前言:

HashMap是在面试中经常会问的一点,很多时候我们仅仅只是知道HashMap他是允许键值对都是Null,并且是非线程安全的,如果在多线程的环境下使用,是很容易出现问题的。 这是我们通常在面试中会说的,但是有时候问到底层的源码分析的时候,为什么允许为Null,为什么不安全,这些问题的时候,如果没有分析过源码的话,好像很难回答, 这样的话我们来研究一下这个源码。看看原因吧。

HashMap最早出现在JDK1.2中,它的底层是基于的散列算法。允许键值对都是Null,并且是非线程安全的,我们先看看这个1.8版本的JDK中HashMap的数据结构吧。

HashMap图解如下

HashMap 底层实现原理,看完面试不再懵逼。-LMLPHP

我们都知道HashMap是数组+链表组成的,bucket数组是HashMap的主体,而链表是为了解决哈希冲突而存在的,但是很多人不知道其实HashMap是包含树结构的,但是得有一点 注意事项,什么时候会出现红黑树这种红树结构的呢?我们就得看源码了,源码解释说默认链表长度大于8的时候会转换为树。我们看看源码说的

结构

​/** * Basic hash bin node, used for most entries.  (See below for * TreeNode subclass, and in LinkedHashMap for its Entry subclass.) */ /**    Node是hash基础的节点,是单向链表,实现了Map.Entry接口 */static class Node<K,V> implements Map.Entry<K,V> {    final int hash;    final K key;    V value;    Node<K,V> next;    //构造函数    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;  }  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;  }}​复制代码

接下来就是树结构了

TreeNode 是红黑树的数据结构。

     /**     * 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);        }     /**      * Returns root of tree containing this node.      */     final TreeNode<K,V> root() {         for (TreeNode<K,V> r = this, p;;) {             if ((p = r.parent) == null)                 return r;             r = p;         }     }​复制代码

我们在看一下类的定义

​public class HashMap<K,V> extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable {​复制代码

继承了抽象的map,实现了Map接口,并且进行了序列化。

在类里还有基础的变量

变量

​/** * The default initial capacity - MUST be a power of two. *  默认初始容量 16 - 必须是2的幂 */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. * 最大容量 2的30次方 */static final int MAXIMUM_CAPACITY = 1 << 30;​/** * The load factor used when none specified in constructor. * 默认加载因子,用来计算threshold */ 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. * 链表转成树的阈值,当桶中链表长度大于8时转成树  * threshold = capacity * loadFactor */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. * 进行resize操作时,若桶中数量少于6则从树转成链表 */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. * 桶中结构转化为红黑树对应的table的最小大小 * 当需要将解决 hash 冲突的链表转变为红黑树时, * 需要判断下此时数组容量, * 若是由于数组容量太小(小于 MIN_TREEIFY_CAPACITY ) * 导致的 hash 冲突太多,则不进行链表转变为红黑树操作, * 转为利用 resize() 函数对 hashMap 扩容 */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.) * 保存Node<K,V>节点的数组 * 该表在首次使用时初始化,并根据需要调整大小。 分配时, * 长度始终是2的幂。 */transient Node<K,V>[] table;​/** * Holds cached entrySet(). Note that AbstractMap fields are used * for keySet() and values(). * 存放具体元素的集 */transient Set<Map.Entry<K,V>> entrySet;​/** * The number of key-value mappings contained in this map. * 记录 hashMap 当前存储的元素的数量 */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). * 每次更改map结构的计数器 */transient int modCount;​/** * The next size value at which to resize (capacity * load factor). * 临界值 当实际大小(容量*填充因子)超过临界值时,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:要调整大小的下一个大小值(容量*加载因子)。 * @serial */final float loadFactor;​复制代码

我们再看看构造方法

构造方法

​/** * 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. * 传入初始容量大小,使用默认负载因子值 来初始化HashMap对象 */public HashMap(int initialCapacity) {    this(initialCapacity, DEFAULT_LOAD_FACTOR);}​/** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). * 默认容量和负载因子 */public HashMap() {    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted}​/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param  initialCapacity the initial capacity * @param  loadFactor      the load factor * @throws IllegalArgumentException if the initial capacity is negative *         or the load factor is nonpositive * 传入初始容量大小和负载因子 来初始化HashMap对象 */public HashMap(int initialCapacity, float loadFactor) {     // 初始容量不能小于0,否则报错    if (initialCapacity < 0)        throw new IllegalArgumentException("Illegal initial capacity: " +                                           initialCapacity);    // 初始容量不能大于最大值,否则为最大值        if (initialCapacity > MAXIMUM_CAPACITY)        initialCapacity = MAXIMUM_CAPACITY;    //负载因子不能小于或等于0,不能为非数字    if (loadFactor <= 0 || Float.isNaN(loadFactor))        throw new IllegalArgumentException("Illegal load factor: " +                                           loadFactor);    // 初始化负载因子    this.loadFactor = loadFactor;    // 初始化threshold大小    this.threshold = tableSizeFor(initialCapacity);}​/** * Returns a power of two size for the given target capacity. * 找到大于或等于 cap 的最小2的整数次幂的数 */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;}​复制代码

在这源码中,loadFactor负载因子是一个非常重要的参数,因为他能够反映HashMap桶数组的使用情况, 这样的话,HashMap的时间复杂度就会出现不同的改变。

当这个负载因子属于低负载因子的时候,HashMap所能够容纳的键值对数量就是偏少的,扩容后,重新将键值对 存储在桶数组中,键与键之间产生的碰撞会下降,链表的长度也会随之变短。

但是如果增加负载因子当这个负载因子大于1的时候,HashMap所能够容纳的键值对就会变多,这样碰撞就会增加, 这样的话链表的长度也会增加,一般情况下负载因子我们都不会去修改。都是默认的0.75。加q群:479499375,可获取一份Java进阶学习资料包,有(Java工程化、分布式架构、高并发、高性能、深入浅出、微服务架构、Spring、MyBatis、Netty、源码分析、JVM原理解析等...这些成为架构师必备的内容)以及Java进阶学习路线图。

扩容机制

resize()这个方法就是重新计算容量的一个方法,我们看看源码:

​/** * 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() {    //引用扩容前的Entry数组    Node<K,V>[] oldTab = table;    int oldCap = (oldTab == null) ? 0 : oldTab.length;    int oldThr = threshold;    int newCap, newThr = 0;    if (oldCap > 0) {​        // 扩容前的数组大小如果已经达到最大(2^30)了        //在这里去判断是否达到最大的大小         if (oldCap >= MAXIMUM_CAPACITY) {               //修改阈值为int的最大值(2^31-1),这样以后就不会扩容了            threshold = Integer.MAX_VALUE;            return oldTab;        }​        // 如果扩容后小于最大值 而且 旧数组桶大于初始容量16, 阈值左移1(扩大2倍)        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&                 oldCap >= DEFAULT_INITIAL_CAPACITY)            newThr = oldThr << 1; // double threshold    }    // 如果数组桶容量<=0 且 旧阈值 >0    else if (oldThr > 0) // initial capacity was placed in threshold        //新的容量就等于旧的阀值        newCap = oldThr;    else {               // zero initial threshold signifies using defaults         // 如果数组桶容量<=0 且 旧阈值 <=0         // 新容量=默认容量         // 新阈值= 负载因子*默认容量        newCap = DEFAULT_INITIAL_CAPACITY;        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);    }    // 如果新阈值为0    if (newThr == 0) {        // 重新计算阈值        float ft = (float)newCap * loadFactor;        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?                  (int)ft : Integer.MAX_VALUE);    }    //在这里就会 更新阈值    threshold = newThr;    @SuppressWarnings({"rawtypes","unchecked"})     //创建新的数组     Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];​    // 覆盖数组桶    table = newTab;     // 如果旧数组桶不是空,则遍历桶数组,并将键值对映射到新的桶数组中    //在这里还有一点诡异的,1.7是不存在后边红黑树的,但是1.8就是有红黑树    if (oldTab != null) {        for (int j = 0; j < oldCap; ++j) {            Node<K,V> e;            if ((e = oldTab[j]) != null) {                oldTab[j] = null;                if (e.next == null)                    newTab[e.hash & (newCap - 1)] = e;​                // 如果是红黑树                else if (e instanceof TreeNode)​                    // 重新映射时,然后对红黑树进行拆分                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);                else { // preserve order                    // 如果不是红黑树,那也就是说他链表长度没有超过8,那么还是链表,                    //那么还是会按照链表处理                    Node<K,V> loHead = null, loTail = null;                    Node<K,V> hiHead = null, hiTail = null;                    Node<K,V> next;​                    // 遍历链表,并将链表节点按原顺序进行分组                    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);                    // 将分组后的链表映射到新桶中                    if (loTail != null) {                        loTail.next = null;                        newTab[j] = loHead;                    }                    if (hiTail != null) {                        hiTail.next = null;                        newTab[j + oldCap] = hiHead;                    }                }            }        }    }    return newTab;}​复制代码

所以说在经过resize这个方法之后,元素的位置要么就是在原来的位置,要么就是在原来的位置移动2次幂的位置上。 源码上的注释也是可以翻译出来的

​/**     * 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​     如果为null,则分配符合字段阈值中保存的初始容量目标。      否则,因为我们使用的是2次幂扩展,     所以每个bin中的元素必须保持相同的索引,或者在新表中以2的偏移量移动。​     */    final Node<K,V>[] resize() .....​复制代码

所以说他的扩容其实很有意思,就有了三种不同的扩容方式了,

在HashMap刚初始化的时候,使用默认的构造初始化,会返回一个空的table,并且 thershold为0,因此第一次扩容的时候默认值就会是16. 同时再去计算thershold = DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY = 16*0.75 = 12.

如果说指定初始容量的初始HashMap的时候,那么这时候计算这个threshold的时候就变成了 threshold = DEFAULT_LOAD_FACTOR * threshold(当前的容量)

如果HashMap不是第一次扩容,已经扩容过了,那么每次table的容量

threshold也会变成原来的2倍。

之前看1.7的源码的时候,是没有这个红黑树的,而是在1.8 之后做了相应的优化。 使用的是2次幂的扩展(指长度扩为原来2倍)。 而且在扩充HashMap的时候,不需要像JDK1.7的实现那样重新计算hash,这样子他就剩下了计算hash的时间了。

07-11 04:20