HashMap、HashTable、ConcurrentHashMap的對比

ConcurrentHashMap：是線程安全的，用於併發，相比於HashTable效率提高顯著，HashTable用synchronized鎖住整個表，ConcurrentHashMap因爲是分段加鎖，可以允許多個修改同時併發進行。底層實現是分段的數組加鏈表。
底層相關源碼：

/**
     * Stripped-down version of helper class used in previous version,
     * declared for the sake of serialization compatibility
     */
    static class Segment<K,V> extends ReentrantLock implements Serializable {
        private static final long serialVersionUID = 2249069246763182397L;
        final float loadFactor;
        Segment(float lf) { this.loadFactor = lf; }
    }

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // For serialization compatibility
        // Emulate segment calculation from previous version of this class
        int sshift = 0;
        int ssize = 1;
        while (ssize < DEFAULT_CONCURRENCY_LEVEL) {
            ++sshift;
            ssize <<= 1;
        }
        int segmentShift = 32 - sshift;
        int segmentMask = ssize - 1;
        @SuppressWarnings("unchecked")
        Segment<K,V>[] segments = (Segment<K,V>[])
            new Segment<?,?>[DEFAULT_CONCURRENCY_LEVEL];
        for (int i = 0; i < segments.length; ++i)
            segments[i] = new Segment<K,V>(LOAD_FACTOR);
        s.putFields().put("segments", segments);
        s.putFields().put("segmentShift", segmentShift);
        s.putFields().put("segmentMask", segmentMask);
        s.writeFields();

        Node<K,V>[] t;
        if ((t = table) != null) {
            Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length);
            for (Node<K,V> p; (p = it.advance()) != null; ) {
                s.writeObject(p.key);
                s.writeObject(p.val);
            }
        }
        s.writeObject(null);
        s.writeObject(null);
        segments = null; // throw away
    }

HashTable：

HashTable是線程安全的，不過相對與性能來說相對較低，因爲其使用synchronized鎖住整個表讓一個線程獨佔，不釋放鎖其他線程只能等待，導致CPU資源利用率低，其底層實現的話，是數組加鏈表，其中key-value不允許有空值

HashMap：

HashMap是線程不安全的，key-vlaue可以爲空值，底層實現的話是數組加鏈表
源碼實現：

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;
    }