源码看JAVA【二十九】AtomicLong

1、VM_SUPPORTS_LONG_CAS：虚拟机是否支持CAS操作

    /**
     * Records whether the underlying JVM supports lockless
     * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
     * method works in either case, some constructions should be
     * handled at Java level to avoid locking user-visible locks.
     */
    static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();

    /**
     * Returns whether underlying JVM supports lockless CompareAndSet
     * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
     */
    private static native boolean VMSupportsCS8();

2、变量定义

valueOffset对应value在内存中的地址

volatile 修饰value使value在多线程下可见。

    private static final long valueOffset;

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

    private volatile long value;

3、boolean compareAndSet(long expect, long update)：设置前比较前值是否一致，一致则设置，不一致则设置失败。设置成功返回true，设置失败返回false

    /**
     * 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(long expect, long update) {
        return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * 
May fail
     * spuriously and does not provide ordering guarantees, 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 final boolean weakCompareAndSet(long expect, long update) {
        return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
    }

4、long getAndSet(long newValue)：设置新值，并获取设置新值前的值。如果设置新值不成功，则会一直执行直到成功并返回

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

5、一系列的自增自减操作方法，原子性与int类似，因为操作的原子性，在设置时可保证多线程的安全性。

    /**
     * Atomically increments by one the current value.
     *
     * @return the previous value
     */
    public final long getAndIncrement() {
        return unsafe.getAndAddLong(this, valueOffset, 1L);
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the previous value
     */
    public final long getAndDecrement() {
        return unsafe.getAndAddLong(this, valueOffset, -1L);
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the previous value
     */
    public final long getAndAdd(long delta) {
        return unsafe.getAndAddLong(this, valueOffset, delta);
    }

    /**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final long incrementAndGet() {
        return unsafe.getAndAddLong(this, valueOffset, 1L) + 1L;
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the updated value
     */
    public final long decrementAndGet() {
        return unsafe.getAndAddLong(this, valueOffset, -1L) - 1L;
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the updated value
     */
    public final long addAndGet(long delta) {
        return unsafe.getAndAddLong(this, valueOffset, delta) + delta;
    }