【Java并发】原子类源码分析之AtomicBoolean

JDK1.8

public class AtomicBoolean implements java.io.Serializable {
    private static final long serialVersionUID = 4654671469794556979L;
    // setup to use Unsafe.compareAndSwapInt for updates
    // 使用Unsafe.compareAndSwapInt进行更新
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    // valueOffset是对象在内存中的偏移量
    private static final long valueOffset;

    //初始阶段，通过unsafe来获取AtomicBoolean类的value字段在内存中的偏移量valueOffset
    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicBoolean.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }
    //value被volatile修饰，保证其内存可见性，后面也会以value来判断AtomicBoolean值
    private volatile int value;

    /**
     * Creates a new {@code AtomicBoolean} with the given initial value.
     *
     * @param initialValue the initial value
     */
    //构造方法：支持设置初始值，并且以int来存储  true -> 1 false -> 0
    public AtomicBoolean(boolean initialValue) {
        value = initialValue ? 1 : 0;
    }

    /**
     * Creates a new {@code AtomicBoolean} with initial value {@code false}.
     */
    //构造方法：没有赋值，没赋值的话，int默认为0， 0 -> false
    public AtomicBoolean() {
    }

    /**
     * Returns the current value.
     *
     * @return the current value
     */
    //获取当前value值 final修饰
    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.
     */
    //如果当前值 == 期望值expect，便会原子地更新成update值，返回true
    //如果当前值 != 期望值expect，更新失败，返回false
    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
     */
    //是的，这里和compareAndSet是一模一样的实现
    //有兴趣的可以百度下
    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
     */
    //不带任何条件地进行赋值，虽然value被volatile修饰，但是set这里修改不是原子方法
    public final void set(boolean newValue) {
        value = newValue ? 1 : 0;
    }

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    //注释的含义是：最终设置成功
    //和set方法对比来看，lazySet底层调用的是unsafe.putOrderedInt
    //从hotspot源码看putOrderedXXX源码分析底层操作
    //可以参考这个同学的分析，https://www.jianshu.com/p/4ed887664b13
    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
     */
    //原子地更新value值为newValue，并返回更新之前的值
    //先获取更新前的值，然后while一直尝试更新直到成功为止
    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());
    }

}