AtomicBoolean

源码导读

java.util.concurrent.atomic.AtomicBoolean类提供了可以原子读取和写入的底层布尔值的操作，并且还包含高级原子操作。
AtomicBoolean支持基础布尔变量上的原子操作。 它具有获取和设置方法，如在volatile变量上的读取和写入。
也就是说，一个集合与同一变量上的任何后续get相关联。 原子compareAndSet方法也具有这些内存一致性功能。

一般情况下，我们使用 AtomicBoolean 高效并发处理 “只初始化一次” 的功能要求
采用volatile int value类型原子变量保证内存可见性
采用Unsafe类 compareAndSwapInt方法实现变量值valueOffset的修改

知识参考点：Unsafe类/ volatile 关键字/ CAS

源码参考

package java.util.concurrent.atomic;
import sun.misc.Unsafe;

/**
 * A {@code boolean} value that may be updated atomically. See the
 * {@link java.util.concurrent.atomic} package specification for
 * description of the properties of atomic variables. An
 * {@code AtomicBoolean} is used in applications such as atomically
 * updated flags, and cannot be used as a replacement for a
 * {@link java.lang.Boolean}.
 *
 * @since 1.5
 * @author Doug Lea
 */
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());
    }

}