on values in the C heap. /** * Fetches a value from a given memory address. If the address is zero, or * does not point into a block obtained from {@link #allocateMemory}, the * results are undefined. * * @see #allocateMemory */ public native byte getByte(long address); /** * Stores a value into a given memory address. If the address is zero, or * does not point into a block obtained from {@link #allocateMemory}, the * results are undefined. * * @see #getByte(long) */ public native void putByte(long address, byte x); /** @see #getByte(long) */ public native short getShort(long address); /** @see #putByte(long, byte) */ public native void putShort(long address, short x); /** @see #getByte(long) */ public native char getChar(long address); /** @see #putByte(long, byte) */ public native void putChar(long address, char x); /** @see #getByte(long) */ public native int getInt(long address); /** @see #putByte(long, byte) */ public native void putInt(long address, int x); /** @see #getByte(long) */ public native long getLong(long address); /** @see #putByte(long, byte) */ public native void putLong(long address, long x); /** @see #getByte(long) */ public native float getFloat(long address); /** @see #putByte(long, byte) */ public native void putFloat(long address, float x); /** @see #getByte(long) */ public native double getDouble(long address); /** @see #putByte(long, byte) */ public native void putDouble(long address, double x); /** * Fetches a native pointer from a given memory address. If the address is * zero, or does not point into a block obtained from {@link * #allocateMemory}, the results are undefined. * * <p> If the native pointer is less than 64 bits wide, it is extended as * an unsigned number to a Java long. The pointer may be indexed by any * given byte offset, simply by adding that offset (as a simple integer) to * the long representing the pointer. The number of bytes actually read * from the target address maybe determined by consulting {@link * #addressSize}. * * @see #allocateMemory */ public native long getAddress(long address); /** * Stores a native pointer into a given memory address. If the address is * zero, or does not point into a block obtained from {@link * #allocateMemory}, the results are undefined. * * <p> The number of bytes actually written at the target address maybe * determined by consulting {@link #addressSize}. * * @see #getAddress(long) */ public native void putAddress(long address, long x); /// wrappers for malloc, realloc, free: /** * Allocates a new block of native memory, of the given size in bytes. The * contents of the memory are uninitialized; they will generally be * garbage. The resulting native pointer will never be zero, and will be * aligned for all value types. Dispose of this memory by calling {@link * #freeMemory}, or resize it with {@link #reallocateMemory}. * * @throws IllegalArgumentException if the size is negative or too large * for the native size_t type * * @throws OutOfMemoryError if the allocation is refused by the system * * @see #getByte(long) * @see #putByte(long, byte) */ public native long allocateMemory(long bytes); /** * Resizes a new block of native memory, to the given size in bytes. The * contents of the new block past the size of the old block are * uninitialized; they will generally be garbage. The resulting native * pointer will be zero if and only if the requested size is zero. The * resulting native pointer will be aligned for all value types. Dispose * of this memory by calling {@link #freeMemory}, or resize it with {@link * #reallocateMemory}. The address passed to this method may be null, in * which case an allocation will be performed. * * @throws IllegalArgumentException if the size is negative or too large * for the native size_t type * * @throws OutOfMemoryError if the allocation is refused by the system * * @see #allocateMemory */ public native long reallocateMemory(long address, long bytes); /** * Sets all bytes in a given block of memory to a fixed value * (usually zero). * * <p>This method determines a block‘s base address by means of two parameters, * and so it provides (in effect) a <em>double-register</em> addressing mode, * as discussed in {@link #getInt(Object,long)}. When the object reference is null, * the offset supplies an absolute base address. * * <p>The stores are in coherent (atomic) units of a size determined * by the address and length parameters. If the effective address and * length are all even modulo 8, the stores take place in ‘long‘ units. * If the effective address and length are (resp.) even modulo 4 or 2, * the stores take place in units of ‘int‘ or ‘short‘. * * @since 1.7 */ public native void setMemory(Object o, long offset, long bytes, byte value); /** * Sets all bytes in a given block of memory to a fixed value * (usually zero). This provides a <em>single-register</em> addressing mode, * as discussed in {@link #getInt(Object,long)}. * * <p>Equivalent to <code>setMemory(null, address, bytes, value)</code>. */ public void setMemory(long address, long bytes, byte value) { setMemory(null, address, bytes, value); } /** * Sets all bytes in a given block of memory to a copy of another * block. * * <p>This method determines each block‘s base address by means of two parameters, * and so it provides (in effect) a <em>double-register</em> addressing mode, * as discussed in {@link #getInt(Object,long)}. When the object reference is null, * the offset supplies an absolute base address. * * <p>The transfers are in coherent (atomic) units of a size determined * by the address and length parameters. If the effective addresses and * length are all even modulo 8, the transfer takes place in ‘long‘ units. * If the effective addresses and length are (resp.) even modulo 4 or 2, * the transfer takes place in units of ‘int‘ or ‘short‘. * * @since 1.7 */ public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes); /** * Sets all bytes in a given block of memory to a copy of another * block. This provides a <em>single-register</em> addressing mode, * as discussed in {@link #getInt(Object,long)}. * * Equivalent to <code>copyMemory(null, srcAddress, null, destAddress, bytes)</code>. */ public void copyMemory(long srcAddress, long destAddress, long bytes) { copyMemory(null, srcAddress, null, destAddress, bytes); } /** * Disposes of a block of native memory, as obtained from {@link * #allocateMemory} or {@link #reallocateMemory}. The address passed to * this method may be null, in which case no action is taken. * * @see #allocateMemory */ public native void freeMemory(long address); /// random queries /** * This constant differs from all results that will ever be returned from * {@link #staticFieldOffset}, {@link #objectFieldOffset}, * or {@link #arrayBaseOffset}. */ public static final int INVALID_FIELD_OFFSET = -1; /** * Returns the offset of a field, truncated to 32 bits. * This method is implemented as follows: * <blockquote><pre> * public int fieldOffset(Field f) { * if (Modifier.isStatic(f.getModifiers())) * return (int) staticFieldOffset(f); * else * return (int) objectFieldOffset(f); * } * </pre></blockquote> * @deprecated As of 1.4.1, use {@link #staticFieldOffset} for static * fields and {@link #objectFieldOffset} for non-static fields. */ @Deprecated public int fieldOffset(Field f) { if (Modifier.isStatic(f.getModifiers())) return (int) staticFieldOffset(f); else return (int) objectFieldOffset(f); } /** * Returns the base address for accessing some static field * in the given class. This method is implemented as follows: * <blockquote><pre> * public Object staticFieldBase(Class c) { * Field[] fields = c.getDeclaredFields(); * for (int i = 0; i < fields.length; i++) { * if (Modifier.isStatic(fields[i].getModifiers())) { * return staticFieldBase(fields[i]); * } * } * return null; * } * </pre></blockquote> * @deprecated As of 1.4.1, use {@link #staticFieldBase(Field)} * to obtain the base pertaining to a specific {@link Field}. * This method works only for JVMs which store all statics * for a given class in one place. */ @Deprecated public Object staticFieldBase(Class<?> c) { Field[] fields = c.getDeclaredFields(); for (int i = 0; i < fields.length; i++) { if (Modifier.isStatic(fields[i].getModifiers())) { return staticFieldBase(fields[i]); } } return null; } /** * Report the location of a given field in the storage allocation of its * class. Do not expect to perform any sort of arithmetic on this offset; * it is just a cookie which is passed to the unsafe heap memory accessors. * * <p>Any given field will always have the same offset and base, and no * two distinct fields of the same class will ever have the same offset * and base. * * <p>As of 1.4.1, offsets for fields are represented as long values, * although the Sun JVM does not use the most significant 32 bits. * However, JVM implementations which store static fields at absolute * addresses can use long offsets and null base pointers to express * the field locations in a form usable by {@link #getInt(Object,long)}. * Therefore, code which will be ported to such JVMs on 64-bit platforms * must preserve all bits of static field offsets. * @see #getInt(Object, long) */ public native long staticFieldOffset(Field f); /** * Report the location of a given static field, in conjunction with {@link * #staticFieldBase}. * <p>Do not expect to perform any sort of arithmetic on this offset; * it is just a cookie which is passed to the unsafe heap memory accessors. * * <p>Any given field will always have the same offset, and no two distinct * fields of the same class will ever have the same offset. * * <p>As of 1.4.1, offsets for fields are represented as long values, * although the Sun JVM does not use the most significant 32 bits. * It is hard to imagine a JVM technology which needs more than * a few bits to encode an offset within a non-array object, * However, for consistency with other methods in this class, * this method reports its result as a long value. * @see #getInt(Object, long) */ public native long objectFieldOffset(Field f); /** * Report the location of a given static field, in conjunction with {@link * #staticFieldOffset}. * <p>Fetch the base "Object", if any, with which static fields of the * given class can be accessed via methods like {@link #getInt(Object, * long)}. This value may be null. This value may refer to an object * which is a "cookie", not guaranteed to be a real Object, and it should * not be used in any way except as argument to the get and put routines in * this class. */ public native Object staticFieldBase(Field f); /** * Detect if the given class may need to be initialized. This is often * needed in conjunction with obtaining the static field base of a * class. * @return false only if a call to {@code ensureClassInitialized} would have no effect */ public native boolean shouldBeInitialized(Class<?> c); /** * Ensure the given class has been initialized. This is often * needed in conjunction with obtaining the static field base of a * class. */ public native void ensureClassInitialized(Class<?> c); /** * Report the offset of the first element in the storage allocation of a * given array class. If {@link #arrayIndexScale} returns a non-zero value * for the same class, you may use that scale factor, together with this * base offset, to form new offsets to access elements of arrays of the * given class. * * @see #getInt(Object, long) * @see #putInt(Object, long, int) */ public native int arrayBaseOffset(Class<?> arrayClass); /** The value of {@code arrayBaseOffset(boolean[].class)} */ public static final int ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class); /** The value of {@code arrayBaseOffset(byte[].class)} */ public static final int ARRAY_BYTE_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class); /** The value of {@code arrayBaseOffset(short[].class)} */ public static final int ARRAY_SHORT_BASE_OFFSET = theUnsafe.arrayBaseOffset(short[].class); /** The value of {@code arrayBaseOffset(char[].class)} */ public static final int ARRAY_CHAR_BASE_OFFSET = theUnsafe.arrayBaseOffset(char[].class); /** The value of {@code arrayBaseOffset(int[].class)} */ public static final int ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset(int[].class); /** The value of {@code arrayBaseOffset(long[].class)} */ public static final int ARRAY_LONG_BASE_OFFSET = theUnsafe.arrayBaseOffset(long[].class); /** The value of {@code arrayBaseOffset(float[].class)} */ public static final int ARRAY_FLOAT_BASE_OFFSET = theUnsafe.arrayBaseOffset(float[].class); /** The value of {@code arrayBaseOffset(double[].class)} */ public static final int ARRAY_DOUBLE_BASE_OFFSET = theUnsafe.arrayBaseOffset(double[].class); /** The value of {@code arrayBaseOffset(Object[].class)} */ public static final int ARRAY_OBJECT_BASE_OFFSET = theUnsafe.arrayBaseOffset(Object[].class); /** * Report the scale factor for addressing elements in the storage * allocation of a given array class. However, arrays of "narrow" types * will generally not work properly with accessors like {@link * #getByte(Object, int)}, so the scale factor for such classes is reported * as zero. * * @see #arrayBaseOffset * @see #getInt(Object, long) * @see #putInt(Object, long, int) */ public native int arrayIndexScale(Class<?> arrayClass); /** The value of {@code arrayIndexScale(boolean[].class)} */ public static final int ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class); /** The value of {@code arrayIndexScale(byte[].class)} */ public static final int ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale(byte[].class); /** The value of {@code arrayIndexScale(short[].class)} */ public static final int ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale(short[].class); /** The value of {@code arrayIndexScale(char[].class)} */ public static final int ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale(char[].class); /** The value of {@code arrayIndexScale(int[].class)} */ public static final int ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale(int[].class); /** The value of {@code arrayIndexScale(long[].class)} */ public static final int ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale(long[].class); /** The value of {@code arrayIndexScale(float[].class)} */ public static final int ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale(float[].class); /** The value of {@code arrayIndexScale(double[].class)} */ public static final int ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale(double[].class); /** The value of {@code arrayIndexScale(Object[].class)} */ public static final int ARRAY_OBJECT_INDEX_SCALE = theUnsafe.arrayIndexScale(Object[].class); /** * Report the size in bytes of a native pointer, as stored via {@link * #putAddress}. This value will be either 4 or 8. Note that the sizes of * other primitive types (as stored in native memory blocks) is determined * fully by their information content. */ public native int addressSize(); /** The value of {@code addressSize()} */ public static final int ADDRESS_SIZE = theUnsafe.addressSize(); /** * Report the size in bytes of a native memory page (whatever that is). * This value will always be a power of two. */ public native int pageSize(); /// random trusted operations from JNI: /** * Tell the VM to define a class, without security checks. By default, the * class loader and protection domain come from the caller‘s class. */ public native Class<?> defineClass(String name, byte[] b, int off, int len, ClassLoader loader, ProtectionDomain protectionDomain); /** * Define a class but do not make it known to the class loader or system dictionary. * <p> * For each CP entry, the corresponding CP patch must either be null or have * the a format that matches its tag: * <ul> * <li>Integer, Long, Float, Double: the corresponding wrapper object type from java.lang * <li>Utf8: a string (must have suitable syntax if used as signature or name) * <li>Class: any java.lang.Class object * <li>String: any object (not just a java.lang.String) * <li>InterfaceMethodRef: (NYI) a method handle to invoke on that call site‘s arguments * </ul> * @params hostClass context for linkage, access control, protection domain, and class loader * @params data bytes of a class file * @params cpPatches where non-null entries exist, they replace corresponding CP entries in data */ public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches); /** Allocate an instance but do not run any constructor. Initializes the class if it has not yet been. */ public native Object allocateInstance(Class<?> cls) throws InstantiationException; /** Lock the object. It must get unlocked via {@link #monitorExit}. */ @Deprecated public native void monitorEnter(Object o); /** * Unlock the object. It must have been locked via {@link * #monitorEnter}. */ @Deprecated public native void monitorExit(Object o); /** * Tries to lock the object. Returns true or false to indicate * whether the lock succeeded. If it did, the object must be * unlocked via {@link #monitorExit}. */ @Deprecated public native boolean tryMonitorEnter(Object o); /** Throw the exception without telling the verifier. */ public native void throwException(Throwable ee); /** * Atomically update Java variable to <tt>x</tt> if it is currently * holding <tt>expected</tt>. * @return <tt>true</tt> if successful */ public final native boolean compareAndSwapObject(Object o, long offset, Object expected, Object x); /** * Atomically update Java variable to <tt>x</tt> if it is currently * holding <tt>expected</tt>. * @return <tt>true</tt> if successful */ public final native boolean compareAndSwapInt(Object o, long offset, int expected, int x); /** * Atomically update Java variable to <tt>x</tt> if it is currently * holding <tt>expected</tt>. * @return <tt>true</tt> if successful */ public final native boolean compareAndSwapLong(Object o, long offset, long expected, long x); /** * Fetches a reference value from a given Java variable, with volatile * load semantics. Otherwise identical to {@link #getObject(Object, long)} */ public native Object getObjectVolatile(Object o, long offset); /** * Stores a reference value into a given Java variable, with * volatile store semantics. Otherwise identical to {@link #putObject(Object, long, Object)} */ public native void putObjectVolatile(Object o, long offset, Object x); /** Volatile version of {@link #getInt(Object, long)} */ public native int getIntVolatile(Object o, long offset); /** Volatile version of {@link #putInt(Object, long, int)} */ public native void putIntVolatile(Object o, long offset, int x); /** Volatile version of {@link #getBoolean(Object, long)} */ public native boolean getBooleanVolatile(Object o, long offset); /** Volatile version of {@link #putBoolean(Object, long, boolean)} */ public native void putBooleanVolatile(Object o, long offset, boolean x); /** Volatile version of {@link #getByte(Object, long)} */ public native byte getByteVolatile(Object o, long offset); /** Volatile version of {@link #putByte(Object, long, byte)} */ public native void putByteVolatile(Object o, long offset, byte x); /** Volatile version of {@link #getShort(Object, long)} */ public native short getShortVolatile(Object o, long offset); /** Volatile version of {@link #putShort(Object, long, short)} */ public native void putShortVolatile(Object o, long offset, short x); /** Volatile version of {@link #getChar(Object, long)} */ public native char getCharVolatile(Object o, long offset); /** Volatile version of {@link #putChar(Object, long, char)} */ public native void putCharVolatile(Object o, long offset, char x); /** Volatile version of {@link #getLong(Object, long)} */ public native long getLongVolatile(Object o, long offset); /** Volatile version of {@link #putLong(Object, long, long)} */ public native void putLongVolatile(Object o, long offset, long x); /** Volatile version of {@link #getFloat(Object, long)} */ public native float getFloatVolatile(Object o, long offset); /** Volatile version of {@link #putFloat(Object, long, float)} */ public native void putFloatVolatile(Object o, long offset, float x); /** Volatile version of {@link #getDouble(Object, long)} */ public native double getDoubleVolatile(Object o, long offset); /** Volatile version of {@link #putDouble(Object, long, double)} */ public native void putDoubleVolatile(Object o, long offset, double x); /** * Version of {@link #putObjectVolatile(Object, long, Object)} * that does not guarantee immediate visibility of the store to * other threads. This method is generally only useful if the * underlying field is a Java volatile (or if an array cell, one * that is otherwise only accessed using volatile accesses). */ public native void putOrderedObject(Object o, long offset, Object x); /** Ordered/Lazy version of {@link #putIntVolatile(Object, long, int)} */ public native void putOrderedInt(Object o, long offset, int x); /** Ordered/Lazy version of {@link #putLongVolatile(Object, long, long)} */ public native void putOrderedLong(Object o, long offset, long x); /** * Unblock the given thread blocked on <tt>park</tt>, or, if it is * not blocked, cause the subsequent call to <tt>park</tt> not to * block. Note: this operation is "unsafe" solely because the * caller must somehow ensure that the thread has not been * destroyed. Nothing special is usually required to ensure this * when called from Java (in which there will ordinarily be a live * reference to the thread) but this is not nearly-automatically * so when calling from native code. * @param thread the thread to unpark. * */ public native void unpark(Object thread); /** * Block current thread, returning when a balancing * <tt>unpark</tt> occurs, or a balancing <tt>unpark</tt> has * already occurred, or the thread is interrupted, or, if not * absolute and time is not zero, the given time nanoseconds have * elapsed, or if absolute, the given deadline in milliseconds * since Epoch has passed, or spuriously (i.e., returning for no * "reason"). Note: This operation is in the Unsafe class only * because <tt>unpark</tt> is, so it would be strange to place it * elsewhere. */ public native void park(boolean isAbsolute, long time); /** * Gets the load average in the system run queue assigned * to the available processors averaged over various periods of time. * This method retrieves the given <tt>nelem</tt> samples and * assigns to the elements of the given <tt>loadavg</tt> array. * The system imposes a maximum of 3 samples, representing * averages over the last 1, 5, and 15 minutes, respectively. * * @params loadavg an array of double of size nelems * @params nelems the number of samples to be retrieved and * must be 1 to 3. * * @return the number of samples actually retrieved; or -1 * if the load average is unobtainable. */ public native int getLoadAverage(double[] loadavg, int nelems); // The following contain CAS-based Java implementations used on // platforms not supporting native instructions /** * Atomically adds the given value to the current value of a field * or array element within the given object <code>o</code> * at the given <code>offset</code>. * * @param o object/array to update the field/element in * @param offset field/element offset * @param delta the value to add * @return the previous value * @since 1.8 */ public final int getAndAddInt(Object o, long offset, int delta) { int v; do { v = getIntVolatile(o, offset); } while (!compareAndSwapInt(o, offset, v, v + delta)); return v; } /** * Atomically adds the given value to the current value of a field * or array element within the given object <code>o</code> * at the given <code>offset</code>. * * @param o object/array to update the field/element in * @param offset field/element offset * @param delta the value to add * @return the previous value * @since 1.8 */ public final long getAndAddLong(Object o, long offset, long delta) { long v; do { v = getLongVolatile(o, offset); } while (!compareAndSwapLong(o, offset, v, v + delta)); return v; } /** * Atomically exchanges the given value with the current value of * a field or array element within the given object <code>o</code> * at the given <code>offset</code>. * * @param o object/array to update the field/element in * @param offset field/element offset * @param newValue new value * @return the previous value * @since 1.8 */ public final int getAndSetInt(Object o, long offset, int newValue) { int v; do { v = getIntVolatile(o, offset); } while (!compareAndSwapInt(o, offset, v, newValue)); return v; } /** * Atomically exchanges the given value with the current value of * a field or array element within the given object <code>o</code> * at the given <code>offset</code>. * * @param o object/array to update the field/element in * @param offset field/element offset * @param newValue new value * @return the previous value * @since 1.8 */ public final long getAndSetLong(Object o, long offset, long newValue) { long v; do { v = getLongVolatile(o, offset); } while (!compareAndSwapLong(o, offset, v, newValue)); return v; } /** * Atomically exchanges the given reference value with the current * reference value of a field or array element within the given * object <code>o</code> at the given <code>offset</code>. * * @param o object/array to update the field/element in * @param offset field/element offset * @param newValue new value * @return the previous value * @since 1.8 */ public final Object getAndSetObject(Object o, long offset, Object newValue) { Object v; do { v = getObjectVolatile(o, offset); } while (!compareAndSwapObject(o, offset, v, newValue)); return v; } /** * Ensures lack of reordering of loads before the fence * with loads or stores after the fence. * @since 1.8 */ public native void loadFence(); /** * Ensures lack of reordering of stores before the fence * with loads or stores after the fence. * @since 1.8 */ public native void storeFence(); /** * Ensures lack of reordering of loads or stores before the fence * with loads or stores after the fence. * @since 1.8 */ public native void fullFence(); /** * Throws IllegalAccessError; for use by the VM. * @since 1.8 */ private static void throwIllegalAccessError() { throw new IllegalAccessError(); } }
参考：《Java魔法类：Unsafe应用解析》
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