接下来再看一个JDK中比较特殊的类Unsafe。

一、概述

Java和C++语言的一个重要区别就是Java中我们无法直接操作一块内存区域，不能像C++中那样可以自己申请内存和释放内存。Java中的Unsafe类为我们提供了类似C++手动管理内存的能力。
Unsafe类，全限定名是sun.misc.Unsafe，从名字中我们可以看出来这个类对普通程序员来说是“危险”的，一般应用开发者不会用到这个类。它不属于Java标准。但是很多Java的基础类库，包括一些被广泛使用的高性能开发库都是基于Unsafe类开发的，比如Netty、Cassandra、Hadoop、Kafka等。Unsafe类在提升Java运行效率，增强Java语言底层操作能力方面起了很大的作用。

Unsafe类使Java拥有了像指针一样操作内存的能力，但同时也带来了指针问题。过度的使用Unsafe类会使得出错的几率变大，因此Java官方并不建议使用的，官方文档也几乎没有。Oracle正在计划从Java中去掉Unsafe类，如果真是如此影响就太大了。

二、类介绍

Unsafe类是"final"的，不允许继承。且构造函数是private的。

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();....
}

Unsafe可以通过静态方法getUnsafe()来进行实例化，源码如下：

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

Unsafe类做了限制，如果是普通的调用的话，它会抛出一个SecurityException异常；只有由主类加载器(BootStrap classLoader)加载的类才能调用这个类中的方法。最简单的使用方式是基于反射获取Unsafe实例。示例如下：

publice static Unsafe getUnsafe(){try {Field field = Unsafe.class.getDeclaredField("theUnsafe");field.setAccessible(true);Unsafe unsafe = (Unsafe) field.get(null);return unsafe;} catch (Exception e) {e.printStackTrace();}return null;
}

三、常见方法介绍

1.直接内存操作。

该部分包括了allocateMemory（分配内存）、reallocateMemory（重新分配内存）、copyMemory（拷贝内存）、freeMemory（释放内存）、getAddress（获取内存地址）、addressSize、pageSize、getInt（获取内存地址指向的整数）、getIntVolatile（获取内存地址指向的整数，并支持volatile语义）、putInt（将整数写入指定内存地址）、putIntVolatile（将整数写入指定内存地址，并支持volatile语义）、putOrderedInt（将整数写入指定内存地址、有序或者有延迟的方法）等方法。getXXX和putXXX包含了各种基本类型的操作。

利用copyMemory方法，我们可以实现一个通用的对象拷贝方法，无需再对每一个对象都实现clone方法，当然这通用的方法只能做到对象浅拷贝。

2.非常规的对象实例化。

allocateInstance()方法提供了另一种创建实例的途径。通常我们可以用new或者反射来实例化对象，使用allocateInstance()方法可以直接生成对象实例，且无需调用构造方法和其它初始化方法。

这在对象反序列化的时候会很有用，能够重建和设置final字段，而不需要调用构造方法。

3.操作类、对象、变量。

这部分包括了staticFieldOffset（静态域偏移）、defineClass（定义类）、defineAnonymousClass（定义匿名类）、ensureClassInitialized（确保类初始化）、objectFieldOffset（对象域偏移）等方法。

通过这些方法我们可以获取对象的指针，通过对指针进行偏移，我们不仅可以直接修改指针指向的数据（即使它们是私有的），甚至可以找到JVM已经认定为垃圾、可以进行回收的对象。

4.数组操作。

这部分包括了arrayBaseOffset（获取数组第一个元素的偏移地址）、arrayIndexScale（获取数组中元素的增量地址）等方法。arrayBaseOffset与arrayIndexScale配合起来使用，就可以定位数组中每个元素在内存中的位置。

由于Java的数组最大值为Integer.MAX_VALUE，使用Unsafe类的内存分配方法可以实现超大数组。实际上这样的数据就可以认为是C数组，因此需要注意在合适的时间释放内存。

5.多线程同步。包括锁机制、CAS操作等。

这部分包括了monitorEnter、tryMonitorEnter、monitorExit、compareAndSwapInt、compareAndSwap等方法。

其中monitorEnter、tryMonitorEnter、monitorExit已经被标记为deprecated，不建议使用。

Unsafe类的CAS操作可能是用的最多的，它为Java的锁机制提供了一种新的解决办法，比如AtomicInteger等类都是通过该方法来实现的。compareAndSwap方法是原子的，可以避免繁重的锁机制，提高代码效率。这是一种乐观锁，通常认为在大部分情况下不出现竞态条件，如果操作失败，会不断重试直到成功。

6.线程相关。

这部分包括了park、unpark等方法。

将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。

7.内存屏障。

这部分包括了loadFence、storeFence、fullFence等方法。这是在Java 8新引入的，用于定义内存屏障，避免代码重排序。

loadFence() 表示该方法之前的所有load操作在内存屏障之前完成。同理storeFence()表示该方法之前的所有store操作在内存屏障之前完成。fullFence()表示该方法之前的所有load、store操作在内存屏障之前完成。

四、Unsafe的使用方法

1.使用Unsafe实例化一个类

假如我们有一个简单的类如下：

class User {int age;public User() {this.age = 10;}
}

如果我们通过构造方法实例化这个类，age属性将会返回10。

User user1 = new User();
// 打印10
System.out.println(user1.age);

如果我们调用Unsafe来实例化呢？

User user2 = (User) unsafe.allocateInstance(User.class);
// 打印0
System.out.println(user2.age);

age将返回0，因为Unsafe.allocateInstance()只会给对象分配内存，并不会调用构造方法，所以这里只会返回int类型的默认值0。

2.修改私有字段的值

public class UnsafeTest {public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, InstantiationException {Field f = Unsafe.class.getDeclaredField("theUnsafe");f.setAccessible(true);Unsafe unsafe = (Unsafe) f.get(null);User user = new User();Field age = user.getClass().getDeclaredField("age");unsafe.putInt(user, unsafe.objectFieldOffset(age), 20);// 打印20System.out.println(user.getAge());}
}class User {private int age;public User() {this.age = 10;}public int getAge() {return age;}
}

一旦我们通过反射调用得到字段age，我们就可以使用Unsafe将其值更改为任何其他int值。（当然，这里也可以通过反射直接修改）

3.抛出checked异常

我们知道如果代码抛出了checked异常，要不就使用try...catch捕获它，要不就在方法签名上定义这个异常，但是，通过Unsafe我们可以抛出一个checked异常，同时却不用捕获或在方法签名上定义它。

    // 使用正常方式抛出IOException需要定义在方法签名上往外抛public static void readFile() throws IOException {throw new IOException();}// 使用Unsafe抛出异常不需要定义在方法签名上往外抛public static void readFileUnsafe() {unsafe.throwException(new IOException());}

4.CompareAndSwap操作

JUC下面大量使用了CAS操作，它们的底层是调用的Unsafe的CompareAndSwapXXX()方法。这种方式广泛运用于无锁算法，与java中标准的悲观锁机制相比，它可以利用CAS处理器指令提供极大的加速。

比如，我们可以基于Unsafe的compareAndSwapInt()方法构建线程安全的计数器。

class Counter {private volatile int count = 0;private static long offset;private static Unsafe unsafe;static {try {Field f = Unsafe.class.getDeclaredField("theUnsafe");f.setAccessible(true);unsafe = (Unsafe) f.get(null);offset = unsafe.objectFieldOffset(Counter.class.getDeclaredField("count"));} catch (NoSuchFieldException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();}}public void increment() {int before = count;// 失败了就重试直到成功为止while (!unsafe.compareAndSwapInt(this, offset, before, before + 1)) {before = count;}}public int getCount() {return count;}
}

我们定义了一个volatile的字段count，以便对它的修改所有线程都可见，并在类加载的时候获取count在类中的偏移地址。

在increment()方法中，我们通过调用Unsafe的compareAndSwapInt()方法来尝试更新之前获取到的count的值，如果它没有被其它线程更新过，则更新成功，否则不断重试直到成功为止。

我们可以通过使用多个线程来测试我们的代码：

    Counter counter = new Counter();ExecutorService threadPool = Executors.newFixedThreadPool(100);// 起100个线程，每个线程自增10000次IntStream.range(0, 100).forEach(i->threadPool.submit(()->IntStream.range(0, 10000).forEach(j->counter.increment())));threadPool.shutdown();Thread.sleep(2000);// 打印1000000System.out.println(counter.getCount());

五、Unsafe的使用建议

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

使用Unsafe要注意以下几个问题：

1、Unsafe有可能在未来的Jdk版本移除或者不允许Java应用代码使用，这一点可能导致使用了Unsafe的应用无法运行在高版本的Jdk。
2、Unsafe的不少方法中必须提供原始地址(内存地址)和被替换对象的地址，偏移量要自己计算，一旦出现问题就是JVM崩溃级别的异常，会导致整个JVM实例崩溃，表现为应用程序直接crash掉。
3、Unsafe提供的直接内存访问的方法中使用的内存不受JVM管理(无法被GC)，需要手动管理，一旦出现疏忽很有可能成为内存泄漏的源头。

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

六、附录

Unsafe类完整源码，来自于jdk-78d2004f65eb：

/** 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.*/@CallerSensitivepublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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.*/@Deprecatedpublic int fieldOffset(Field f) {if (Modifier.isStatic(f.getModifiers()))return (int) staticFieldOffset(f);elsereturn (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.*/@Deprecatedpublic 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}. */@Deprecatedpublic native void monitorEnter(Object o);/*** Unlock the object.  It must have been locked via {@link* #monitorEnter}.*/@Deprecatedpublic 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}.*/@Deprecatedpublic 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();}}

参考资料：

https://www.cnblogs.com/pkufork/p/java_unsafe.html
https://www.zhihu.com/question/29266773?sort=created
https://www.cnblogs.com/throwable/p/9139947.html
https://www.jianshu.com/p/db8dce09232d
https://www.jianshu.com/p/cda24891f9e4
https://www.jb51.net/article/161858.htm

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