Jdk1.8的优化（相比1.7）

数组+链表改成了数组 + 链表/红黑树

链表插入由头插法改为尾插法

扩容时1.7对原数组中的元素重新hash定位，1.8是位置不变或者是索引+旧容量大小

插入与扩容的顺序。1.8是先插入再扩容。

线程安全的做法

hashmap有数据覆盖的问题。

不是线程安全。例子：putval 比如线程1符合判断条件，正好挂起；而线程2开始写。这是A恢复现场，赋值操作。还有重复扩容。

<code>final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node[] tab; Node p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break; 

                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }/<code>

HashTable

直接在操作方法上加上synchronized，锁住整个数组,粗粒度。

<code>public class Hashtable
    extends Dictionary
    implements Map, Cloneable, java.io.Serializable {
\t\t......................................
\t\tpublic Hashtable(Map extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }
 
    public synchronized int size() {
        return count;
    }
    public synchronized boolean isEmpty() {
        return count == 0;
    } 

    public synchronized Enumeration keys() {
        return this.getEnumeration(KEYS);
    }
    public synchronized Enumeration elements() {
        return this.getEnumeration(VALUES);
    }
    public synchronized boolean contains(Object value) {
        if (value == null) {
            throw new NullPointerException();
        }
        Entry,?> tab[] = table;
        for (int i = tab.length ; i-- > 0 ;) {
            for (Entry,?> e = tab[i] ; e != null ; e = e.next) {
                if (e.value.equals(value)) {
                    return true;
                }
            }
        }
        return false;
    }
\t\t
    }
    .......................................
}/<code>

Collections.synchronizedMap

内部定义一个对象锁mutex

<code>public static  Map synchronizedMap(Map m) {
        return new SynchronizedMap<>(m);
    }
    /**
     * @serial include
     */
    private static class SynchronizedMap 

        implements Map, Serializable {
        private static final long serialVersionUID = 1978198479659022715L;
        private final Map m;     // Backing Map
        final Object      mutex;        // Object on which to synchronize
        SynchronizedMap(Map m) {
            this.m = Objects.requireNonNull(m);
            mutex = this;
        }
        SynchronizedMap(Map m, Object mutex) {
            this.m = m;
            this.mutex = mutex;
        }
        public int size() {
            synchronized (mutex) {return m.size();}
        }
        public boolean isEmpty() {
            synchronized (mutex) {return m.isEmpty();}
        }
        public boolean containsKey(Object key) {
            synchronized (mutex) {return m.containsKey(key);}
        }
        public boolean containsValue(Object value) {
            synchronized (mutex) {return m.containsValue(value);}
        }
        public V get(Object key) {
            synchronized (mutex) {return m.get(key);}
        }
        public V put(K key, V value) {
            synchronized (mutex) {return m.put(key, value);}
        }
        public V remove(Object key) {
            synchronized (mutex) {return m.remove(key);}
        }
        public void putAll(Map extends K, ? extends V> map) {
            synchronized (mutex) {m.putAll(map);}
        }
        public void clear() {
            synchronized (mutex) {m.clear();}
        }
        private transient Set keySet;
        private transient Set<map.entry>> entrySet;
        private transient Collection values;
        public Set 
 keySet() {
            synchronized (mutex) {
                if (keySet==null)
                    keySet = new SynchronizedSet<>(m.keySet(), mutex);
                return keySet;
            }
        }
        public Set<map.entry>> entrySet() {
            synchronized (mutex) {
                if (entrySet==null)
                    entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
                return entrySet;
            }
        }
        public Collection values() {
            synchronized (mutex) {
                if (values==null)
                    values = new SynchronizedCollection<>(m.values(), mutex);
                return values;
            }
        }
        public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized (mutex) {return m.equals(o);}
        }
        public int hashCode() {
            synchronized (mutex) {return m.hashCode();}
        }
        public String toString() {
            synchronized (mutex) {return m.toString();}
        }
        // Override default methods in Map
        @Override
        public V getOrDefault(Object k, V defaultValue) {
            synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
        }
        @Override
        public void forEach(BiConsumer super K, ? super V> action) {
            synchronized (mutex) {m.forEach(action);}
        }
        @Override
        public void replaceAll(BiFunction super K, ? super V, ? extends V> function) {
            synchronized (mutex) {m.replaceAll(function);}
        }
        @Override
        public V putIfAbsent(K key, V value) {
            synchronized (mutex) {return m.putIfAbsent(key, value);}
        } 

        @Override
        public boolean remove(Object key, Object value) {
            synchronized (mutex) {return m.remove(key, value);}
        }
        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            synchronized (mutex) {return m.replace(key, oldValue, newValue);}
        }
        @Override
        public V replace(K key, V value) {
            synchronized (mutex) {return m.replace(key, value);}
        }
        ...........................
      
    }/<map.entry>/<map.entry>/<code>

ConcurrentHashMap

使用分段锁，降低锁的粒度。

ConcurrentHashMap成员变量使用volatile 修饰。

使用CAS操作和synchronized结合实现赋值操作，多线程操作只会锁住当前操作索引的节点。

<code>public class ConcurrentHashMap extends AbstractMap
    implements ConcurrentMap, Serializable {
    .............................................
    transient volatile Node[] table;
    /** 

     * The next table to use; non-null only while resizing.
     */
    private transient volatile Node[] nextTable;
    /**
     * Base counter value, used mainly when there is no contention,
     * but also as a fallback during table initialization
     * races. Updated via CAS.
     */
    private transient volatile long baseCount;
    /**
     * Table initialization and resizing control.  When negative, the
     * table is being initialized or resized: -1 for initialization,
     * else -(1 + the number of active resizing threads).  Otherwise,
     * when table is null, holds the initial table size to use upon
     * creation, or 0 for default. After initialization, holds the
     * next element count value upon which to resize the table.
     */
    private transient volatile int sizeCtl;
    /**
     * The next table index (plus one) to split while resizing.
     */
    private transient volatile int transferIndex;
    /**
     * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
     */
    private transient volatile int cellsBusy;
    /**
     * Table of counter cells. When non-null, size is a power of 2.
     */
    private transient volatile CounterCell[] counterCells;
    ...................................................
    }/<code>

HashMap

初始化

JDK1.8版本的，内部使用数组 + 链表/红黑树。 如果自己传入初始大小k，初始化大小为大于k的2的整数次方，例如如果传10，大小为16。默认初始值是16，最大容量是2^31次方。

<code>/**
 * The default initial capacity - MUST be a power of two.
 */
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.
  */
 static final int MAXIMUM_CAPACITY = 1 << 30;/<code>

哈希函数

先拿到key的hashcode，然后让hashcode的前16位与后16位进行异或

尽可能降低hash碰撞，越分散越好

尽可能高效，这是高频操作，所以采用位运算

不直接使用hashcode的原因是hashcode函数的返回类型是int型散列值。初始化数组只有16，容易出现哈希冲突

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

Node

<code>final Node getNode(int hash, Object key) {
        Node[] tab; Node first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode) 

                    return ((TreeNode)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }/<code>

get函数

<code>public V get(Object key) {
        Node 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 getNode(int hash, Object key) {
    Node[] tab; Node first, e; int n; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null); 

        }
    }
    return null;
}/<code>

数据插入原理

判断数组是否为空，为空进行初始化;

不为空，计算 k 的 hash 值，通过(n - 1) & hash计算应当存放在数组中的下标 index;

查看 table[index] 是否存在数据，没有数据就构造一个Node节点存放在 table[index] 中；

存在数据，说明发生了hash冲突(存在二个节点key的hash值一样), 继续判断key是否相等，相等，用新的value替换原数据(onlyIfAbsent为false)；

如果不相等，判断当前节点类型是不是树型节点，如果是树型节点，创造树型节点插入红黑树中；

如果不是树型节点，创建普通Node加入链表中；判断链表长度是否大于 8， 大于的话链表转换为红黑树；

插入完成之后判断当前节点数是否大于阈值，如果大于开始扩容为原数组的二倍。

LinkedHashMap

LinkedHashMap内部维护了一个单链表，有头尾节点。

LinkedHashMap节点Entry内部除了继承HashMap的Node属性，before 和 after用于标识前置节点和后置节点。

实现按插入的顺序或访问顺序排序。

<code>public class LinkedHashMap
    extends HashMap
    implements Map
{
    /**
     * HashMap.Node subclass for normal LinkedHashMap entries.
     */
    static class Entry extends HashMap.Node {
        Entry before, after;
        Entry(int hash, K key, V value, Node next) {
            super(hash, key, value, next);
        }
    }
    .....................................................
}/<code>

TreeMap

默认是自然排序。

key所属的类实现Comparable接口进行比较。

<code>public class TreeMap
    extends AbstractMap
    implements NavigableMap, Cloneable, java.io.Serializable
{
    /**
     * The comparator used to maintain order in this tree map, or
     * null if it uses the natural ordering of its keys.
     *
     * @serial
     */
    private final Comparator super K> comparator;
    ...........................................
    public TreeMap(Comparator super K> comparator) {
        this.comparator = comparator;
    }
}/<code>

总结

HashMap是一个有趣的数据类型，设计的知识很多。本文只选取了几个重要的点进行分析，包括存储结构里的数据红黑树，哈希函数中的冲突避免和线程安全问题。另外HashMap在面试中出现的次数也非常多，几乎是后端开发岗的必问题目，值得结合源码深入研究。

参考链接：https://blog.csdn.net/zhengwangzw/article/details/104889549?utm_source=app

原文链接：https://blog.csdn.net/JAck_chen0309/java/article/details/105213625

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