AtomicLongArray介紹和函數列表
在"Java多線程系列--“JUC原子類”02之 AtomicLong原子類"中介紹過， AtomicLong是作用是對長整形進行原子操作。而AtomicLongArray的作用則是對"長整形數組" 進行原子操作。
AtomicLongArray函數列表
// 創建給定長度的新 AtomicLongArray。
AtomicLongArray(int length)
// 創建與給定數組具有相同長度的新 AtomicLongArray，並從給定數組復制其所有元素。
AtomicLongArray(long[] array)
// 以原子方式將給定值添加到索引 i 的元素。
long addAndGet(int i, long delta)
// 如果當前值 == 預期值，則以原子方式將該值設置為給定的更新值。
boolean compareAndSet(int i, long expect, long update)
// 以原子方式將索引 i 的元素減1。
long decrementAndGet(int i)
// 獲取位置 i 的當前值。
long get(int i)
// 以原子方式將給定值與索引 i 的元素相加。
long getAndAdd(int i, long delta)
// 以原子方式將索引 i 的元素減 1。
long getAndDecrement(int i)
// 以原子方式將索引 i 的元素加 1。
long getAndIncrement(int i)
// 以原子方式將位置 i 的元素設置為給定值，並返回舊值。
long getAndSet(int i, long newValue)
// 以原子方式將索引 i 的元素加1。
long incrementAndGet(int i)
// 最終將位置 i 的元素設置為給定值。
void lazySet(int i, long newValue)
// 返回該數組的長度。
int length()
// 將位置 i 的元素設置為給定值。
void set(int i, long newValue)
// 返回數組當前值的字符串表示形式。
String toString()
// 如果當前值 == 預期值，則以原子方式將該值設置為給定的更新值。
boolean weakCompareAndSet(int i, long expect, long update)
AtomicLongArray源碼分析(基於JDK1.7.0_40)
AtomicLongArray的完整源碼
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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* 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.*;
/**
* 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 1.5
* @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 - 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 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">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的代碼很簡單，下面僅以incrementAndGet()為例，對AtomicLong的原理進行說明。
incrementAndGet()源碼如下：
public final long incrementAndGet(int i) {
return addAndGet(i, 1);
}
說明：incrementAndGet()的作用是以原子方式將long數組的索引 i 的元素加1，並返回加1之後的值 。
addAndGet()源碼如下：
public long addAndGet(int i, long delta) {
// 檢查數組是否越界
long offset = checkedByteOffset(i);
while (true) {
// 獲取long型數組的索引 offset 的原始值
long current = getRaw(offset);
// 修改long型值
long next = current + delta;
// 通過CAS更新long型數組的索引 offset的值。
if (compareAndSetRaw(offset, current, next))
return next;
}
}
說明：addAndGet()首先檢查數組是否越界。如果沒有越界的話，則先獲取數組索引i的值；然後通過 CAS函數更新i的值。
getRaw()源碼如下：
private long getRaw(long offset) {
return unsafe.getLongVolatile(array, offset);
}
說明：unsafe是通過Unsafe.getUnsafe()返回的一個Unsafe對象。通過Unsafe的CAS函數對long型數組 的元素進行原子操作。如compareAndSetRaw()就是調用Unsafe的CAS函數，它的源碼如下：
private boolean compareAndSetRaw(long offset, long expect, long update) {
return unsafe.compareAndSwapLong(array, offset, expect, update);
}
AtomicLongArray示例
1 // LongArrayTest.java的源碼
2 import java.util.concurrent.atomic.AtomicLongArray;
3
4 public class LongArrayTest {
5 6 public static void main(String[] args){
7
8 // 新建AtomicLongArray對象
9 long[] arrLong = new long[] {10, 20, 30, 40, 50};
10 AtomicLongArray ala = new AtomicLongArray(arrLong);
11
12 ala.set(0, 100);
13 for (int i=0, len=ala.length(); i<len; i++)
14 System.out.printf("get(%d) : %s\n", i, ala.get(i));
15
16 System.out.printf("%20s : %s\n", "getAndDecrement(0)", ala.getAndDecrement(0));
17 System.out.printf("%20s : %s\n", "decrementAndGet(1)", ala.decrementAndGet(1));
18 System.out.printf("%20s : %s\n", "getAndIncrement(2)", ala.getAndIncrement(2));
19 System.out.printf("%20s : %s\n", "incrementAndGet(3)", ala.incrementAndGet(3));
20
21 System.out.printf("%20s : %s\n", "addAndGet(100)", ala.addAndGet(0, 100));
22 System.out.printf("%20s : %s\n", "getAndAdd(100)", ala.getAndAdd(1, 100));
23
24 System.out.printf("%20s : %s\n", "compareAndSet()", ala.compareAndSet(2, 31, 1000));
25 System.out.printf("%20s : %s\n", "get(2)", ala.get(2));
26 }
27 }
運行結果：
get(0) : 100
get(1) : 20
get(2) : 30
get(3) : 40
get(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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