java.util.concurrent.atomic.AtomicIntegerJava

文章发布较早，内容可能过时，阅读注意甄别。

# 1. 成员变量

static final Unsafe unsafe = Unsafe.getUnsafe()设置unsafe.compareAndSwapInt更新，

    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

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

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# 2. 成员函数

getAndIncrement方法的实现方式是：

    public final int getAndAddInt(Object o, long offset, int delta) {
        int v;
        do {
            v = getIntVolatile(o, offset);
        } while (!compareAndSwapInt(o, offset, v, v + delta));
        return v;
    }

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do{}while()循环中得到v返回，其中getIntVolatile方法为native原生方法，

    /** Volatile version of {@link #getInt(Object, long)}  */
    public native int     getIntVolatile(Object o, long offset);

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其中compareAndSwapInt方法也为native原生方法，

    /**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapInt(Object o, long offset,
                                                  int expected,
                                                  int x);

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加一时offset为1，减一时offset为-1。

#Java