Java多线程：“JUC原子类”02之AtomicLong原子类
副标题#e#
AtomicLong先容和函数列表
AtomicLong是浸染是对长整形举办原子操纵。
在32位操纵系统中，64位的long 和 double 变量由 于会被JVM看成两个疏散的32位来举办操纵，所以不具有原子性。而利用AtomicLong能让long的操纵保持 原子型。
AtomicLong函数列表
// 结构函数 AtomicLong() // 建设值为initialValue的AtomicLong工具 AtomicLong(long initialValue) // 以原子方法配置当前值为newValue。 final void set(long newValue) // 获取当前值 final long get() // 以原子方法将当前值减 1，并返回减1后的值。等价于“--num” final long decrementAndGet() // 以原子方法将当前值减 1，并返回减1前的值。等价于“num--” final long getAndDecrement() // 以原子方法将当前值加 1，并返回加1后的值。等价于“++num” final long incrementAndGet() // 以原子方法将当前值加 1，并返回加1前的值。等价于“num++” final long getAndIncrement() // 以原子方法将delta与当前值相加，并返回相加后的值。 final long addAndGet(long delta) // 以原子方法将delta添加到当前值，并返回相加前的值。 final long getAndAdd(long delta) // 假如当前值 == expect，则以原子方法将该值配置为update。乐成返回true，不然返回false，而且不 修改原值。 final boolean compareAndSet(long expect, long update) // 以原子方法配置当前值为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() // 最后配置为给定值。延时配置变量值，这个等价于set()要领，可是由于字段是volatile范例的，因此 次字段的修改会比普通字段（非volatile字段）有稍微的机能延时（尽量可以忽略），所以假如不是想立 即读取配置的新值，答允在“靠山”修改值，那么此要领就很有用。假如照旧难以领略，这里 就雷同于启动一个靠山线程如执行修改新值的任务，原线程就不期待修改功效当即返回（这种表明其实是 不正确的，可是可以这么领略）。 final void lazySet(long newValue) // 假如当前值 == 预期值，则以原子方法将该配置为给定的更新值。JSR类型中说：以原子方法读取和有 条件地写入变量但不 建设任何 happen-before 排序，因此不提供与除 weakCompareAndSet 方针外任何 变量以前或后续读取或写入操纵有关的任何担保。大意就是说挪用weakCompareAndSet时并不能担保不存 在happen-before的产生（也就是大概存在指令重排序导致此操纵失败）。可是从Java源码来看，其实此 要领并没有实现JSR类型的要求，最后结果和compareAndSet是等效的，都挪用了 unsafe.compareAndSwapInt()完成操纵。 final boolean weakCompareAndSet(long expect, long update)
AtomicLong源码阐明(基于JDK1.7.0_40)
#p#副标题#e#
AtomicLong的完整源码
/*
* 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();
}
}
查察本栏目
AtomicLong的代码很简朴，下面仅以incrementAndGet()为例，对AtomicLong的道理举办说明。
#p#分页标题#e#
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()首先会按照get()获取AtomicLong对应的long值。该值是volatile 范例的变量，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()的浸染是更新AtomicLong对应的long值。它会较量AtomicLong的原始值是否与expect 相等，若相等的话，则配置AtomicLong的值为update。
#p#分页标题#e#
AtomicLong示例
1 // LongTest.java的源码
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工具
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 }
运行功效：
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
