【轉載】HashMap源碼分析

HashMap最先出如今JDK1.2中，它的底層是基於的散列算法。容許鍵值對都是Null，而且是非線程安全的，咱們先看看這個1.8版本的JDK中HashMap的數據結構把。

HashMap圖解以下

咱們都知道HashMap是數組+鏈表組成的，bucket數組是HashMap的主體，而鏈表是爲了解決哈希衝突而存在的，可是不少人不知道其實HashMap是包含樹結構的，可是得有一點 注意事項，何時會出現紅黑樹這種紅樹結構的呢？咱們就得看源碼了，源碼解釋說默認鏈表長度大於8的時候會轉換爲樹。咱們看看源碼說的

結構

/**
 * Basic hash bin node, used for most entries.  (See below for
 * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
 */
 /**
    Node是hash基礎的節點，是單向鏈表，實現了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;
    //構造函數
    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;
  }
}

接下來就是樹結構了

TreeNode 是紅黑樹的數據結構。

/**
     * 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;
         }
     }

咱們在看一下類的定義

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

繼承了抽象的map，實現了Map接口，而且進行了序列化。

在類裏還有基礎的變量

變量

/**
 * The default initial capacity - MUST be a power of two.
 *  默認初始容量 16 - 必須是2的冪
 */
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.
 * 默認加載因子，用來計算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.
 * 鏈表轉成樹的閾值，當桶中鏈表長度大於8時轉成樹 
 * 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.
 * 進行resize操做時，若桶中數量少於6則從樹轉成鏈表
 */
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.
 * 桶中結構轉化爲紅黑樹對應的table的最小大小
 * 當須要將解決 hash 衝突的鏈表轉變爲紅黑樹時，
 * 須要判斷下此時數組容量，
 * 如果因爲數組容量過小（小於　MIN_TREEIFY_CAPACITY　）
 * 致使的 hash 衝突太多，則不進行鏈表轉變爲紅黑樹操做，
 * 轉爲利用　resize() 函數對　hashMap 擴容
 */
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>節點的數組
 * 該表在首次使用時初始化，並根據須要調整大小。 分配時，
 * 長度始終是2的冪。
 */
transient Node<K,V>[] table;

/**
 * Holds cached entrySet(). Note that AbstractMap fields are used
 * for keySet() and values().
 * 存放具體元素的集
 */
transient Set<Map.Entry<K,V>> entrySet;

/**
 * The number of key-value mappings contained in this map.
 * 記錄 hashMap 當前存儲的元素的數量
 */
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結構的計數器
 */
transient int modCount;

/**
 * The next size value at which to resize (capacity * load factor).
 * 臨界值 當實際大小(容量*填充因子)超過臨界值時，會進行擴容
 * @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.
 * 負載因子：要調整大小的下一個大小值（容量*加載因子）。
 * @serial
 */
final float loadFactor;

咱們再看看構造方法

構造方法

/**
 * 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.
 * 傳入初始容量大小，使用默認負載因子值 來初始化HashMap對象
 */
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).
 * 默認容量和負載因子
 */
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
 * 傳入初始容量大小和負載因子 來初始化HashMap對象
 */
public HashMap(int initialCapacity, float loadFactor) {
     // 初始容量不能小於0，不然報錯
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    // 初始容量不能大於最大值，不然爲最大值    
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    //負載因子不能小於或等於0，不能爲非數字
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    // 初始化負載因子
    this.loadFactor = loadFactor;
    // 初始化threshold大小
    this.threshold = tableSizeFor(initialCapacity);
}

/**
 * Returns a power of two size for the given target capacity.
 * 找到大於或等於 cap 的最小2的整數次冪的數
 */
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;
}

在這源碼中，loadFactor負載因子是一個很是重要的參數，由於他可以反映HashMap桶數組的使用狀況， 這樣的話，HashMap的時間複雜度就會出現不一樣的改變。

當這個負載因子屬於低負載因子的時候，HashMap所可以容納的鍵值對數量就是偏少的，擴容後，從新將鍵值對 存儲在桶數組中，鍵與鍵之間產生的碰撞會降低，鏈表的長度也會隨之變短。

可是若是增長負載因子當這個負載因子大於1的時候，HashMap所可以容納的鍵值對就會變多，這樣碰撞就會增長， 這樣的話鏈表的長度也會增長，

通常狀況下負載因子咱們都不會去修改。都是默認的0.75。

擴容機制

resize()這個方法就是從新計算容量的一個方法，咱們看看源碼：

/**
 * 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() {
    //引用擴容前的Entry數組
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
       
        // 擴容前的數組大小若是已經達到最大(2^30)了
        //在這裏去判斷是否達到最大的大小 
        if (oldCap >= MAXIMUM_CAPACITY) {
               //修改閾值爲int的最大值(2^31-1)，這樣之後就不會擴容了
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        
        // 若是擴容後小於最大值 並且 舊數組桶大於初始容量16， 閾值左移1(擴大2倍)
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    // 若是數組桶容量<=0 且 舊閾值 >0
    else if (oldThr > 0) // initial capacity was placed in threshold
        //新的容量就等於舊的閥值
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
         // 若是數組桶容量<=0 且 舊閾值 <=0
         // 新容量=默認容量
         // 新閾值= 負載因子*默認容量
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    // 若是新閾值爲0
    if (newThr == 0) {
        // 從新計算閾值
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    //在這裏就會 更新閾值
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
     //建立新的數組
     Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    
    // 覆蓋數組桶
    table = newTab;
     // 若是舊數組桶不是空，則遍歷桶數組，並將鍵值對映射到新的桶數組中
    //在這裏還有一點詭異的，1.7是不存在後邊紅黑樹的，可是1.8就是有紅黑樹
    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;
                
                // 若是是紅黑樹
                else if (e instanceof TreeNode)
                
                    // 從新映射時，而後對紅黑樹進行拆分
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    // 若是不是紅黑樹，那也就是說他鏈表長度沒有超過8，那麼仍是鏈表，
                    //那麼仍是會按照鏈表處理
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    
                    // 遍歷鏈表，並將鏈表節點按原順序進行分組
                    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);
                    // 將分組後的鏈表映射到新桶中
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

因此說在通過resize這個方法以後，元素的位置要麼就是在原來的位置，要麼就是在原來的位置移動2次冪的位置上。 源碼上的註釋也是能夠翻譯出來的

/**
     * 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
     
     若是爲null，則分配符合字段閾值中保存的初始容量目標。 
     不然，由於咱們使用的是2次冪擴展，
     因此每一個bin中的元素必須保持相同的索引，或者在新表中以2的偏移量移動。
     
     */
    final Node<K,V>[] resize() .....

因此說他的擴容其實頗有意思，就有了三種不一樣的擴容方式了，

1. 在HashMap剛初始化的時候，使用默認的構造初始化，會返回一個空的table，而且 thershold爲0，所以第一次擴容的時候默認值就會是16. 同時再去計算thershold = DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY = 16*0.75 = 12.
2. 若是說指定初始容量的初始HashMap的時候，那麼這時候計算這個threshold的時候就變成了 threshold = DEFAULT_LOAD_FACTOR * threshold(當前的容量)
3. 若是HashMap不是第一次擴容，已經擴容過了，那麼每次table的容量和threshold也會變成原來的2倍。

以前看1.7的源碼的時候，是沒有這個紅黑樹的，而是在1.8 以後作了相應的優化。 使用的是2次冪的擴展(指長度擴爲原來2倍)。 並且在擴充HashMap的時候，不須要像JDK1.7的實現那樣從新計算hash，這樣子他就剩下了計算hash的時間了。