了解AtomicBoolean

這裡我們就從AtomicBoolean開始說吧，自己正好也複習一下。對于官方的說明是：

可以用原子方式更新的 boolean 值。有關原子變量屬性的描述，請參閱 java.util.concurrent.atomic

包規範。AtomicBoolean 可用在應用程式中（如以原子方式更新的标志），但不能用于替換 Boolean。

換一句話說，Atomic就是原子性的意思，即能夠保證在高并發的情況下隻有一個線程能夠通路這個屬性值。（類似我們之前所說的volatile）

一般情況下，我們使用 AtomicBoolean 高效并發處理 “隻初始化一次” 的功能要求：

private static AtomicBoolean initialized = new AtomicBoolean(false);
public void init()
{
   if( initialized.compareAndSet(false, true) )
   {
       // 這裡放置初始化代碼....
   }
}
           

如果沒有AtomicBoolean，我們可以使用volatile做如下操作：

public static volatile initialized = false;
public void init()
{
    if( initialized == false ){
        initialized = true;
        // 這裡初始化代碼....
    }
}
           

既然如此神奇，那麼我們看看AtomicBoolean的源碼時如何實作的，檢視源碼如下：

public class AtomicBoolean implements java.io.Serializable {
    private static final long serialVersionUID = 4654671469794556979L;
    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicBoolean.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile int value;

    /**
     * Creates a new {@code AtomicBoolean} with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicBoolean(boolean initialValue) {
        value = initialValue ? 1 : 0;
    }

    /**
     * Creates a new {@code AtomicBoolean} with initial value {@code false}.
     */
    public AtomicBoolean() {
    }

    /**
     * Returns the current value.
     *
     * @return the current value
     */
    public final boolean get() {
        return value != 0;
    }

    /**
     * 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(boolean expect, boolean update) {
        int e = expect ? 1 : 0;
        int u = update ? 1 : 0;
        return unsafe.compareAndSwapInt(this, valueOffset, e, u);
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, 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 boolean weakCompareAndSet(boolean expect, boolean update) {
        int e = expect ? 1 : 0;
        int u = update ? 1 : 0;
        return unsafe.compareAndSwapInt(this, valueOffset, e, u);
    }

    /**
     * Unconditionally sets to the given value.
     *
     * @param newValue the new value
     */
    public final void set(boolean newValue) {
        value = newValue ? 1 : 0;
    }

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(boolean newValue) {
        int v = newValue ? 1 : 0;
        unsafe.putOrderedInt(this, valueOffset, v);
    }

    /**
     * Atomically sets to the given value and returns the previous value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    public final boolean getAndSet(boolean newValue) {
        boolean prev;
        do {
            prev = get();
        } while (!compareAndSet(prev, newValue));
        return prev;
    }

    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value
     */
    public String toString() {
        return Boolean.toString(get());
    }

}
           

你猜的沒錯，AtomicBoolean就是使用了Volatile屬性來完成的。

Java6以後出現的很多的原子行的類，除了上述我們所說的AtomicBoolean以外，AtomicBoolean家族還是比較強大的，後面我們有時間在一一介紹。包括：

基本類：

AtomicInteger、AtomicLong、AtomicBoolean；

引用類型：

AtomicReference、AtomicReference的ABA執行個體、AtomicStampedRerence、AtomicMarkableReference；

數組類型：

AtomicIntegerArray、AtomicLongArray、AtomicReferenceArray

屬性原子修改器（Updater）：

AtomicIntegerFieldUpdater、AtomicLongFieldUpdater、AtomicReferenceFieldUpdater