相关疑难解决方法(0)

在ArrayBlockingQueue,所有需要锁的方法final在调用之前将其复制到局部变量lock().

public boolean offer(E e) {
    if (e == null) throw new NullPointerException();
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        if (count == items.length)
            return false;
        else {
            insert(e);
            return true;
        }
    } finally {
        lock.unlock();
    }
}

当字段是什么时,有没有理由复制this.lock到局部变量？lockthis.lockfinal

此外,它还在使用E[]之前使用本地副本:

private E extract() {
    final E[] items = this.items;
    E x = items[takeIndex];
    items[takeIndex] = null;
    takeIndex = inc(takeIndex);
    --count;
    notFull.signal();
    return x;
}

有没有理由将最终字段复制到本地最终变量？

我查看了源代码java.util.HashMap并看到了以下代码:

public Set<K> keySet() {
    Set<K> ks;
    return (ks = keySet) == null ? (keySet = new KeySet()) : ks;
}

(Windows,java版"1.8.0_111")

在我的MacBook上它看起来像这样:

public Set<K> keySet() {
    Set<K> ks = keySet;
    if (ks == null) {
        ks = new KeySet();
        keySet = ks;
    }
    return ks;
}

(MacOs X Sierra,java版"1.8.0_121")

为什么两个变体都声明了局部变量ks？为什么不这样写:

public Set<K> keySet() {
    if (keySet == null) {
        keySet = new KeySet();
    }
    return keySet;
}

要么

public Set<K> keySet() {
    return keySet == null ? (keySet = …