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Analysis and understanding of atomicintegerarray source code

2022-04-23 13:56:00 【InfoQ】

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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 equ
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al 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);

}

/**