AtomicLongArray介绍和函数列表

AtomicLongArray函数列表

// 创建给定长度的新 AtomicLongArray。AtomicLongArray(int length)// 创建与给定数组具有相同长度的新 AtomicLongArray，并从给定数组复制其所有元素。AtomicLongArray(long[] array)// 以原子方式将给定值添加到索引 i 的元素。long addAndGet(int i, long delta)// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。boolean compareAndSet(int i, long expect, long update)// 以原子方式将索引 i 的元素减1。long decrementAndGet(int i)// 获取位置 i 的当前值。long get(int i)// 以原子方式将给定值与索引 i 的元素相加。long getAndAdd(int i, long delta)// 以原子方式将索引 i 的元素减 1。long getAndDecrement(int i)// 以原子方式将索引 i 的元素加 1。long getAndIncrement(int i)// 以原子方式将位置 i 的元素设置为给定值，并返回旧值。long getAndSet(int i, long newValue)// 以原子方式将索引 i 的元素加1。long incrementAndGet(int i)// 最终将位置 i 的元素设置为给定值。void lazySet(int i, long newValue)// 返回该数组的长度。int length()// 将位置 i 的元素设置为给定值。void set(int i, long newValue)// 返回数组当前值的字符串表示形式。String toString()// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。boolean weakCompareAndSet(int i, long expect, long update)

AtomicLongArray源码分析(基于JDK1.7.0_40)

AtomicLongArray的完整源码

/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * Written by Doug Lea with assistance from members of JCP JSR- * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/./ */ package java.util.concurrent.atomic; import sun.misc.Unsafe; import java.util.*; /** * A {@code long} 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 . * @author Doug Lea */ public class AtomicLongArray implements java.io.Serializable { private static final long serialVersionUID = -2308431214976778248L; private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final int base = unsafe.arrayBaseOffset(long[].class); private static final int shift; private final long[] array; static { int scale = unsafe.arrayIndexScale(long[].class); if ((scale & (scale - )) != ) throw new Error("data type scale not a power of two"); shift = - Integer.numberOfLeadingZeros(scale); } private long checkedByteOffset(int i) { if (i < || 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 AtomicLongArray of the given length, with all * elements initially zero. * * @param length the length of the array */ public AtomicLongArray(int length) { array = new long[length]; } /** * Creates a new AtomicLongArray 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 AtomicLongArray(long[] 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 long get(int i) { return getRaw(checkedByteOffset(i)); } private long getRaw(long offset) { return unsafe.getLongVolatile(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, long newValue) { unsafe.putLongVolatile(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, long newValue) { unsafe.putOrderedLong(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 long getAndSet(int i, long newValue) { long offset = checkedByteOffset(i); while (true) { long 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, long expect, long update) { return compareAndSetRaw(checkedByteOffset(i), expect, update); } private boolean compareAndSetRaw(long offset, long expect, long update) { return unsafe.compareAndSwapLong(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" rel="external nofollow" >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, long expect, long 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 long 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 long 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 long getAndAdd(int i, long delta) { long offset = checkedByteOffset(i); while (true) { long 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 long 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 long 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 long addAndGet(int i, long delta) { long offset = checkedByteOffset(i); while (true) { long current = getRaw(offset); long 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(' '); } } }

AtomicLongArray的代码很简单，下面仅以incrementAndGet()为例，对AtomicLong的原理进行说明。

incrementAndGet()源码如下：

public final long incrementAndGet(int i) { return addAndGet(i, 1);}

说明：incrementAndGet()的作用是以原子方式将long数组的索引 i 的元素加1，并返回加1之后的值。

addAndGet()源码如下：

public long addAndGet(int i, long delta) { // 检查数组是否越界 long offset = checkedByteOffset(i); while (true) { // 获取long型数组的索引 offset 的原始值 long current = getRaw(offset); // 修改long型值 long next = current + delta; // 通过CAS更新long型数组的索引 offset的值。 if (compareAndSetRaw(offset, current, next)) return next; }}

说明：addAndGet()首先检查数组是否越界。如果没有越界的话，则先获取数组索引i的值；然后通过CAS函数更新i的值。

getRaw()源码如下：

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

说明：unsafe是通过Unsafe.getUnsafe()返回的一个Unsafe对象。通过Unsafe的CAS函数对long型数组的元素进行原子操作。如compareAndSetRaw()就是调用Unsafe的CAS函数，它的源码如下：

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

AtomicLongArray示例

// LongArrayTest.java的源码 import java.util.concurrent.atomic.AtomicLongArray; public class LongArrayTest { public static void main(String[] args){ // 新建AtomicLongArray对象 long[] arrLong = new long[] {10, 20, 30, 40, 50}; AtomicLongArray ala = new AtomicLongArray(arrLong); ala.set(0, 100); for (int i=0, len=ala.length(); i<len; i++) System.out.printf("get(%d) : %s\n", i, ala.get(i)); System.out.printf("%20s : %s\n", "getAndDecrement(0)", ala.getAndDecrement(0)); System.out.printf("%20s : %s\n", "decrementAndGet(1)", ala.decrementAndGet(1)); System.out.printf("%20s : %s\n", "getAndIncrement(2)", ala.getAndIncrement(2)); System.out.printf("%20s : %s\n", "incrementAndGet(3)", ala.incrementAndGet(3)); System.out.printf("%20s : %s\n", "addAndGet(100)", ala.addAndGet(0, 100)); System.out.printf("%20s : %s\n", "getAndAdd(100)", ala.getAndAdd(1, 100)); System.out.printf("%20s : %s\n", "compareAndSet()", ala.compareAndSet(2, 31, 1000)); System.out.printf("%20s : %s\n", "get(2)", ala.get(2)); } }

运行结果：

get(0) : 100get(1) : 20get(2) : 30get(3) : 40get(4) : 50 getAndDecrement(0) : 100 decrementAndGet(1) : 19 getAndIncrement(2) : 30 incrementAndGet(3) : 41 addAndGet(100) : 199 getAndAdd(100) : 19 compareAndSet() : true get(2) : 1000

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