Java concurrency之AtomicLong原子类_动力节点Java学院整理

Java concurrency之AtomicLong原子类_动力节点Java学院整理

AtomicLong介绍和函数列表

AtomicLong是作用是对长整形进行原子操作。

在32位操作系统中，64位的long 和 double 变量由于会被JVM当作两个分离的32位来进行操作，所以不具有原子性。而使用AtomicLong能让long的操作保持原子型。

AtomicLong函数列表

// 构造函数

AtomicLong()

// 创建值为initialValue的AtomicLong对象

AtomicLong(long initialValue)

// 以原子方式设置当前值为newValue。

final void set(long newValue)

// 获取当前值

final long get()

// 以原子方式将当前值减 1，并返回减1后的值。等价于“--num”

finahttp://l long decrementAndGet()

// 以原子方式将当前值减 1，并返回减1前的值。等价于“num--”

final long getAndDecrement()

// 以原子方式将当前值加 1，并返回加1后的值。等价于“++num”

final long incrementAndGet()

// 以原子方式将当前值加 1，并返回加1前的值。等价于“num++”

final long getAndIncrement()

// 以原子方式将delta与当前值相加，并返回相加后的值。

final long addAndGet(long delta)

// 以原子方式将delta添加到当前值，并返回相加前的值。

final long getAndAdd(long delta)

// 如果当前值 == expect，则以原子方式将该值设置为update。成功返回true，否则返回false，并且不修改原值。

final boolean compareAndSet(long expect, long update)

// 以原子方式设置当前值为newValue，并返回旧值。

final long getAndSet(long newValue)

// 返回当前值对应的int值

int intValue()

// 获取当前值对应的long值

long longValue()

// 以 float 形式返回当前值

float floatValue()

// 以 double 形式返回当前值

double doubleValue()

// 最后设置为给定值。延时设置变量值，这个等价于set()方法，但是由于字段是volatile类型的，因此次字段的修改会比普通字段（非volatile字段）有稍微的性能延时（尽管可以忽略），所以如果不是想立即读取设置的新值，允许在“后台”修改值，那么此方法就很有用。如果还是难以理解，这里就类似于启动一个后台线程如执行修改新值的任务，原线程就不等待修改结果立即返回（这种解释其实是不正确的，但是可以这么理解）。

final void lazySet(long newValue)

// 如果当前值 == 预期值，则以原子方式将该设置为给定的更新值。jsR规范中说：以原子方式读取和有条件地写入变量但不 创建任何 happen-before 排序，因此不提供与除 weakCompareAndSet 目标外任何变量以前或后续读取或写入操作有关的任何保证。大意就是说调用weakCompareAndSet时并不能保证不存在happen-before的发生（也就是可能存在指令重排序导致此操作失败）。但是从java源码来看，其实此方法并没有实现JSR规范的要求，最后效果和compareAndSet是等效的，都调用了unsafe.compareAndSwapInt()完成操作。

final boolean weakCompareAndSet(long expect, long update)

AtomicLong源码分析(基于JDK1.7.0_40)

AtomicLong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;

/**

* 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 .

* @author Doug Lea

*/

public class AtomicLong extends Number implements java.io.Serializable {

private static final long serialVersionUID = L;

// 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 = VMSupportsCS();

/**

* Returns whether underlying JVM supports lockless CompareAndSet

* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.

*/

private static native boolean VMSupportsCS();

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 }.

*/

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.

*

*

May fail spuriously

* 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();

}

}

AtomicLong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() {

for (;;) {

// 获取AtomicLong当前对应的long值

long current = get();

// 将current加1

long next = current + 1;

// 通过CAS函数，更新current的值

if (compareAndSet(current, next))

return next;

}

}

说明：

(01) incrementAndGet()首先会根据get()获取AtomicLong对应的long值。该值是volatile类型的变量，get()的源码如下：

// value是AtomicLong对应的long值

private volatile long value;

// 返回AtomicLong对应的long值

public final long get() {

return value;

}

(02) incrementAndGet()接着将current加1,然后通过CAS函数，将新的值赋值给value。

compareAndSet()的源码如下：

public final boolean compareAndSet(long expect, long update) {

return unsafe.compareAndSwapLong(this, valueOffset, expect, update);

}

compareAndSet()的作用是更新AtomicLong对应的long值。它会比较AtomicLong的原始值是否与expect相等，若相等的话，则设置AtomicLong的值为update。

AtomicLong示例

// LongTest.java的源码

import java.util.concurrent.atomic.AtomicLong;

public class LongTest {

public static void main(String[] args){

// 新建AtomicLong对象

AtomicLong mAtoLong = new AtomicLong();

mAtoLong.set(0x0123456789ABCDEFL);

System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());

System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());

System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());

System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());

System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());

System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());

System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());

System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());

System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());

System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));

System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));

System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());

System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0http://xFEDCBA9876543210L));

System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());

}

}

运行结果：

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

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