集合

ArrayBlockingQueue和LinkedBlockingQueue区别

• 都实现BlockingQueue接口
• 都是阻塞队列，通过ReetrantLock和Condition实现同步，Condition的await()和signal()实现线程通信
  • LinkedBlockingQueue内部两把锁，读锁takeLock和写锁putLock
  • ArrayBlockingQueue内部读写使用一把锁，可分为公平锁非公平锁

    /** Lock held by take, poll, etc */
    private final ReentrantLock takeLock = new ReentrantLock();

    /** Wait queue for waiting takes */
    private final Condition notEmpty = takeLock.newCondition();

    /** Lock held by put, offer, etc */
    private final ReentrantLock putLock = new ReentrantLock();

    /** Wait queue for waiting puts */
    private final Condition notFull = putLock.newCondition();

    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

• LinkedBlockingQueue通过链表实现，ArrayBlockingQueue通过数组实现
• LinkedBlockingQueue clear()方法使用两把锁

HashMap

• hash异或移位运算作用
  扰动函数，使key均匀分布

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

• 数组长度是2的n次幂
  1. 计算hash时，由&代替%，提高效率
  2. 扩容

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node[] tab; Node p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

从图中可知，(n-1)&hash来确定tab索引，2的n次幂-1转成二进制最高位是0其余全为1，与key的hash值做&运算，结果是key的低位保持不变，即平均分在各个数组里面。

注：两数&，都是1为1，否则为0

• 1.8之后链表长度>=8并且数组长度>=64，才转为红黑树，否则只进行扩容
• 扩容：1.7 扩容使用的是头插法，会造成死循环
  1.8 使用尾插法，扩容之后元素要么在原位置，要么在原位置+原数组长度，通过倒数第5位即可确定。
  
• 数组长度计算：如果初始化传入的长度不是2的n次幂，hashmap会转成比他大的最近的2的n次幂。

ConcurrentHashMap

• 通过CAS+synchronized实现线程安全