AtomicLong介紹和函數列表
AtomicLong是作用是對長整形進行原子操作。
在32位操作系統中，64位的long 和 double 變量由 於會被JVM當作兩個分離的32位來進行操作，所以不具有原子性。而使用AtomicLong能讓long的操作保持 原子型。
AtomicLong函數列表
// 構造函數 AtomicLong() // 創建值為initialValue的AtomicLong對象 AtomicLong(long initialValue) // 以原子方式設置當前值為newValue。 final void set(long newValue) // 獲取當前值 final long get() // 以原子方式將當前值減 1，並返回減1後的值。等價於“--num” final long decrementAndGet() // 以原子方式將當前值減 1，並返回減1前的值。等價於“num--” final long getAndDecrement() // 以原子方式將當前值加 1，並返回加1後的值。等價於“++num” final long incrementAndGet() // 以原子方式將當前值加 1，並返回加1前的值。等價於“num++” final long getAndIncrement() // 以原子方式將delta與當前值相加，並返回相加後的值。 final long addAndGet(long delta) // 以原子方式將delta添加到當前值，並返回相加前的值。 final long getAndAdd(long delta) // 如果當前值 == expect，則以原子方式將該值設置為update。成功返回true，否則返回false，並且不 修改原值。 final boolean compareAndSet(long expect, long update) // 以原子方式設置當前值為newValue，並返回舊值。 final long getAndSet(long newValue) // 返回當前值對應的int值 int intValue() // 獲取當前值對應的long值 long longValue() // 以 float 形式返回當前值 float floatValue() // 以 double 形式返回當前值 double doubleValue() // 最後設置為給定值。延時設置變量值，這個等價於set()方法，但是由於字段是volatile類型的，因此 次字段的修改會比普通字段（非volatile字段）有稍微的性能延時（盡管可以忽略），所以如果不是想立 即讀取設置的新值，允許在“後台”修改值，那麼此方法就很有用。如果還是難以理解，這裡 就類似於啟動一個後台線程如執行修改新值的任務，原線程就不等待修改結果立即返回（這種解釋其實是 不正確的，但是可以這麼理解）。 final void lazySet(long newValue) // 如果當前值 == 預期值，則以原子方式將該設置為給定的更新值。JSR規范中說：以原子方式讀取和有 條件地寫入變量但不 創建任何 happen-before 排序，因此不提供與除 weakCompareAndSet 目標外任何 變量以前或後續讀取或寫入操作有關的任何保證。大意就是說調用weakCompareAndSet時並不能保證不存 在happen-before的發生（也就是可能存在指令重排序導致此操作失敗）。但是從Java源碼來看，其實此 方法並沒有實現JSR規范的要求，最後效果和compareAndSet是等效的，都調用了 unsafe.compareAndSwapInt()完成操作。 final boolean weakCompareAndSet(long expect, long update)
AtomicLong源碼分析(基於JDK1.7.0_40)
AtomicLong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;
/**
* A {@code long} value that may be updated atomically. See the
* {@link java.util.concurrent.atomic} package specification for
* description of the properties of atomic variables. An
* {@code AtomicLong} is used in applications such as atomically
* incremented sequence numbers, and cannot be used as a replacement
* for a {@link java.lang.Long}. However, this class does extend
* {@code Number} to allow uniform access by tools and utilities that
* deal with numerically-based classes.
*
* @since 1.5
* @author Doug Lea
*/
public class AtomicLong extends Number implements java.io.Serializable {
private static final long serialVersionUID = 1927816293512124184L;
// setup to use Unsafe.compareAndSwapLong for updates
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;
/**
* Records whether the underlying JVM supports lockless
* compareAndSwap for longs. While the Unsafe.compareAndSwapLong
* method works in either case, some constructions should be
* handled at Java level to avoid locking user-visible locks.
*/
static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
/**
* Returns whether underlying JVM supports lockless CompareAndSet
* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
*/
private static native boolean VMSupportsCS8();
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicLong.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
private volatile long value;
/**
* Creates a new AtomicLong with the given initial value.
*
* @param initialValue the initial value
*/
public AtomicLong(long initialValue) {
value = initialValue;
}
/**
* Creates a new AtomicLong with initial value {@code 0}.
*/
public AtomicLong() {
}
/**
* Gets the current value.
*
* @return the current value
*/
public final long get() {
return value;
}
/**
* Sets to the given value.
*
* @param newValue the new value
*/
public final void set(long newValue) {
value = newValue;
}
/**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(long newValue) {
unsafe.putOrderedLong(this, valueOffset, newValue);
}
/**
* Atomically sets to the given value and returns the old value.
*
* @param newValue the new value
* @return the previous value
*/
public final long getAndSet(long newValue) {
while (true) {
long current = get();
if (compareAndSet(current, newValue))
return current;
}
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* @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(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
/**
* Atomically sets the value 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 expect the expected value
* @param update the new value
* @return true if successful.
*/
public final boolean weakCompareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
/**
* Atomically increments by one the current value.
*
* @return the previous value
*/
public final long getAndIncrement() {
while (true) {
long current = get();
long next = current + 1;
if (compareAndSet(current, next))
return current;
}
}
/**
* Atomically decrements by one the current value.
*
* @return the previous value
*/
public final long getAndDecrement() {
while (true) {
long current = get();
long next = current - 1;
if (compareAndSet(current, next))
return current;
}
}
/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the previous value
*/
public final long getAndAdd(long delta) {
while (true) {
long current = get();
long next = current + delta;
if (compareAndSet(current, next))
return current;
}
}
/**
* Atomically increments by one the current value.
*
* @return the updated value
*/
public final long incrementAndGet() {
for (;;) {
long current = get();
long next = current + 1;
if (compareAndSet(current, next))
return next;
}
}
/**
* Atomically decrements by one the current value.
*
* @return the updated value
*/
public final long decrementAndGet() {
for (;;) {
long current = get();
long next = current - 1;
if (compareAndSet(current, next))
return next;
}
}
/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the updated value
*/
public final long addAndGet(long delta) {
for (;;) {
long current = get();
long next = current + delta;
if (compareAndSet(current, next))
return next;
}
}
/**
* Returns the String representation of the current value.
* @return the String representation of the current value.
*/
public String toString() {
return Long.toString(get());
}
public int intValue() {
return (int)get();
}
public long longValue() {
return get();
}
public float floatValue() {
return (float)get();
}
public double doubleValue() {
return (double)get();
}
}
查看本欄目
AtomicLong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() {
for (;;) {
// 獲取AtomicLong當前對應的long值
long current = get();
// 將current加1
long next = current + 1;
// 通過CAS函數，更新current的值
if (compareAndSet(current, next))
return next;
}
}
說明：
(01) incrementAndGet()首先會根據get()獲取AtomicLong對應的long值。該值是volatile 類型的變量，get()的源碼如下：
// value是AtomicLong對應的long值
private volatile long value;
// 返回AtomicLong對應的 long值
public final long get() {
return value;
}
(02) incrementAndGet()接著將current加1,然後通過CAS函數，將新的值賦值給value。
compareAndSet()的源碼如下：
public final boolean compareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
compareAndSet()的作用是更新AtomicLong對應的long值。它會比較AtomicLong的原始值是否與expect 相等，若相等的話，則設置AtomicLong的值為update。
AtomicLong示例
1 // LongTest.java的源碼
2 import java.util.concurrent.atomic.AtomicLong;
3
4 public class LongTest {
5
6 public static void main(String[] args) {
7
8 // 新建AtomicLong對象
9 AtomicLong mAtoLong = new AtomicLong();
10
11 mAtoLong.set(0x0123456789ABCDEFL);
12 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
13 System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());
14 System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());
15 System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());
16 System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());
17
18 System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());
19 System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());
20 System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());
21 System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());
22
23 System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));
24 System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));
25
26 System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());
27 28 System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L));
29 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
30 }
31 }
運行結果：
get() : 0x0123456789ABCDEF
intValue() : 0x0000000089ABCDEF
longValue() : 0x0123456789ABCDEF
doubleValue() : 8.1985529216486896E16
floatValue() : 8.1985531E16
getAndDecrement() : 0x0123456789ABCDEF
decrementAndGet() : 0x0123456789ABCDED
getAndIncrement() : 0x0123456789ABCDED
incrementAndGet() : 0x0123456789ABCDEF
addAndGet(0x10) : 0x0123456789ABCDFF
getAndAdd(0x10) : 0x0123456789ABCDFF
get() : 0x0123456789ABCE0F
compareAndSet() : false
get() : 0x0123456789ABCE0F
