Java8集合框架——HashMap源码分析

java.util.HashMap

本文目录：

• 一、HashMap 的特点概述和说明
• 二、HashMap 的内部实现：从内部属性和构造函数说起
• 三、HashMap 的 put 操作
• 四、HashMap 的扩容
• 五、HashMap 的 get 操作
• 六、HashMap 的 remove 操作
• 七、参考

一、HashMap的特点概述和说明

关注点	HashMap的相关结论
是否允许空的 key	是
是否允许重复的 key	否，实际上可能会进行覆盖更新
元素有序：读取数据和存放数据的顺序一致	否，读取和存放都无序
是否线程安全	否
通过 key 进行随机访问的效率	较快
添加元素的效率	较快 涉及扩容、列表转红黑树、遍历列表时相对慢
删除元素的效率	较快

这里主要提几点：

1. Java8 中 HashMap 源码的大方向就是：数组 + 单向链表(数组的元素，Node 实例，包含四个属性：key, value, hash 值和用于单向链表的 next) + 红黑树(链表超过8个元素且总元素个数超过 64 时转换为红黑树)，对于hash冲突的元素，使用链表进行存储，每次存储在链表末尾。
2. capacity：当前数组容量，默认值是 16，自动扩容，但始终保持 2^n，即扩容后数组大小为当前的 2 倍。
3. loadFactor：负载因子，默认为 0.75。
4. threshold：扩容的阈值，等于 capacity * loadFactor，当元素实际个数 size 大于等于 threshold 时，进行扩容。

　　Java8的 HashMap，最大的改变，是使用了数组 + 链表 + 红黑树。当链表中的元素达到了 8 个且总元素个数超过64个时，会将链表转换为红黑树，在这些位置进行查找的时候可以由原来的耗时 O(N)，降低到时间复杂度为 O(logN)。另附上简要示意图：

二、HashMap的内部实现：从内部属性和构造函数说起

1、常用的类属性

　　常用的类属性如下，比如默认容量、负载因子等。

    /**
     * The default initial capacity - MUST be a power of two.
     * 默认的初始容量 16 = 2 ^ 4
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
 
    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     * 允许的最大容量
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;
 
    /**
     * The load factor used when none specified in constructor.
     * 默认的负载因子
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
 
    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     * 达到需要转化为红黑树时的链表容量阈值
     */
    static final int TREEIFY_THRESHOLD = 8;
 
    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     * 红黑树转回链表的下限阈值
     */
    static final int UNTREEIFY_THRESHOLD = 6;
 
    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     * 达到需要转化为红黑树时的Map总容量最低阈值
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

2、实例属性

　　实例属性，包括内部实际存储元素的数组、Map 的实际大小、实际的负载因子、修改次数等

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     * 内部实际存储元素的数组
     */
    transient Node<K,V>[] table;
 
    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;
 
    /**
     * The number of key-value mappings contained in this map.
     * map的大小，即实际的元素个数
     */
    transient int size;
 
    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     * 修改次数，用于 fail-fast 机制校验
     */
    transient int modCount;
 
    /**
     * The next size value at which to resize (capacity * load factor).
     * 容量阈值，元素个数超过阈值是会进行扩容
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;
 
    /**
     * The load factor for the hash table.
     * 负载因子
     * @serial
     */
    final float loadFactor;

3、节点内部类

　　这个是实际的元素存储节点。

    /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;
        ...
    }

4、构造函数

　　几个构造函数中最主要的还是要设定初始的负载因子 loadFactor。

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     * 指定 初始容量 和 负载因子
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    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);
        // 设定负载因子
        this.loadFactor = loadFactor;
        // 设定阈值为 大于等于指定初始容量且最小 的 2^n
        this.threshold = tableSizeFor(initialCapacity);
    }
 
    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity the initial capacity.
     * @throws IllegalArgumentException if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        // 使用默认的负载因子 0.75
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
 
    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        // 使用默认负载因子 0.75，而其他使用默认值，包括 阈值 threshold = 0
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
 
    /**
     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified <tt>Map</tt>.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        // 使用默认的负载因子 0.75
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }
 
    /**
     * Implements Map.putAll and Map constructor
     *
     * @param m the map
     * @param evict false when initially constructing this map, else
     * true (relayed to method afterNodeInsertion).
     */
    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        if (s > 0) {
            if (table == null) { // pre-size，说明还没有初始化，进行阈值的预设定
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                         (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)
                    threshold = tableSizeFor(t);    // 大于等于指定容量且最小 的 2^n
            }
            else if (s > threshold)
                resize();    // 先进行一次扩容
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);    // 内部还会检查是否需要扩容
            }
        }
    }

三、HashMap 的 put 操作

　　put 操作的源码如下：

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
 
    /**
     * Implements Map.put and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * onlyIfAbsent 为 true 时表示不修改已存在的（key 对应的）value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        // 因为前面构造器都没有对数组 table 进行初始化，因此第一次进行put操作时需要进行扩容
        // 由于 table 本身为 null，最终也只是新建大小为默认容量 16 的数组而已
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        // 当前 key 对应的具体数组下标，如果对应数组元素为 null，则直接初始化 Node 元素并设置即可
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {  // 说明该下标位置存在相关的元素数据
            Node<K,V> e; K k;
            // 数组对应位置的元素的 key 与 put 操作的 key “相等”
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            // 否则需要判断是链表还是红黑树来执行 put 操作
            // 红黑树节点则按照红黑树的插值方法进行
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                // 执行到这里说明数组的该位置是一个链表
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        // 插入到链表的最后位置
                        p.next = newNode(hash, key, value, null);
                        // binCount 为 7 时触发红黑树转化，明显此时0-6已经有节点了，
                        // 再加上原来的 tab[i]（相当于 -1），新插入的是链表的第9个位置
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    // put 操作的 key 与链表中的该位置的 key “相等”
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        // 说明在链表中存在并找到了与 key 一致的节点
                        break;
                    p = e;
                }
            }
            // 存在旧的 key 与要 put 的 key 一致，考虑是否进行值覆盖，然后返回旧值
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        // 记录修改次数
        ++modCount;
        // 超过阈值，则进行扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

 四、HashMap 的扩容

　　HashMap扩容的过程也就是内部数组的扩容，源码如下：

    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     * 初始化 或者 倍增扩容
     * @return the table
     */
    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;
            }
            // 将数组大小扩大一倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold 阈值扩大一倍
        }
        // 有进行初始化容量的设定时会设置 threshold，此时的第一次 put 操作进入这里
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            // 没有指定初始容量的 new HashMap()，第一次 put 操作进入这里
            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);
        }
        threshold = newThr;
        // 初始化数组或者创建容量翻倍的数组
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {    // 初始化的则会跳过这里直接返回
            // 遍历数组进行节点的迁移
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    // 该位置是单个节点元素，不是数组也不是红黑树，则直接迁移
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)    // 红黑树时的节点迁移
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        // 链表情况下的迁移
                        // 将链表拆成两个链表，放到新的数组中，并且保留原来的先后顺序
                        // loHead、loTail 对应一条链表，hiHead、hiTail 则对应另一条链表
                        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;
                            // 第一条链表
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            // 第二条链表的新的位置是 j + oldCap，也比较好理解
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

五、HashMap 的 get 操作

　　get 操作比较直接，流程大致如下：

1. 计算 key 的 hash 值，根据 hash 找到对应的数组下标即 (table.length - 1) & hash；
2. 判断对应下标处的节点元素是否正好是要寻找，如是即返回；否，继续下一步；
3. 如果是红黑树，则用红黑树的方法获取数据；
4. 如果是链表，则按照链表的方式寻找相应的节点；
5. 找不到的，返回 null。

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            // 第一个节点的判断
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                // 红黑树的走法
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                // 链表的遍历
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

六、HashMap 的 remove 操作

　　源码走起：

    /**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key whose mapping is to be removed from the map
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }
 
    /**
     * Implements Map.remove and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to match if matchValue, else ignored
     * @param matchValue if true only remove if value is equal
     * @param movable if false do not move other nodes while removing
     * @return the node, or null if none
     */
    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;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {    // 先找到对应的数组下标
            Node<K,V> node = null, e; K k; V v;
            // 总是先判断第一个是否是要找的
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                // 红黑树的找法
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {    // 链表的找法
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                // 红黑树按照红黑树的方式移除
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                // 刚好是数组节点的移除
                else if (node == p)
                    tab[index] = node.next;
                // 链表的移除
                else
                    p.next = node.next;
                // 记录 修改次数 和元素节点的 实际个数
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

七、参考

　　Java8 中 HashMap 的相关基本操作源码介绍，这里也可以直接参考【Java7/8中的 HashMap 和 ConcurrentHashMap 全解析】，介绍得还是挺详细的。

备注：关于红黑树和ConcurrentHashMap，有待后续的进一步研究。