AtomicReference 源碼學(xué)習(xí)

前面寫了原子類的使用(傳送門),現(xiàn)在直接擼一下源碼吧

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類只有200多行源代碼,一下子就能擼完,先看一下成員變量吧:

    
    private static final long serialVersionUID = -1848883965231344442L;
    //unsafe類,提供cas操作的功能
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //value變量的偏移地址,說(shuō)的就是下面那個(gè)value,這個(gè)偏移地址在static塊里初始化,見(jiàn)下面
    private static final long valueOffset;
    //實(shí)際傳入需要原子操作的那個(gè)類實(shí)例
    private volatile V value;

類裝載的時(shí)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

/**
*也沒(méi)什么好說(shuō)的,就是調(diào)用Unsafe的cas操作,傳入對(duì)象,expect值,偏移地址,需要更新的值,即可,如果更新成功,返回true,如果失敗,返回false
public final boolean compareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

這里有個(gè)坑就是,對(duì)于String變量來(lái)說(shuō),必須是對(duì)象相同才視為相同,而不是字符串的內(nèi)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

   //沒(méi)看出和上面那個(gè)有啥區(qū)別
   public final boolean weakCompareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

還有寫奇怪的自旋set的方法,沒(méi)找到UnaryOperator的實(shí)現(xiàn)類...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;
    }