AtomicLong 简介
- 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AtomicLong能让long的加1,减1操作,设置新值等操作在多线程中保持原子性；
- AtomicLong 虽然继承了Number 但不是 Long的替代品，即不要滥用；
- AtomicLong的原子性操作并不由加锁支持的，而是有CompareAndSwap(简称 CAS )支持的原子性;
AtomicLong UML
AtomicLong 关键技术分析
CAS关键技术(CompareAndSwap 又称作比较交换)
我们对一个数字A做修改前（注意是前，修改的动作还没发生，我们修改前需要做些准备工作），首先会得到 存储 A 的地址，即为addrA， A的值，即为addA(A)（即为当前addA 中记录的A的值）。得到addA addA（A）就是修改前的准备工作，此期间别的线程也可以读写A哦。
当对A开始修改了，流程如下:
第一步：去addA中去看一下此刻此地址的A值的，即为\(V_{A}\)；
第二步：比较 \(V_{A}\) 和 addA(A) 是否相等，若相等，则执行更新，如将 A+B的值写入addA，结束。(这步操作是原子性的，是由操作系统的指令支持的，任何一条单独的指令都是原子的，要不执行，要不不执行，设想若指令不是原子性的，那计算机组成中的多级流水线等一系列优化的大厦会轰然倒塌，若是一系列指令完成的，必然要在这些指令的执行“加锁”，如锁住总线，独占CPU,或者让现在线程栈(java虚拟机栈）中修改好缓存，修改缓存期间，其他线程不能修改该缓存对应的主存位置，缓存写好后立即强制写回主存，放开主存锁定，让其他所有线程可见，这一块都是计算机组成的一些知识，仅做抛砖引玉）
第三步： 若 \(V_{A}\) 和 addA(A) 不相等（说明修改前的准备阶段到准备修改期间，有别的线程在操作A，改了A的值)， 然后记住此刻此地址A 记作addA(A)，再从第一步开始，直到该线程成功更新A.（一直循环判断直到成功修改，我们称为自旋）
AtomicLong 源码解读
package java.util.concurrent.atomic;
import java.util.function.LongUnaryOperator;
import java.util.function.LongBinaryOperator;
import sun.misc.Unsafe;
/**
* @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) {
return unsafe.getAndSetLong(this, valueOffset, newValue);
}
/**
* 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 {@code 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 {@code 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() {
return unsafe.getAndAddLong(this, valueOffset, 1L);
}
/**
* Atomically decrements by one the current value.
*
* @return the previous value
*/
public final long getAndDecrement() {
return unsafe.getAndAddLong(this, valueOffset, -1L);
}
/**
* 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) {
return unsafe.getAndAddLong(this, valueOffset, delta);
}
/**
* Atomically increments by one the current value.
*
* @return the updated value
*/
public final long incrementAndGet() {
return unsafe.getAndAddLong(this, valueOffset, 1L) + 1L;
}
/**
* Atomically decrements by one the current value.
*
* @return the updated value
*/
public final long decrementAndGet() {
return unsafe.getAndAddLong(this, valueOffset, -1L) - 1L;
}
/**
* 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) {
return unsafe.getAndAddLong(this, valueOffset, delta) + delta;
}
/**
* Atomically updates the current value with the results of
* applying the given function, returning the previous value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the previous value
* @since 1.8
*/
public final long getAndUpdate(LongUnaryOperator updateFunction) {
long prev, next;
do {
prev = get();
next = updateFunction.applyAsLong(prev);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function, returning the updated value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the updated value
* @since 1.8
*/
public final long updateAndGet(LongUnaryOperator updateFunction) {
long prev, next;
do {
prev = get();
next = updateFunction.applyAsLong(prev);
} while (!compareAndSet(prev, next));
return next;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the previous value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the previous value
* @since 1.8
*/
public final long getAndAccumulate(long x,
LongBinaryOperator accumulatorFunction) {
long prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsLong(prev, x);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the updated value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the updated value
* @since 1.8
*/
public final long accumulateAndGet(long x,
LongBinaryOperator accumulatorFunction) {
long prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsLong(prev, x);
} while (!compareAndSet(prev, 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());
}
/**
* Returns the value of this {@code AtomicLong} as an {@code int}
* after a narrowing primitive conversion.
* @jls 5.1.3 Narrowing Primitive Conversions
*/
public int intValue() {
return (int)get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code long}.
*/
public long longValue() {
return get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code float}
* after a widening primitive conversion.
* @jls 5.1.2 Widening Primitive Conversions
*/
public float floatValue() {
return (float)get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code double}
* after a widening primitive conversion.
* @jls 5.1.2 Widening Primitive Conversions
*/
public double doubleValue() {
return (double)get();
}
}
AtomicLong 示例
//单线程下线程不安全和AtomicLong的线程安全
面试session
- CAS 操作一定是线程安全的吗？
是线程相对安全的，没有绝对的线程安全，但是CAS一般情况下比传统的加锁并发度会更好，性能更加。但是可能会引起一些问题，如ABA问题，自旋引起的性能问题
ABA问题：线程得到 A 时，是其他线程先将 A 改为 B ,再将 B 改回 为 A, 线程也认为此刻没有其他线程在修改A值 也会成功更新 A.这种情况，大部分情况都是没问题的，有些具体的业务对ABA问题敏感，就要注意。反正和
相关的都想想一想，不然干就完了。996谁顶住鸭......
ABA问题可以在每个操作数加一个版本号，如 A1 B1 A2,这样线程得到的是A1,和A2比较时，就会发现有其他线程在操作A,Java并发包为了解决这个问题，提供了一个带有标记的原子引用类“AtomicStampedReference”，它可以通过控制变量值的版本来保证CAS的正确性。当然改成传统的同步加锁也可以哦。
自旋： 就是指一直循环判断直到成功修改为止。长时间的自旋操作，特别是多个线程都在自旋，很影响性能的。可考虑换成锁来操作。