AtomicIntegerArray源码分析与感悟

在讲AtomicIntegerArray之前，我先通过AtomicInteger阐述一下juc原子包的实现原理：   

// 注意是volatile类型的变量，可见性（一个线程对该值修改后，会马上同步到主内存，其

//他线程能马上看到新值）
private volatile int value; 

public final int getAndSet(int newValue) { 
   for (;;) {  // 使用循环重试与(比较并设置 cas),
      int current = get();   // 首先先获取当前值
      if (compareAndSet(current, newValue))  
           // 然后原子性执行比较并设置，如果在设置值的时候，原先的值是                                      // 刚获取的值，表示，该线程在调用 int current = get()方法后，执行compareAndSet
           // 之前没有其他线程修改过该值，则原子性更新为新的值，否则，重试。
           return current;
      }
}

AtomicIntegerArray，提供了一个原子访问数据中元素方法的一种途径。

对于这类原子操作，在理解上应该没什么问题，通过AtomitoIntegerArray，我学到如下几个关键点：

• int  sacle = unsafe.arrayIndexScale（int[].class）与  int base = unsafe.arrayBaseOffset(Int[].class);
  其中 unsafe.arrayBaseOffset 获取该类型的数组，在对象存储时，存放第一个元素的内存地址，相对于数组对象起始地址的内存偏移量。unsafe.arrayIndexSacle(int[].class) 获取该类型的数组中元素的大小，占用多少个字节。
• 根据scale ，base 如何准备定位到任意一个下标的地址呢？请查看如下关键代码块

static {
        int scale = unsafe.arrayIndexScale(int[].class);
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        shift = 31 - Integer.numberOfLeadingZeros(scale);
    }

    private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }

    private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }

用于定位元素偏移量的控制值。

举例说明，int scale = 4;1个int类型，在java中占用4个字节。

Integer.numberOfLeadingZeros(scale); 返回 scale 高位连续0的个数，得出shift = 2, 而shift在如下方法使用

得出结论了吧，shift就是 用来定位数组中的内存位置，用来移位用的，每向左移动移位，在不越界的情况下，想当于乘以2。也就是int类型的长度为4，也就是第0个位置是0，第1(i)个位置是4,，第二个(i)位置是8，也就是偏移位置等于  i * 4,也就是  i << 2;总结出一个乘法转换成移位操作的案例： a * (一个2的幂(n)的数)  =  a << n; 给出一个指定2的幂的数，怎么算成n,,参照shift的计算方法。 

附上AtomicIntegerArray 源码

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 
 */

/*
 *

 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;
import sun.misc.Unsafe;
import java.util.*;

/**
 * An {@code int} array in which elements may be updated atomically.
 * See the {@link java.util.concurrent.atomic} package
 * specification for description of the properties of atomic
 * variables.
 * @since 1.5
 * @author Doug Lea
 */
public class AtomicIntegerArray implements java.io.Serializable {
    private static final long serialVersionUID = 2862133569453604235L;

    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final int base = unsafe.arrayBaseOffset(int[].class);
    private static final int shift;
    private final int[] array;

    static {
        int scale = unsafe.arrayIndexScale(int[].class);
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        shift = 31 - Integer.numberOfLeadingZeros(scale);
    }

    private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }

    private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }

    /**
     * Creates a new AtomicIntegerArray of the given length, with all
     * elements initially zero.
     *
     * @param length the length of the array
     */
    public AtomicIntegerArray(int length) {
        array = new int[length];
    }

    /**
     * Creates a new AtomicIntegerArray with the same length as, and
     * all elements copied from, the given array.
     *
     * @param array the array to copy elements from
     * @throws NullPointerException if array is null
     */
    public AtomicIntegerArray(int[] array) {
        // Visibility guaranteed by final field guarantees
        this.array = array.clone();
    }

    /**
     * Returns the length of the array.
     *
     * @return the length of the array
     */
    public final int length() {
        return array.length;
    }

    /**
     * Gets the current value at position {@code i}.
     *
     * @param i the index
     * @return the current value
     */
    public final int get(int i) {
        return getRaw(checkedByteOffset(i));
    }

    private int getRaw(long offset) {
        return unsafe.getIntVolatile(array, offset);
    }

    /**
     * Sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     */
    public final void set(int i, int newValue) {
        unsafe.putIntVolatile(array, checkedByteOffset(i), newValue);
    }

    /**
     * Eventually sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(int i, int newValue) {
        unsafe.putOrderedInt(array, checkedByteOffset(i), newValue);
    }

    /**
     * Atomically sets the element at position {@code i} to the given
     * value and returns the old value.
     *
     * @param i the index
     * @param newValue the new value
     * @return the previous value
     */
    public final int getAndSet(int i, int newValue) {
        long offset = checkedByteOffset(i);
        while (true) {
            int current = getRaw(offset);
            if (compareAndSetRaw(offset, current, newValue))
                return current;
        }
    }

    /**
     * Atomically sets the element at position {@code i} to the given
     * updated value if the current value {@code ==} the expected value.
     *
     * @param i the index
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(int i, int expect, int update) {
        return compareAndSetRaw(checkedByteOffset(i), expect, update);
    }

    private boolean compareAndSetRaw(long offset, int expect, int update) {
        return unsafe.compareAndSwapInt(array, offset, expect, update);
    }

    /**
     * Atomically sets the element at position {@code i} to the given
     * updated value if the current value {@code ==} the expected value.
     *
     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param i the index
     * @param expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public final boolean weakCompareAndSet(int i, int expect, int update) {
        return compareAndSet(i, expect, update);
    }

    /**
     * Atomically increments by one the element at index {@code i}.
     *
     * @param i the index
     * @return the previous value
     */
    public final int getAndIncrement(int i) {
        return getAndAdd(i, 1);
    }

    /**
     * Atomically decrements by one the element at index {@code i}.
     *
     * @param i the index
     * @return the previous value
     */
    public final int getAndDecrement(int i) {
        return getAndAdd(i, -1);
    }

    /**
     * Atomically adds the given value to the element at index {@code i}.
     *
     * @param i the index
     * @param delta the value to add
     * @return the previous value
     */
    public final int getAndAdd(int i, int delta) {
        long offset = checkedByteOffset(i);
        while (true) {
            int current = getRaw(offset);
            if (compareAndSetRaw(offset, current, current + delta))
                return current;
        }
    }

    /**
     * Atomically increments by one the element at index {@code i}.
     *
     * @param i the index
     * @return the updated value
     */
    public final int incrementAndGet(int i) {
        return addAndGet(i, 1);
    }

    /**
     * Atomically decrements by one the element at index {@code i}.
     *
     * @param i the index
     * @return the updated value
     */
    public final int decrementAndGet(int i) {
        return addAndGet(i, -1);
    }

    /**
     * Atomically adds the given value to the element at index {@code i}.
     *
     * @param i the index
     * @param delta the value to add
     * @return the updated value
     */
    public final int addAndGet(int i, int delta) {
        long offset = checkedByteOffset(i);
        while (true) {
            int current = getRaw(offset);
            int next = current + delta;
            if (compareAndSetRaw(offset, current, next))
                return next;
        }
    }

    /**
     * Returns the String representation of the current values of array.
     * @return the String representation of the current values of array
     */
    public String toString() {
        int iMax = array.length - 1;
        if (iMax == -1)
            return "[]";

        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(getRaw(byteOffset(i)));
            if (i == iMax)
                return b.append(']').toString();
            b.append(',').append(' ');
        }
    }

}

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