java多線程系類：JUC原子類：03之AtomicLongArray原子類

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概要

AtomicIntegerArray, AtomicLongArray, AtomicReferenceArray這3個數群組類型的原子類的原理和用法相似。本章以AtomicLongArray對數群組類型的原子類進行介紹。內容包括：
AtomicLongArray介紹和函數列表
AtomicLongArray源碼分析(基於JDK1.7.0_40)
AtomicLongArray樣本

轉載請註明出處：http://www.cnblogs.com/skywang12345/p/3514604.html

 

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

// 建立給定長度的新 AtomicLongArray。AtomicLongArray(int length)// 建立與給定數組具有相同長度的新 AtomicLongArray，並從給定數組複製其所有元素。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定為給定的更新值。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)// 擷取位置 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 的元素設定為給定值，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 的元素設定為給定值。void lazySet(int i, long newValue)// 返回該數組的長度。int length()// 將位置 i 的元素設定為給定值。void set(int i, long newValue)// 返回數組當前值的字串表示形式。String toString()// 如果當前值 == 預期值，則以原子方式將該值設定為給定的更新值。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) {        // 擷取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，則先擷取數組索引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樣本

 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         // 建立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) : 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

java多線程系類：JUC原子類：03之AtomicLongArray原子類