一、原子操作類簡介

JDK1.5開始提供了java.util.concurrent.atomic包，其中有一系列用法簡單、效能高效、可以執行緒安全更新變數的原子操作類，目前（JDK1.7）大概有這麼些：

二、原子操作類實現原理

以AtomicInteger為例看下原始碼，其中的兩個方法getAndSet，getAndIncrement，都是無限迴圈呼叫compareAndSet，直到成功，CAS方法呼叫的是unsafe的方法，目前unsafe限制越來越嚴，也不建議程式設計師使用了，這裡不做分析。原始碼如下：

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

    /**
     * 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 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);
    }



}