AtomicIntegerArray源码解析

Posted on | In , ,
Words count in article: 2.8k | Reading time ≈ 14

AtomicIntegerArray源码解析

顾名思义，AtomicIntegerArray就是JDK提供的提供原子操作能力的整型数组。之所以提供这个类是因为整型数组在多线程环境下是线程不安全的。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
public class UseIntegerArrayUnsafe implements Runnable {
	private static int[] value = new int[2];

	@Override
	public void run() {
		for (int i = 0;i < 10000;i++) {
			value[0]++;
			value[1]++;
		}
	}
	
	public static void main(String[] args) {
		UseIntegerArrayUnsafe unsafe = new UseIntegerArrayUnsafe();
		
		for (int i = 0;i < 10;i++) {
			Thread thread = new Thread(unsafe);
			thread.start();
		}
		
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
		System.out.println(value[0]);
		System.out.println(value[1]);
	}
}

上述类UseIntegerArrayUnsafe，实现了Runnable接口，在其实现的run方法中，对私有数组变量的两个值分别自增1.

而后在main方法中，起了10个线程，分别执行。

运行结果1

运行结果2

运行结果3

AtomicIntegerArray类介绍

JDK提供了AtomicIntegerArray类用于原子的更新int类型的数组。不同于其他的Atomic×××，AtomicInterArray并没有使用CAS+Volatile关键字来实现原子操作。

类图

AtomicIntegerArray类图

主要属性

1
2
3
4
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;

AtomicIntegerArray主要有上面4个属性：

unsafe：Sun提供的CAS算法的相关操作，AtomicIntegerArray底层就是通过它来完成值的修改的；

base：数组在ArrayIntegerArray中的内存起始地址

shift：左移偏移量。用来计算数组中的元素在整个AtomicIntegerArray对象中的偏移量。数组中第i个元素的地址就是 i << shift + base.

array：数组对象，保存数值

主要方法

初始化

1
2
3
4
5
6
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);
    }

首先看下这个静态代码块，在这个代码块中，首先定义一个int变量scale，然后赋值为int[].class占用的字节数。

如果占用的字节数不是2的幂次，抛出异常。

最后再通过scale计算出来左移的偏移量。

例如，int一般占用4个字节，那么scale就等于4，而shift就等于31减去4变为二进制之后的前导0的个数，也就是2.

那么，数组第一个位置的元素就是base + 0 << 2, A[base]，第二个就是base + 1 << 2.即A[base + 4],第三个就是A[base + 8]

checkedByteOffset(int)

1
2
3
4
5
6
private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }

校验索引i是否越界，越界抛出异常，否则调用byteOffset方法计算出偏移量返回。

byteOffset(int)

1
2
3
private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }

将索引i左移shift位，加上base返回目标位置的内存地址

AtomicIntegerArray(int)

1
2
3
4
5
6
7
8
9
/**
     * 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];
    }

创建一个指定数组长度的AtomicIntegerArray对象

AtomicIntegerArray(int[])

1
2
3
4
5
6
7
8
9
10
11
/**
     * 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();
    }

根据指定的数组，创建一个AtomicIntegerArray对象

length()

1
2
3
4
5
6
7
8
/**
     * Returns the length of the array.
     *
     * @return the length of the array
     */
    public final int length() {
        return array.length;
    }

返回数组的长度

get(int)

1
2
3
4
5
6
7
8
9
/**
     * 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));
    }

获取数组指定位置的值。调用checkedByteOffset检查越界情况，并在方法内部调用byteOffset计算出位置i的内存偏移量。传给getRaw方法，获取值

getRaw(long offset)

1
2
3
private int getRaw(long offset) {
        return unsafe.getIntVolatile(array, offset);
    }

直接调用unsafe的方法，获取对应内存偏移量上的值

set(int , int)

1
2
3
4
5
6
7
8
9
/**
     * 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);
    }

调用unsafe的方法直接将位置i上的值设置为给定的新值

lazySet(int, int)

1
2
3
4
5
6
7
8
9
10
/**
     * 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);
    }

调用unsafe的方法直接将位置i上的值设置为给定的新值。但是这个方法有别于上面的set方法，它在一段时间内是有可能让其他线程查到原来的旧值，而不是立马查到新值。因此，它可以算是一个“最终一致性”的方法

getAndSet(int, int)

1
2
3
4
5
6
7
8
9
10
11
/**
     * 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);
    }

返回位置i上的旧值，并将新值赋值给位置i

compareAndSet(int, int, int)

1
2
3
4
5
6
7
8
9
10
11
12
13
/**
     * 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);
    }

如果位置i上的旧值等于期望值expect，那么就将位置i上的值赋值为update，并返回true，否则返回false

compareAndSetRaw(long, int, int)

1
2
3
private boolean compareAndSetRaw(long offset, int expect, int update) {
        return unsafe.compareAndSwapInt(array, offset, expect, update);
    }

compareAndSet方法实际调用的方法，直接调用unsafe设置对应偏移位置上的值

weakCompareAndSet(int, int, int)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
/**
     * 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);
    }

这个方法的本意是，调用weakCompareAndSet方法时不能保证指令重排的发生，因此，这个方法有时候会毫无理由地失败。

但是从实现上看，和compareAndSet没什么区别

getAndIncrement(int)

1
2
3
4
5
6
7
8
9
/**
     * 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);
    }

获取位置i上的值返回，并对该值自增.

getAndDecrement(int)

1
2
3
4
5
6
7
8
9
/**
     * 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);
    }

获取位置i上的值返回，并对该值自减.

getAndAdd(int, int)

1
2
3
4
5
6
7
8
9
10
/**
     * 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);
    }

返回位置i上对应的值，并加上delta

incrementAndGet(int)

1
2
3
4
5
6
7
8
9
/**
     * 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;
    }

对位置i上的值增1，并返回

decrementAndGet(int)

1
2
3
4
5
6
7
8
9
/**
     * 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;
    }

对位置i上的值减1，并返回

addAndGet(int, int)

1
2
3
4
5
6
7
8
9
10
/**
     * 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;
    }

对位置i上的值加上delta，并返回最新的值

getAndUpdate(int, IntUnaryOperator)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
/**
     * Atomically updates the element at index {@code i} with the results
     * of applying the given function, returning the previous value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.
     *
     * @param i the index
     * @param updateFunction a side-effect-free function
     * @return the previous value
     * @since 1.8
     */
    public final int getAndUpdate(int i, IntUnaryOperator updateFunction) {
        long offset = checkedByteOffset(i);
        int prev, next;
        do {
            prev = getRaw(offset);
            next = updateFunction.applyAsInt(prev);
        } while (!compareAndSetRaw(offset, prev, next));
        return prev;
    }

返回位置i上的值，并对位置i上的值应用单元函数updateFunction，更新位置i上的值。这里要求函数updateFunction是幂等的，即，多次执行结果是一致的，因为CAS操作可能失败。

updateAndGet(int, IntUnaryOperator)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
/**
     * Atomically updates the element at index {@code i} with the results
     * of applying the given function, returning the updated value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.
     *
     * @param i the index
     * @param updateFunction a side-effect-free function
     * @return the updated value
     * @since 1.8
     */
    public final int updateAndGet(int i, IntUnaryOperator updateFunction) {
        long offset = checkedByteOffset(i);
        int prev, next;
        do {
            prev = getRaw(offset);
            next = updateFunction.applyAsInt(prev);
        } while (!compareAndSetRaw(offset, prev, next));
        return next;
    }

对位置i上的值应用单元函数updateFunction，并将应用后的值更新到位置i。要求单元函数是幂等的，因为CAS操作可能失败

getAndAccumulate(int, int, IntBinaryOperator)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
/**
     * Atomically updates the element at index {@code i} with the
     * results of applying the given function to the current and
     * given values, returning the previous value. The function should
     * be side-effect-free, since it may be re-applied when attempted
     * updates fail due to contention among threads.  The function is
     * applied with the current value at index {@code i} as its first
     * argument, and the given update as the second argument.
     *
     * @param i the index
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the previous value
     * @since 1.8
     */
    public final int getAndAccumulate(int i, int x,
                                      IntBinaryOperator accumulatorFunction) {
        long offset = checkedByteOffset(i);
        int prev, next;
        do {
            prev = getRaw(offset);
            next = accumulatorFunction.applyAsInt(prev, x);
        } while (!compareAndSetRaw(offset, prev, next));
        return prev;
    }

返回位置i上的值，并对位置i上的值应用二元函数accumulatorFunction，设置在位置i上。这里同样要求二元函数具有幂等性，因为CAS操作可能会失败。

accumulateAndGet(int, int, IntBinaryOperator)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
/**
     * Atomically updates the element at index {@code i} with the
     * results of applying the given function to the current and
     * given values, returning the updated value. The function should
     * be side-effect-free, since it may be re-applied when attempted
     * updates fail due to contention among threads.  The function is
     * applied with the current value at index {@code i} as its first
     * argument, and the given update as the second argument.
     *
     * @param i the index
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the updated value
     * @since 1.8
     */
    public final int accumulateAndGet(int i, int x,
                                      IntBinaryOperator accumulatorFunction) {
        long offset = checkedByteOffset(i);
        int prev, next;
        do {
            prev = getRaw(offset);
            next = accumulatorFunction.applyAsInt(prev, x);
        } while (!compareAndSetRaw(offset, prev, next));
        return next;
    }

对位置i上的值应用二元函数accumulatorFunction，设置在位置i上，然后返回位置i上的值。这里同样要求二元函数具有幂等性，因为CAS操作可能会失败。

toString()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
/**
     * 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(' ');
        }
    }

返回**[value, value,value]**，这样的字符串。

应用

下面就是用AtomicIntegerArray修改在多线程环境下存在问题的代码。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
public class UseAtomicIntegerArray implements Runnable {
	private static AtomicIntegerArray integerArray = new AtomicIntegerArray(2);

	@Override
	public void run() {
		for (int i = 0;i < 10000;i++) {
			integerArray.addAndGet(0, 1);
			integerArray.addAndGet(1, 1);
		}
	}
	
	public static void main(String[] args) {
		UseAtomicIntegerArray unsafe = new UseAtomicIntegerArray();
		
		for (int i = 0;i < 10;i++) {
			Thread thread = new Thread(unsafe);
			thread.start();
		}
		
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
		System.out.println(integerArray.get(0));
		System.out.println(integerArray.get(1));
	}
}

再执行多少次都不会影响结果了。

0%