HashMap源码分析

1. HashMap介绍

HashMap是一个Hash表，通过key-value来存储数据，并允许使用 null 值和 null 键。HashMap并不保证映射顺序，而是通过Hash算法将key-value保存到对应的索引位置。

还有一点就是HashMap不是线程安全的，但是可以通过Collections类的静态方法synchronizedMap变成线程安全的Map。HashMap之所以线程不安全是因为多个线程修改map时，可能会导致环状结构，形成死循环。

2. HashMap继承的接口

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable

3. HashMap属性

    /**
     * The default initial capacity - MUST be a power of two.
     * ---------------------------------------------------------
     * 默认的初始容量，必须是2的次幂
     */
    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.
     * -----------------------------------------
     * 最大的容量值，指定的最大容量值也要小于等于2^29
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     * -------------------------------------------
     * 默认的加载因子
     */
    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.
     * -------------------------------------------
     * 树化链表的阈值，如果一个链表长度大于或等于8，扩容或者树化链表
     */
    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.
     * ------------------------------------------
     * 取消树化的阈值
     */
    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.
     * -----------------------------------------------
     * 当某一位置的链表长度到达8时，就会进行树化。
     * 树化的过程 当table小于64，则进行扩容 ; 否则进行树化
     * 后面会讲解到
     */
    static final int MIN_TREEIFY_CAPACITY = 64;    

    /**
     * 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.)
     * --------------------------------------------------------
     * 存储数据的数组
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     * --------------------------------------------------------------
     * keySet返回的结果集
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     * ----------------------------------------------------------------
     * Hash表中key-value的个数
     */
    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
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     * ---------------------------------------------------------
     * 下一个要调整的大小值
     * @serial
     */
    int threshold;

    /**
     * The load factor for the hash table.
     * ----------------------------------------------
     * 装载因子
     * @serial
     */
    final float loadFactor;

4. 内部类

    /**
     * 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;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }

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

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }
       //判断key-value是否相等
       //判断相等首先要看是否是地址相同，然后判断equals判断key和value是否相同
        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

    //返回key的集合
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

    final class KeySet extends AbstractSet<K> {
        public final int size()                 { return size; }
        public final void clear()               { HashMap.this.clear(); }
        public final Iterator<K> iterator()     { return new KeyIterator(); }
        public final boolean contains(Object o) { return containsKey(o); }
        public final boolean remove(Object key) {
            return removeNode(hash(key), key, null, false, true) != null;
        }
        public final Spliterator<K> spliterator() {
            return new KeySpliterator<>(HashMap.this, 0, -1, 0, 0);
        }
        public final void forEach(Consumer<? super K> action) {
            Node<K,V>[] tab;
            if (action == null)
                throw new NullPointerException();
            if (size > 0 && (tab = table) != null) {
                int mc = modCount;
                for (int i = 0; i < tab.length; ++i) {
                    for (Node<K,V> e = tab[i]; e != null; e = e.next)
                        action.accept(e.key);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

    //返回value的集合
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    final class Values extends AbstractCollection<V> {
        public final int size()                 { return size; }
        public final void clear()               { HashMap.this.clear(); }
        public final Iterator<V> iterator()     { return new ValueIterator(); }
        public final boolean contains(Object o) { return containsValue(o); }
        public final Spliterator<V> spliterator() {
            return new ValueSpliterator<>(HashMap.this, 0, -1, 0, 0);
        }
        public final void forEach(Consumer<? super V> action) {
            Node<K,V>[] tab;
            if (action == null)
                throw new NullPointerException();
            if (size > 0 && (tab = table) != null) {
                int mc = modCount;
                for (int i = 0; i < tab.length; ++i) {
                    for (Node<K,V> e = tab[i]; e != null; e = e.next)
                        action.accept(e.value);
                }
                if (modCount != mc)
                    throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
     * extends Node) so can be used as extension of either regular or
     * linked node.
     */
    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // red-black tree links
        TreeNode<K,V> left;
        TreeNode<K,V> right;
        TreeNode<K,V> prev;    // needed to unlink next upon deletion
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }

        //…………一堆方法
     }

5. 构造方法

    /*
     * 从此方法可以看出，构造函数传进来{初始容量}和{装载因子}，通过修改threshold来在resize()方法中
     * 改变table数组的大小。
     */
    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);
        //装载因子
        this.loadFactor = loadFactor;
        //找到大于initialCapacity最小2的次幂数
        this.threshold = tableSizeFor(initialCapacity);
    }

6. 重要方法

1. resize

    /**
     * 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.
     *
     * @return the table
     */

    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        //oldCap为旧数组长度
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //oldThr为旧下次要调整的大小
        int oldThr = threshold;
        int newCap, newThr = 0;

        //主要是给newCap和newThr赋值
        if (oldCap > 0) {
            //如果数组超过最大值，则将
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                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);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    //只有一个节点
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //红黑树
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    //链表
                    else { // preserve order
                        //将每个链分为两份
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        //循环，将一个链表分割成两个链表
                        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);
                        //loHead链表还在原来位置
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        //hiTail链表迁移到j+oldCap位置
                        if (hiTail != null) {
                            hiTail.next = null;
                            //非常灵性
                            //为什么说j+oldCap有灵性呢？
                            //因为计算机都是二进制存储，扩容之后的新的位置的bit位变成1即可
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

总结一下

resize方法是HashMap中table的初始化方法和扩容方法，扩容后的大小为threshold。然后分割红黑树，或者分割链表。最后返回一个新数组。

2. hash

计算key的hash值

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

3. put

put操作是最最常规用的方法之一

put -> putVal -> 链表尾插入 || putTreeVal ->

    /**
     * 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.
     * ---------------------------------------------------------------------
     * 在map里将特殊的key和value联系起来，如果map中包含这个key，则将旧的value替换成新的value
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

put方法调用的是putVal方法

    /**
     * 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
     * @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;
        //判断数组是否需要初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //1.没有hash冲突
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        //2.有hash冲突，使用拉链法
        else {
            Node<K,V> e; K k;
            //p是一个链表的首元素（头节点）
            //1.key已经存在，但是要根据onlyIfAbsent来决定是否更新
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //2. 红黑树
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //3. 链表
            else {
                //binCount记录链表长度
                for (int binCount = 0; ; ++binCount) {
                    //不存在，插入链表结尾
                    if ((e = p.next) == null) {
                        //新节点
                        p.next = newNode(hash, key, value, null);
                        //是否树化
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    //已存在
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            //存在待更新的值
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                //返回替换的旧节点
                return oldValue;
            }
        }
        //修改次数+1
        ++modCount;
        //判断是否扩容
        if (++size > threshold)
            resize();
        //LinkedHashMap实现的方法，而HashMap中是空方法
        afterNodeInsertion(evict);
        return null;
    }

再了解一下treeifyBin方法

    /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     * ------------------------------------------------------
     * 树化方法
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        //重点
        //table长度小于64就会扩容
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        //树化
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

总结一下putVal

putVal是HashMap的插入方法，首先table是否初始化。接着判断key值是否出现hash冲突，不冲突直接加入table中；冲突则采用拉链法，一种加入到红黑树，一种加入链表。

链表长度大于等于8时，会转成红黑树
转化为红黑树的必要条件是table数组长度大于等于64，否则不会转化为红黑树，而是进行扩容

4. get

    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

get方法调用getNode方法

    /**
     * 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) {
            //判断首元素
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                //树取值
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                //链表取值
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

5. remove

remove为删除方法

    /**
     * Removes the mapping for the specified key from this map if present.
     */
    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }

    public boolean remove(Object key, Object value) {
        return removeNode(hash(key), key, value, true, true) != null;
    }

removeNode方法

    /**
     * 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 
     * matchValue 如果为true equals(value) 则删除 ; 否则不关心value
     * @param movable if false do not move other nodes while removing
     * movable 删除后是否移动节点 true 移动 false 不移动
     * @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) {
            Node<K,V> node = null, e; K k; V v;
            //头节点
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            //在链表或红黑树查找
            else if ((e = p.next) != null) {
                //红黑树
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                //链表
                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);
                }
            }
            //开始移除
            //node为空 --- matchValue=false --- 不需要关心value的值
            //         --- matchValue=true --- key对应的v == value
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                //红黑树移除
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                //链表头节点移除
                else if (node == p)
                    tab[index] = node.next;
                //链中移除
                else
                    p.next = node.next;
                //修改次数
                ++modCount;
                --size;
                //LinkedHashMap实现
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

6. contains

contains判断map中是否包含key或者value

containsKey

如果此映射包含指定键的映射，则返回 true 。

    /**
     * Returns <tt>true</tt> if this map contains a mapping for the
     * specified key.
     *
     * @param   key   The key whose presence in this map is to be tested
     * @return <tt>true</tt> if this map contains a mapping for the specified
     * key.
     */
    public boolean containsKey(Object key) {
        //前面说到getNode方法
        return getNode(hash(key), key) != null;
    }

containsValue

如果此地图将一个或多个键映射到指定值，则返回 true 。

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     *
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value
     */
    public boolean containsValue(Object value) {
        Node<K,V>[] tab; V v;
        if ((tab = table) != null && size > 0) {
            //两层循环判断value是否存在，就像二维数组查找
            for (int i = 0; i < tab.length; ++i) {
                for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                    if ((v = e.value) == value ||
                        (value != null && value.equals(v)))
                        return true;
                }
            }
        }
        return false;
    }

7. replace

    /**
     * 判断oldVaule和key对应的value是否一样，如果一样则替换，返回true，否则返回false
     */
    public boolean replace(K key, V oldValue, V newValue) {
        Node<K,V> e; V v;
        if ((e = getNode(hash(key), key)) != null &&
            ((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {
            e.value = newValue;
            afterNodeAccess(e);
            return true;
        }
        return false;
    }

    /**
     * 找到key，直接替换，将替换后的value返回
     */
    public V replace(K key, V value) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) != null) {
            V oldValue = e.value;
            e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
        return null;
    }

7. 总结

仅是看了部分的HashMap的源码，从中学习到了很多知识

外部public方法调用内部private方法，防止外部直接调用内部逻辑方法
()内赋值再判断，减少代码量，但是可读性变差了，小白可能会崩溃的
内部类和外部类的完美应用，相互依靠，相得益彰
一个方法的设计要尽可能降低耦合度，提高方法的复用
方法中可以提供一些功能，让子类来实现

    // Callbacks to allow LinkedHashMap post-actions
    void afterNodeAccess(Node<K,V> p) { }
    void afterNodeInsertion(boolean evict) { }
    void afterNodeRemoval(Node<K,V> p) { }