CAS

CAS 底层实现

AtomicInteger 为例

• 利用 unsafe 工具拿到 value 的内存地址
• 并且利用 volatile 保证 value 的可见性 ```java 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); } }

private volatile long value;

```java
public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}

Unsafe

• 这里的 getAndAddInt 就是 do while + cms

  // class Unsafe
  public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
        var5 = this.getIntVolatile(var1, var2);
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
    return var5;
  }

• cms 是 native 方法，由 os 保证原子性

  public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);