ArrayBlockingQueue和LinkedBlockingQueue區(qū)別

• 都實現(xiàn)BlockingQueue接口
• 都是阻塞隊列本冲，通過ReetrantLock和Condition實現(xiàn)同步，Condition的await()和signal()實現(xiàn)線程通信
  • LinkedBlockingQueue內(nèi)部兩把鎖升酣，讀鎖takeLock和寫鎖putLock
  • ArrayBlockingQueue內(nèi)部讀寫使用一把鎖诫睬，可分為公平鎖非公平鎖

    /** 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通過鏈表實現(xiàn)煞茫，ArrayBlockingQueue通過數(shù)組實現(xiàn)
• LinkedBlockingQueue clear()方法使用兩把鎖

HashMap

• hash異或移位運算作用
  擾動函數(shù)，使key均勻分布

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

• 數(shù)組長度是2的n次冪
  1. 計算hash時，由&代替%续徽，提高效率
  2. 擴容

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> 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<K,V> 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<K,V>)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;
    }

從圖中可知蚓曼，(n-1)&hash來確定tab索引，2的n次冪-1轉(zhuǎn)成二進制最高位是0其余全為1钦扭，與key的hash值做&運算纫版，結(jié)果是key的低位保持不變，即平均分在各個數(shù)組里面客情。

注：兩數(shù)&其弊，都是1為1，否則為0

• 1.8之后鏈表長度>=8并且數(shù)組長度>=64膀斋，才轉(zhuǎn)為紅黑樹梭伐，否則只進行擴容
• 擴容：1.7 擴容使用的是頭插法，會造成死循環(huán)
  1.8 使用尾插法仰担，擴容之后元素要么在原位置糊识，要么在原位置+原數(shù)組長度，通過倒數(shù)第5位即可確定摔蓝。
  
• 數(shù)組長度計算：如果初始化傳入的長度不是2的n次冪赂苗，hashmap會轉(zhuǎn)成比他大的最近的2的n次冪。

ConcurrentHashMap

• 通過CAS+synchronized實現(xiàn)線程安全