AtomicReference 源码学习
前面写了原子类的使用(传送门),现在直接撸一下源码吧
先看一下如何使用AR
AtomicReference<String> ar = new AtomicReference<String>();
ar.set("senninha");
//CAS操作更新
ar.compareAndSet("senninha", "senninha1");
AR类只有200多行源代码,一下子就能撸完,先看一下成员变量吧:
private static final long serialVersionUID = -1848883965231344442L;
//unsafe类,提供cas操作的功能
private static final Unsafe unsafe = Unsafe.getUnsafe();
//value变量的偏移地址,说的就是下面那个value,这个偏移地址在static块里初始化,见下面
private static final long valueOffset;
//实际传入需要原子操作的那个类实例
private volatile V value;
类装载的时候初始化偏移地址:
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicReference.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
compareAndSet方法
/**
*也没什么好说的,就是调用Unsafe的cas操作,传入对象,expect值,偏移地址,需要更新的值,即可,如果更新成功,返回true,如果失败,返回false
public final boolean compareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
这里有个坑就是,对于String变量来说,必须是对象相同才视为相同,而不是字符串的内容相同就可以相同,如下:
AtomicReference<String> ar = new AtomicReference<String>();
ar.set("senninha");
System.out.println(ar.compareAndSet(new String("senninha"), "senninha1"));//false
weakCompareAndSet方法
//没看出和上面那个有啥区别
public final boolean weakCompareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
还有写奇怪的自旋set的方法,没找到UnaryOperator的实现类...反正就是自旋操作
public final V getAndUpdate(UnaryOperator<V> updateFunction) {
V prev, next;
do {
prev = get();
next = updateFunction.apply(prev);
} while (!compareAndSet(prev, next));
return prev;
}
