Java中Unsafe类详解

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http://www.cnblogs.com/mickole/articles/3757278.html

Java不能直接访问操作系统底层,而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作,主要提供了以下功能:

1、通过Unsafe类可以分配内存,可以释放内存;

类中提供的3个本地方法allocateMemory、reallocateMemory、freeMemory分别用于分配内存,扩充内存和释放内存,与C语言中的3个方法对应。

public native long allocateMemory(long l);
public native long reallocateMemory(long l, long l1);
public native void freeMemory(long l);

 

2、可以定位对象某字段的内存位置,也可以修改对象的字段值,即使它是私有的;

字段的定位:

JAVA中对象的字段的定位可能通过staticFieldOffset方法实现,该方法返回给定field的内存地址偏移量,这个值对于给定的filed是唯一的且是固定不变的。

getIntVolatile方法获取对象中offset偏移地址对应的整型field的值,支持volatile load语义。

getLong方法获取对象中offset偏移地址对应的long型field的值

数组元素定位:

Unsafe类中有很多以BASE_OFFSET结尾的常量,比如ARRAY_INT_BASE_OFFSET,ARRAY_BYTE_BASE_OFFSET等,这些常量值是通过arrayBaseOffset方法得到的。arrayBaseOffset方法是一个本地方法,可以获取数组第一个元素的偏移地址。Unsafe类中还有很多以INDEX_SCALE结尾的常量,比如 ARRAY_INT_INDEX_SCALE , ARRAY_BYTE_INDEX_SCALE等,这些常量值是通过arrayIndexScale方法得到的。arrayIndexScale方法也是一个本地方法,可以获取数组的转换因子,也就是数组中元素的增量地址。将arrayBaseOffset与arrayIndexScale配合使用,可以定位数组中每个元素在内存中的位置。

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public final class Unsafe {
    public static final int ARRAY_INT_BASE_OFFSET;
    public static final int ARRAY_INT_INDEX_SCALE;

    public native long staticFieldOffset(Field field);
    public native int getIntVolatile(Object obj, long l);
    public native long getLong(Object obj, long l);
    public native int arrayBaseOffset(Class class1);
    public native int arrayIndexScale(Class class1);

    static 
    {
        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset([I);
        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale([I);
    }
}
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3、挂起与恢复

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。

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public class LockSupport {
    public static void unpark(Thread thread) {
        if (thread != null)
            unsafe.unpark(thread);
    }

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        unsafe.park(false, 0L);
        setBlocker(t, null);
    }

    public static void parkNanos(Object blocker, long nanos) {
        if (nanos > 0) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            unsafe.park(false, nanos);
            setBlocker(t, null);
        }
    }

    public static void parkUntil(Object blocker, long deadline) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        unsafe.park(true, deadline);
        setBlocker(t, null);
    }

    public static void park() {
        unsafe.park(false, 0L);
    }

    public static void parkNanos(long nanos) {
        if (nanos > 0)
            unsafe.park(false, nanos);
    }

    public static void parkUntil(long deadline) {
        unsafe.park(true, deadline);
    }
}
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4、CAS操作

是通过compareAndSwapXXX方法实现的

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/**
* 比较obj的offset处内存位置中的值和期望的值,如果相同则更新。此更新是不可中断的。
* 
* @param obj 需要更新的对象
* @param offset obj中整型field的偏移量
* @param expect 希望field中存在的值
* @param update 如果期望值expect与field的当前值相同,设置filed的值为这个新值
* @return 如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);
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CAS操作有3个操作数,内存值M,预期值E,新值U,如果M==E,则将内存值修改为B,否则啥都不做。

参考资料:

Unsafe.h

源码剖析之sun.misc.Unsafe

注:转自http://blog.csdn.net/aesop_wubo/article/details/7537278


首先介绍一下什么是Compare And Swap(CAS)?简单的说就是比较并交换。

CAS 操作包含三个操作数 —— 内存位置(V)、预期原值(A)和新值(B)。如果内存位置的值与预期原值相匹配,那么处理器会自动将该位置值更新为新值。否则,处理器不做任何操作。无论哪种情况,它都会在 CAS 指令之前返回该位置的值。CAS 有效地说明了“我认为位置 V 应该包含值 A;如果包含该值,则将 B 放到这个位置;否则,不要更改该位置,只告诉我这个位置现在的值即可。” Java并发包(java.util.concurrent)中大量使用了CAS操作,涉及到并发的地方都调用了sun.misc.Unsafe类方法进行CAS操作。

在看一下volatile, Volatile修饰的成员变量在每次被线程访问时,都强迫从共享内存中重读该成员变量的值。而且,当成员变量发生变化时,强迫线程将变化值回写到共享内存。这样在任何时刻,两个不同的线程总是看到某个成员变量的值是相同的,更简单一点理解就是volatile修饰的变量值发生变化时对于另外的线程是可见的。

如何正确使用volatile可以参考下面这篇文章:

http://www.ibm.com/developerworks/cn/java/j-jtp06197.html Java 理论与实践: 正确使用 Volatile 变量

下面来看看java中具体的CAS操作类sun.misc.Unsafe。Unsafe类提供了硬件级别的原子操作,Java无法直接访问到操作系统底层(如系统硬件等),为此Java使用native方法来扩展Java程序的功能。具体实现使用c++,详见文件sun.misc.natUnsafe.cc();sun.misc包的源代码可以在这里找到:

http://www.oschina.net/code/explore/gcc-4.5.2/libjava/sun/misc

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//下面是sun.misc.Unsafe.java类源码
package sun.misc;
import java.lang.reflect.Field;
/***
 * This class should provide access to low-level operations and its
 * use should be limited to trusted code.  Fields can be accessed using
 * memory addresses, with undefined behaviour occurring if invalid memory
 * addresses are given.
 * 这个类提供了一个更底层的操作并且应该在受信任的代码中使用。可以通过内存地址
 * 存取fields,如果给出的内存地址是无效的那么会有一个不确定的运行表现。
 * 
 * @author Tom Tromey (tromey@redhat.com)
 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
 */
public class Unsafe
{
  // Singleton class.
  private static Unsafe unsafe = new Unsafe();
  /***
   * Private default constructor to prevent creation of an arbitrary
   * number of instances.
   * 使用私有默认构造器防止创建多个实例
   */
  private Unsafe()
  {
  }
  /***
   * Retrieve the singleton instance of <code>Unsafe</code>.  The calling
   * method should guard this instance from untrusted code, as it provides
   * access to low-level operations such as direct memory access.
   * 获取<code>Unsafe</code>的单例,这个方法调用应该防止在不可信的代码中实例,
   * 因为unsafe类提供了一个低级别的操作,例如直接内存存取。
   * 
   * @throws SecurityException if a security manager exists and prevents
   *                           access to the system properties.
   *                           如果安全管理器不存在或者禁止访问系统属性
   */
  public static Unsafe getUnsafe()
  {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null)
      sm.checkPropertiesAccess();
    return unsafe;
  }
  
  /***
   * Returns the memory address offset of the given static field.
   * The offset is merely used as a means to access a particular field
   * in the other methods of this class.  The value is unique to the given
   * field and the same value should be returned on each subsequent call.
   * 返回指定静态field的内存地址偏移量,在这个类的其他方法中这个值只是被用作一个访问
   * 特定field的一个方式。这个值对于 给定的field是唯一的,并且后续对该方法的调用都应该
   * 返回相同的值。
   *
   * @param field the field whose offset should be returned.
   *              需要返回偏移量的field
   * @return the offset of the given field.
   *         指定field的偏移量
   */
  public native long objectFieldOffset(Field field);
  /***
   * Compares the value of the integer field at the specified offset
   * in the supplied object with the given expected value, and updates
   * it if they match.  The operation of this method should be atomic,
   * thus providing an uninterruptible way of updating an integer field.
   * 在obj的offset位置比较integer field和期望的值,如果相同则更新。这个方法
   * 的操作应该是原子的,因此提供了一种不可中断的方式更新integer field。
   * 
   * @param obj the object containing the field to modify.
   *            包含要修改field的对象
   * @param offset the offset of the integer field within <code>obj</code>.
   *               <code>obj</code>中整型field的偏移量
   * @param expect the expected value of the field.
   *               希望field中存在的值
   * @param update the new value of the field if it equals <code>expect</code>.
   *           如果期望值expect与field的当前值相同,设置filed的值为这个新值
   * @return true if the field was changed.
   *                             如果field的值被更改
   */
  public native boolean compareAndSwapInt(Object obj, long offset,
                                          int expect, int update);
  /***
   * Compares the value of the long field at the specified offset
   * in the supplied object with the given expected value, and updates
   * it if they match.  The operation of this method should be atomic,
   * thus providing an uninterruptible way of updating a long field.
   * 在obj的offset位置比较long field和期望的值,如果相同则更新。这个方法
   * 的操作应该是原子的,因此提供了一种不可中断的方式更新long field。
   * 
   * @param obj the object containing the field to modify.
   *              包含要修改field的对象 
   * @param offset the offset of the long field within <code>obj</code>.
   *               <code>obj</code>中long型field的偏移量
   * @param expect the expected value of the field.
   *               希望field中存在的值
   * @param update the new value of the field if it equals <code>expect</code>.
   *               如果期望值expect与field的当前值相同,设置filed的值为这个新值
   * @return true if the field was changed.
   *              如果field的值被更改
   */
  public native boolean compareAndSwapLong(Object obj, long offset,
                                           long expect, long update);
  /***
   * Compares the value of the object field at the specified offset
   * in the supplied object with the given expected value, and updates
   * it if they match.  The operation of this method should be atomic,
   * thus providing an uninterruptible way of updating an object field.
   * 在obj的offset位置比较object field和期望的值,如果相同则更新。这个方法
   * 的操作应该是原子的,因此提供了一种不可中断的方式更新object field。
   * 
   * @param obj the object containing the field to modify.
   *    包含要修改field的对象 
   * @param offset the offset of the object field within <code>obj</code>.
   *         <code>obj</code>中object型field的偏移量
   * @param expect the expected value of the field.
   *               希望field中存在的值
   * @param update the new value of the field if it equals <code>expect</code>.
   *               如果期望值expect与field的当前值相同,设置filed的值为这个新值
   * @return true if the field was changed.
   *              如果field的值被更改
   */
  public native boolean compareAndSwapObject(Object obj, long offset,
                                             Object expect, Object update);
  /***
   * Sets the value of the integer field at the specified offset in the
   * supplied object to the given value.  This is an ordered or lazy
   * version of <code>putIntVolatile(Object,long,int)</code>, which
   * doesn\'t guarantee the immediate visibility of the change to other
   * threads.  It is only really useful where the integer field is
   * <code>volatile</code>, and is thus expected to change unexpectedly.
   * 设置obj对象中offset偏移地址对应的整型field的值为指定值。这是一个有序或者
   * 有延迟的<code>putIntVolatile</cdoe>方法,并且不保证值的改变被其他线程立
   * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
   * 使用才有用。
   * 
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the integer field within <code>obj</code>.
   *       <code>obj</code>中整型field的偏移量
   * @param value the new value of the field.
   *      field将被设置的新值
   * @see #putIntVolatile(Object,long,int)
   */
  public native void putOrderedInt(Object obj, long offset, int value);
  /***
   * Sets the value of the long field at the specified offset in the
   * supplied object to the given value.  This is an ordered or lazy
   * version of <code>putLongVolatile(Object,long,long)</code>, which
   * doesn\'t guarantee the immediate visibility of the change to other
   * threads.  It is only really useful where the long field is
   * <code>volatile</code>, and is thus expected to change unexpectedly.
   * 设置obj对象中offset偏移地址对应的long型field的值为指定值。这是一个有序或者
   * 有延迟的<code>putLongVolatile</cdoe>方法,并且不保证值的改变被其他线程立
   * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
   * 使用才有用。
   * 
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the long field within <code>obj</code>.
   *       <code>obj</code>中long型field的偏移量
   * @param value the new value of the field.
   *      field将被设置的新值
   * @see #putLongVolatile(Object,long,long)
   */
  public native void putOrderedLong(Object obj, long offset, long value);
  /***
   * Sets the value of the object field at the specified offset in the
   * supplied object to the given value.  This is an ordered or lazy
   * version of <code>putObjectVolatile(Object,long,Object)</code>, which
   * doesn\'t guarantee the immediate visibility of the change to other
   * threads.  It is only really useful where the object field is
   * <code>volatile</code>, and is thus expected to change unexpectedly.
   * 设置obj对象中offset偏移地址对应的object型field的值为指定值。这是一个有序或者
   * 有延迟的<code>putObjectVolatile</cdoe>方法,并且不保证值的改变被其他线程立
   * 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
   * 使用才有用。
   *
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the object field within <code>obj</code>.
   *       <code>obj</code>中long型field的偏移量
   * @param value the new value of the field.
   *      field将被设置的新值
   */
  public native void putOrderedObject(Object obj, long offset, Object value);
  /***
   * Sets the value of the integer field at the specified offset in the
   * supplied object to the given value, with volatile store semantics.
   * 设置obj对象中offset偏移地址对应的整型field的值为指定值。支持volatile store语义
   * 
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the integer field within <code>obj</code>.
   *       <code>obj</code>中整型field的偏移量
   * @param value the new value of the field.
   *       field将被设置的新值
   */
  public native void putIntVolatile(Object obj, long offset, int value);
  /***
   * Retrieves the value of the integer field at the specified offset in the
   * supplied object with volatile load semantics.
   * 获取obj对象中offset偏移地址对应的整型field的值,支持volatile load语义。
   * 
   * @param obj the object containing the field to read.
   *    包含需要去读取的field的对象
   * @param offset the offset of the integer field within <code>obj</code>.
   *       <code>obj</code>中整型field的偏移量
   */
  public native int getIntVolatile(Object obj, long offset);
  /***
   * Sets the value of the long field at the specified offset in the
   * supplied object to the given value, with volatile store semantics.
   * 设置obj对象中offset偏移地址对应的long型field的值为指定值。支持volatile store语义
   *
   * @param obj the object containing the field to modify.
   *            包含需要修改field的对象
   * @param offset the offset of the long field within <code>obj</code>.
   *               <code>obj</code>中long型field的偏移量
   * @param value the new value of the field.
   *              field将被设置的新值
   * @see #putLong(Object,long,long)
   */
  public native void putLongVolatile(Object obj, long offset, long value);
  /***
   * Sets the value of the long field at the specified offset in the
   * supplied object to the given value.
   * 设置obj对象中offset偏移地址对应的long型field的值为指定值。
   * 
   * @param obj the object containing the field to modify.
   *     包含需要修改field的对象
   * @param offset the offset of the long field within <code>obj</code>.
   *     <code>obj</code>中long型field的偏移量
   * @param value the new value of the field.
   *     field将被设置的新值
   * @see #putLongVolatile(Object,long,long)
   */
  public native void putLong(Object obj, long offset, long value);
  /***
   * Retrieves the value of the long field at the specified offset in the
   * supplied object with volatile load semantics.
   * 获取obj对象中offset偏移地址对应的long型field的值,支持volatile load语义。
   * 
   * @param obj the object containing the field to read.
   *    包含需要去读取的field的对象
   * @param offset the offset of the long field within <code>obj</code>.
   *       <code>obj</code>中long型field的偏移量
   * @see #getLong(Object,long)
   */
  public native long getLongVolatile(Object obj, long offset);
  /***
   * Retrieves the value of the long field at the specified offset in the
   * supplied object.
   * 获取obj对象中offset偏移地址对应的long型field的值
   * 
   * @param obj the object containing the field to read.
   *    包含需要去读取的field的对象
   * @param offset the offset of the long field within <code>obj</code>.
   *       <code>obj</code>中long型field的偏移量
   * @see #getLongVolatile(Object,long)
   */
  public native long getLong(Object obj, long offset);
  /***
   * Sets the value of the object field at the specified offset in the
   * supplied object to the given value, with volatile store semantics.
   * 设置obj对象中offset偏移地址对应的object型field的值为指定值。支持volatile store语义
   * 
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the object field within <code>obj</code>.
   *     <code>obj</code>中object型field的偏移量
   * @param value the new value of the field.
   *       field将被设置的新值
   * @see #putObject(Object,long,Object)
   */
  public native void putObjectVolatile(Object obj, long offset, Object value);
  /***
   * Sets the value of the object field at the specified offset in the
   * supplied object to the given value.
   * 设置obj对象中offset偏移地址对应的object型field的值为指定值。
   * 
   * @param obj the object containing the field to modify.
   *    包含需要修改field的对象
   * @param offset the offset of the object field within <code>obj</code>.
   *     <code>obj</code>中object型field的偏移量
   * @param value the new value of the field.
   *       field将被设置的新值
   * @see #putObjectVolatile(Object,long,Object)
   */
  public native void putObject(Object obj, long offset, Object value);
  /***
   * Retrieves the value of the object field at the specified offset in the
   * supplied object with volatile load semantics.
   * 获取obj对象中offset偏移地址对应的object型field的值,支持volatile load语义。
   * 
   * @param obj the object containing the field to read.
   *    包含需要去读取的field的对象
   * @param offset the offset of the object field within <code>obj</code>.
   *       <code>obj</code>中object型field的偏移量
   */
  public native Object getObjectVolatile(Object obj, long offset);
  /***
   * Returns the offset of the first element for a given array class.
   * To access elements of the array class, this value may be used along with
   * with that returned by 
   * <a href="#arrayIndexScale"><code>arrayIndexScale</code></a>,
   * if non-zero.
   * 获取给定数组中第一个元素的偏移地址。
   * 为了存取数组中的元素,这个偏移地址与<a href="#arrayIndexScale"><code>arrayIndexScale
   * </code></a>方法的非0返回值一起被使用。
   * @param arrayClass the class for which the first element\'s address should
   *                   be obtained.
   *                   第一个元素地址被获取的class
   * @return the offset of the first element of the array class.
   *    数组第一个元素 的偏移地址
   * @see arrayIndexScale(Class)
   */
  public native int arrayBaseOffset(Class arrayClass);
  /***
   * Returns the scale factor used for addressing elements of the supplied
   * array class.  Where a suitable scale factor can not be returned (e.g.
   * for primitive types), zero should be returned.  The returned value
   * can be used with 
   * <a href="#arrayBaseOffset"><code>arrayBaseOffset</code></a>
   * to access elements of the class.
   * 获取用户给定数组寻址的换算因子.一个合适的换算因子不能返回的时候(例如:基本类型),
   * 返回0.这个返回值能够与<a href="#arrayBaseOffset"><code>arrayBaseOffset</code>
   * </a>一起使用去存取这个数组class中的元素
   * 
   * @param arrayClass the class whose scale factor should be returned.
   * @return the scale factor, or zero if not supported for this array class.
   */
  public native int arrayIndexScale(Class arrayClass);
  
  /***
   * Releases the block on a thread created by 
   * <a href="#park"><code>park</code></a>.  This method can also be used
   * to terminate a blockage caused by a prior call to <code>park</code>.
   * This operation is unsafe, as the thread must be guaranteed to be
   * live.  This is true of Java, but not native code.
   * 释放被<a href="#park"><code>park</code></a>创建的在一个线程上的阻塞.这个
   * 方法也可以被使用来终止一个先前调用<code>park</code>导致的阻塞.
   * 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。
   * @param thread the thread to unblock.
   *           要解除阻塞的线程
   */
  public native void unpark(Thread thread);
  /***
   * Blocks the thread until a matching 
   * <a href="#unpark"><code>unpark</code></a> occurs, the thread is
   * interrupted or the optional timeout expires.  If an <code>unpark</code>
   * call has already occurred, this also counts.  A timeout value of zero
   * is defined as no timeout.  When &

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