版权声明： https://blog.csdn.net/jiangxuexuanshuang/article/details/88141056
说明：AtomicIntegerArray与AtomicLongArray的实现逻辑一致，Integer的shift为2，Long的shift为8，然后操作时各自调用相应的Integer/Long的原子操作。
1、成员变量定义
array：实际保存的数据
base：int[]开辟地址空间的起始地址。这个有点不明白的是，base为什么每个都一样，那是怎么区分不同对象保存的位置。
shift：该类型的字节位移数。比如int四个字节，则shift为2，即1<<2，也就是每个int起始位置都是4的倍数，与int占四个字节吻合。
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;2、初始化
构造函数一开始便指定了此对象的大小。
并初始化了原子数组的每个位需要移位的数量
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);
}
/**
* 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();
}3、long checkedByteOffset(int i)：计算下标的偏移字节量
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;
}4、虽然array定义时并没有使用volatile，但是取值设置值时使用了与volatile相关的方法getIntVolatile/putIntVolatile，保证了取值过程在多线程间的可见性。取值也是使用数组内存中的偏移字节。
/**
* 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);
}
5、其余值的设置与AtomicInteger类似，不一样的是多了一步计算元素主存中偏移量的操作。如果数组长度为1实际便退化成类似AtomicInteger的操作了。
/**
* 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) {
return unsafe.getAndSetInt(array, checkedByteOffset(i), newValue);
}
/**
* 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 {@code 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><a href="package-summary.html#weakCompareAndSet">May fail
* spuriously and does not provide ordering guarantees</a>, 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 {@code 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) {
return unsafe.getAndAddInt(array, checkedByteOffset(i), delta);
}
/**
* 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 getAndAdd(i, 1) + 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 getAndAdd(i, -1) - 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) {
return getAndAdd(i, delta) + delta;
}