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：
@CallerSensitive public 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20 System.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1000000 System.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. */ @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(); } }
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