Java NIO -AtomicInteger

概述

通常情况下，在Java中我们对++i,--i的操作都是非线程安全的。对于一个++i存在三个独立的操作，获取变量i当前的值，为该值+1。如下代码，多线程操作i++。

public class UnsafeI {
public static int num = 0;
public static void main(String[] args) throws InterruptedException {
final int threadSize = 10000;
Thread[] ts = new Thread[threadSize];
for (int i = 0; i < threadSize; i++) {
ts[i] = new Thread() {
public void run() {
num++;
}
};
}
for (Thread t : ts) {
t.start();
}
System.out.println(num);
}
}

多次运行，在可能的情况下，我们会看到输出的num存在不同的值。

AtomicInteger提供一系列的线程安全的方法，供我们实现i++操作。

  /**
     * Atomically increments by one the current value.
     *
     * @return the previous value
     */
    public final int getAndIncrement() {
        for (;;) {
            int current = get();
            int next = current + 1;
            if (compareAndSet(current, next))
                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(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }



底层通过Unsafe方法实现。具体Unsafe的各个防范，有兴趣的可以参考如下博客，写的比较详细：http://ifeve.com/sun-misc-unsafe/

我们多线程环境对AtomicInteger进行操作。

import java.util.concurrent.atomic.AtomicInteger;
public class SafeAtomicInteger {
  static final AtomicInteger value = new AtomicInteger(0);
  
  
  public static void main(String[] args) throws InterruptedException {
final int threadSize = 10000;
Thread[] ts = new Thread[threadSize];
for (int i = 0; i < threadSize; i++) {
ts[i] = new Thread() {
public void run() {
value.incrementAndGet();
}
};
}
for (Thread t : ts) {
t.start();
}
System.out.println(value.get());
}
}

参考列表：http://www.blogjava.net/xylz/archive/2010/07/01/324988.html

http://ifeve.com/sun-misc-unsafe/