HashMap源码解析

2020-06-30

HashMap是Java中极其频繁的、非常重要的一个集合类，在JDK8改动也比较大，本文主要基于JDK8下HashMap的实现

源码分析

类视图
类注释
1. 允许NULL值，NULL键
2. 不要轻易改变负载因子，负载因子过高会导致链表过长，查找键值对时间复杂度就会增高，负载因子过低会导致hash桶的数量过多，空间复杂度会增高
3. Hash表每次会扩容长度为以前的2倍
4. HashMap是多线程不安全的，在JDK1.7进行多线程put操作，之后遍历，直接死循环，CPU飙到100%，在JDK 1.8中进行多线程操作会出现节点和value值丢失，为什么JDK1.7与JDK1.8多线程操作会出现很大不同，是因为JDK 1.8的作者对resize方法进行了优化不会产生链表闭环。这也是本章的重点之一，具体的细节大家可以去查阅资料。这里就不解释太多了

类常量

  /**
   * The default initial capacity - MUST be a power of two.
   */
  static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
  
/**
   * The load factor used when none specified in constructor.
   */
  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.
   */
  static final int TREEIFY_THRESHOLD = 8;
  /**
   * 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.
   */
  static final int MIN_TREEIFY_CAPACITY = 64;
  /**
   * 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.
   */
  static final int MAXIMUM_CAPACITY = 1 << 30;

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;
    //下面介绍一下这行代码的作用
    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);
}

HashMap有4个构造函数.

重点介绍下tableSizeFor(initialCapacity)方法，该方法作用，将你传入的initialCapacity进行计算，返回一个大于等于initialCapacity最小的2的幂次方，比如输入6，结算结果为8，源码如下:

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;
}

插入源码

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

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,                                     
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //当table为空时，这里初始化table，不是通过构造函数初始化，而是在插入时通过扩容初始化，有效防止了初始化HashMap没有数据插入造成空间浪费可能造成内存泄露的情况
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //存放新键值对
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //旧键值对的覆盖
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //在红黑树中查找旧键值对更新
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //将新键值对放在链表的最后
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    //当链表的长度大于等于树化阀值，并且hash桶的长度大于等于MIN_TREEIFY_CAPACITY，链表转化为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //链表中包含键值对
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        //map中含有旧key，返回旧值
        if (e != null) { 
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    //map调整次数加1
    ++modCount;
    //键值对的数量达到阈值需要扩容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

上述代码总结如下：

首次插入进行hash表的初始化操作，扩容初始化，插入键值对
插入的键值对中key已经存在，更新键值对
插入链表，如果链表长度大于MIN_TREEIFY_CAPACITY（默认值为8），转化为红黑树，否则直接插入
检查是否需要扩容，当键值对的个数大于threshold阈值进行扩容操作，其中threshold=size*loadFactor

扩容

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    //如果旧hash桶不为空
    if (oldCap > 0) {
        //超过hash桶的最大长度，将阀值设为最大值
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //新的hash桶的长度2被扩容没有超过最大长度，将新容量阀值扩容为以前的2倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    //如果hash表阈值已经初始化过
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    //如果旧hash桶，并且hash桶容量阈值没有初始化，那么需要初始化新的hash桶的容量和新容量阀值
    else {              
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    //新的局部变量阀值赋值
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    //为当前容量阀值赋值
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    //初始化hash桶
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    //如果旧的hash桶不为空，需要将旧的hash表里的键值对重新映射到新的hash桶中
    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，通过索引位置直接映射
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                //如果是红黑树，需要进行树拆分然后映射
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { 
                    //如果是多个节点的链表，将原链表拆分为两个链表，两个链表的索引位置，一个为原索引，一个为原索引加上旧Hash桶长度的偏移量       
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        //链表1
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        //链表2
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    //链表1存于原索引
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    //链表2存于原索引加上原hash桶长度的偏移量
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

如下情况会产生扩容操作：

初始化HashMap，第一次进行put操作
当键值对的个数大于threshold阀值时产生扩容，threshold=size*loadFactor

源码中关于红黑树的操作、旋转、着色，这里不做介绍，有兴趣可以另行查看

总结

HashMap允许NULL值，NULL键
不要轻易改变负载因子，负载因子过高会导致链表过长，查找键值对时间复杂度就会增高，负载因子过低会导致hash桶的数量过多，空间复杂度会增高
Hash表每次会扩容长度为以前的2倍
HashMap是多线程不安全的，我在JDK 1.7进行多线程put操作，之后遍历，直接死循环，CPU飙到100%，在JDK 1.8中
进行多线程操作会出现节点和value值丢失，为什么JDK1.7与JDK1.8多线程操作会出现很大不同，是因为JDK 1.8的作者对resize
方法进行了优化不会产生链表闭环。这也是本章的重点之一，具体的细节大家可以去查阅资料。这里就不解释太多了
尽量设置HashMap的初始容量，尤其在数据量大的时候，防止多次resize
HashMap在JDK 1.8在做了很好性能的提升，我看到过在JDK1.7和JDK1.8 get操作性能对比JDK1.8是要优于JDK 1.7的，大家感兴趣的可以自己做个测试。