java.util.concurrent.atomic

AtomicInteger是一个常用作与计数器的原子Integer类,该类实现了并发下高效率的加减操作.

atomic意思为原子性的,顾名思义,该包下面的类都是与原子性操作有关:不可中断的操作.
java.util.concurrent.atomic包下面的实现原理与AtomicInteger基本相似.

源码分析

java.util.concurrent.atomic.AtomicInteger的源码比较容易理解:

1. 成员属性有volatile修饰的值value,可以保证多线程下面的可见性.valueOffset存储value字段的内存位置,通过反射获取.unsafe则是底层硬件的实现.

    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;
   	
    static {
      try {
        valueOffset = unsafe.objectFieldOffset
            (AtomicInteger.class.getDeclaredField("value"));
      } catch (Exception ex) { throw new Error(ex); }
    }
   	
    private volatile int value;
   

2. 当值进行更新时候,采用比较交换模式:即不断循环仅当value为当前线程获取到的value相等时才进行更新操作(若不相等,则表明已被其他线程更新了)

    /**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    public final int getAndSet(int newValue) {
        for (;;) {
            int 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(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }
   

3. 比较交换的参考实现 简单参考实现,帮助解读,并非真实实现

    class SimulatedCAS {
        private int value;
   	
        public synchronized int getValue() {
            return value;
        }
   	
        public synchronized int compareAndSwap(int expectedValue, int newValue) {
            int oldValue = value;
            if (value == expectedValue)
                value = newValue;
            return oldValue;
        }
    }
   

来源

1. 聊聊并发（五）原子操作的实现原理
2. Java之美[从菜鸟到高手演练]之atomic包的原理及分析