sun.misc.Unsafe's APIs

Compare And Swap(CAS):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。

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。

Unsafe的APIS：

1.取得Unsafe实例。

    private static final sun.misc.Unsafe UNSAFE;

        try {            UNSAFE = sun.misc.Unsafe.getUnsafe();        } catch (Exception e) {            throw new Error(e);        }

2.native methods。
// **********************************************************************offset**************************************************************************

2.1 返回指定field的内存偏移地址，该值用于访问该field的特定方式，该值唯一的。

  /***   * 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.   *   * @param field the field whose offset should be returned.   * @return the offset of the given field.   */  public native long objectFieldOffset(Field field);

2.2 获取给定数组的第一个元素的偏移地址。

  /***   * 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.   * @param arrayClass the class for which the first element's address should   *                   be obtained.   * @return the offset of the first element of the array class.   * @see arrayIndexScale(Class)   */  public native int arrayBaseOffset(Class arrayClass);

2.3 获取用户给定数组寻址的换算因子。

  /***   * 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.   *    * @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);

计算数组元素的偏移地址方法：

Class tc = Object[].class;

final long BASE = UNSAFE.arrayBaseOffset(tc);

final int SHIFT = 31 - Integer.numberOfLeadingZeros(ts);

long u = (h << SHIFT) + BASE;

// **********************************************************************int**************************************************************************
2.4 CAS给定Object的int整型变量的值。

  /***   * 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.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the integer field within <code>obj</code>.   * @param expect the expected value of the field.   * @param update the new value of the field if it equals <code>expect</code>.   * @return true if the field was changed.   */  public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);

2.5 设置给定Object的int整型变量的值。该设置，其他线程不能立刻可见。

  /***   * 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.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the integer field within <code>obj</code>.   * @param value the new value of the field.   * @see #putIntVolatile(Object,long,int)   */  public native void putOrderedInt(Object obj, long offset, int value);

2.6 设置给定Object的int整型变量的值。该设置，其他线程立刻可见。

  /***   * Sets the value of the integer field at the specified offset in the   * supplied object to the given value, with volatile store semantics.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the integer field within <code>obj</code>.   * @param value the new value of the field.   */  public native void putIntVolatile(Object obj, long offset, int value);

2.7 获取给定Object的int整型变量的值，volatile加载。

  /***   * Retrieves the value of the integer field at the specified offset in the   * supplied object with volatile load semantics.   *    * @param obj the object containing the field to read.   * @param offset the offset of the integer field within <code>obj</code>.   */  public native int getIntVolatile(Object obj, long offset);

// **********************************************************************long**************************************************************************
2.8 CAS给定Object的long长整型变量的值。

  /***   * 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.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the long field within <code>obj</code>.   * @param expect the expected value of the field.   * @param update the new value of the field if it equals <code>expect</code>.   * @return true if the field was changed.   */  public native boolean compareAndSwapLong(Object obj, long offset, long expect, long update);

2.9 设置给定Object的long长整型变量的值。该设置，其他线程不能立刻可见。

  /***   * 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.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the long field within <code>obj</code>.   * @param value the new value of the field.   * @see #putLongVolatile(Object,long,long)   */  public native void putOrderedLong(Object obj, long offset, long value);

2.10 设置给定Object的long长整型变量的值。该设置，其他线程立刻可见。

  /***   * Sets the value of the long field at the specified offset in the   * supplied object to the given value, with volatile store semantics.   *   * @param obj the object containing the field to modify.   * @param offset the offset of the long field within <code>obj</code>.   * @param value the new value of the field.   * @see #putLong(Object,long,long)   */  public native void putLongVolatile(Object obj, long offset, long value);

2.11 获取给定Object的long整型变量的值，volatile加载。

  /***   * Retrieves the value of the long field at the specified offset in the   * supplied object with volatile load semantics.   *    * @param obj the object containing the field to read.   * @param offset the offset of the long field within <code>obj</code>.   * @see #getLong(Object,long)   */  public native long getLongVolatile(Object obj, long offset);

// **********************************************************************Object**************************************************************************

2.12 CAS给定Object的对象类型变量的值。

  /***   * 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.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the object field within <code>obj</code>.   * @param expect the expected value of the field.   * @param update the new value of the field if it equals <code>expect</code>.   * @return true if the field was changed.   */  public native boolean compareAndSwapObject(Object obj, long offset, Object expect, Object update);

2.13 设置给定Object的对象类型变量的值。该设置，其他线程不能立刻可见。

  /***   * 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.   *   * @param obj the object containing the field to modify.   * @param offset the offset of the object field within <code>obj</code>.   * @param value the new value of the field.   */  public native void putOrderedObject(Object obj, long offset, Object value);

2.14 设置给定Object的对象类型变量的值。该设置，其他线程立刻可见。

  /***   * Sets the value of the object field at the specified offset in the   * supplied object to the given value, with volatile store semantics.   *    * @param obj the object containing the field to modify.   * @param offset the offset of the object field within <code>obj</code>.   * @param value the new value of the field.   * @see #putObject(Object,long,Object)   */  public native void putObjectVolatile(Object obj, long offset, Object value);

2.15 获取给定Object的对象类型变量的值，volatile加载。

  /***   * Retrieves the value of the object field at the specified offset in the   * supplied object with volatile load semantics.   *    * @param obj the object containing the field to read.   * @param offset the offset of the object field within <code>obj</code>.   */  public native Object getObjectVolatile(Object obj, long offset);

// **********************************************************************block-thread**************************************************************************
2.16 阻塞线程。

  /***   * 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 <code>isAbsolute</code> is   * <code>true</code>, the timeout is in milliseconds relative to the   * epoch.  Otherwise, the value is the number of nanoseconds which must   * occur before timeout.  This call may also return spuriously (i.e.   * for no apparent reason).   *    * @param isAbsolute true if the timeout is specified in milliseconds from   *                   the epoch.   * @param time either the number of nanoseconds to wait, or a time in   *             milliseconds from the epoch to wait for.   */  public native void park(boolean isAbsolute, long time);

2.17 释放park的阻塞。

  /***   * 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.   * @param thread the thread to unblock.   */  public native void unpark(Thread thread);