HashMap源码解读

1. 字段

        字段设置为 $transient$ ,因为 $HashMap$ 通过 $Object.hashCode(\ )$ 获取哈希值,并通过哈希值与桶个数取模确定对象。$Object.hashCode(\ )$ 是一个本地方法,依赖于JVM实现,存在跨平台问题,所有 $HashMap$ 在序列化时会先保存所有 $Key$ ,再在反序列化时重新插入。

// 最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;

// 默认负载因子,大小/容量超过这个比例就会扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 当桶中节点数不小于该值时树化
// 树节点大小是常规节点两倍,应该在有足够的节点后树化
static final int TREEIFY_THRESHOLD = 8;

// 当桶中节点数小于该值时链表化
// 不设为TREEIFY_THRESHOLD,避免抖动
static final int UNTREEIFY_THRESHOLD = 6;

// 当容量不小于该值时才会树化,否则会优先扩容
// 应当至少为 4 * TREEIFY_THRESHOLD
static final int MIN_TREEIFY_CAPACITY = 64;

// 初次使用时初始化,长度为 2 的幂次
transient Node<K,V>[] table;

// entrySet() 缓存
transient Set<Map.Entry<K,V>> entrySet;

// 改变次数,每次修改都会增加
transient int modCount;

2. 构造函数

public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                            initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                            loadFactor);
    this.loadFactor = loadFactor;
    // tableSizeFor会获得不小于当前数字的最小的2的幂次
    this.threshold = tableSizeFor(initialCapacity);
}

public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

/** 计算大于等于输入参数的最小的 2 的幂次 */
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

3. 内部类

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;

    public final int hashCode() { return Objects.hashCode(key) ^ Objects.hashCode(value); }
}

4. 插入

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

public V put(K key, V value) { return putVal(hash(key), key, value, false, true); }

// @param onlyIfAbsent 如果为true,则不会改变已存在的value
// @param evict        如果为false,说明处于构造模式
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)
        // 表为空,创建表
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        // 对应位置为空,直接创建节点
        tab[i] = newNode(hash, key, value, null);
    // 碰撞处理
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            // 节点key相同
            e = p;
        else if (p instanceof TreeNode)
            // 树状节点,向树中插入
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    // 尾插法
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        // 超过阈值,树化
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    // 节点key相同
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            // 更新值
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        // 超过阈值,扩容
        resize();
    afterNodeInsertion(evict);
    return null;
}

// 扩容方法
final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                    oldCap >= DEFAULT_INITIAL_CAPACITY)
            // 两倍扩容
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                    (int)ft : Integer.MAX_VALUE);
    }
    // 设置新阈值并创建新表
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        // 迁移节点
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                    // 没有碰撞
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                    // 树分裂,这一步会判断是否链表化
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    // 链表分裂
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

5. 删除

public V remove(Object key) {
    Node<K,V> e;
    return (e = removeNode(hash(key), key, null, false, true)) == null ?
        null : e.value;
}

// @param value      值
// @param matchValue true表示只有value相同时才会移除
// @param movable    false则移除时不会移动其他节点
final Node<K,V> removeNode(int hash, Object key, Object value,
                            boolean matchValue, boolean movable) {
    Node<K,V>[] tab; Node<K,V> p; int n, index;
    // 表存在、节点存在
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (p = tab[index = (n - 1) & hash]) != null) {
        Node<K,V> node = null, e; K k; V v;
        // 查找节点
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            node = p;
        else if ((e = p.next) != null) {
            if (p instanceof TreeNode)
                node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
            else {
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key ||
                            (key != null && key.equals(k)))) {
                        node = e;
                        break;
                    }
                    p = e;
                } while ((e = e.next) != null);
            }
        }
        // 找到对应节点
        if (node != null && (!matchValue || (v = node.value) == value ||
                                (value != null && value.equals(v)))) {
            if (node instanceof TreeNode)
                // 树移除节点,这一步如果红黑树太小会链表化
                ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
            else if (node == p)
                tab[index] = node.next;
            else
                p.next = node.next;
            ++modCount;
            --size;
            afterNodeRemoval(node);
            return node;
        }
    }
    return null;
}

6. 区别

  1. JDK1.7使用头插法,JDK1.8使用尾插法,避免多线程同时扩容时的死循环;
  2. JDK1.7是先扩容后插入的,JDK1.8反过来,避免无效扩容;
  3. JDK1.7只有数组+链表,JDK1.8引入红黑树;

HashMap源码解读