JDK容器类Map源码解读

2019-08-09 00:00:00 源码 容器 解读

java.util.Map接口是JDK1.2开始提供的一个基于键值对的散列表接口,其设计的初衷是为了替换JDK1.0中的java.util.Dictionary抽象类。Dictionary是JDK最初的键值对类,它不可以存储null作为key和value,目前这个类早已不被使用了。目前都是在使用Map接口,它是可以存储null值作为key和value,但Map的key是不可以重复的。其常用的实现类主要有HashMap,TreeMap,ConcurrentHashMap等

HashMap源码解读

目前JDK已经发布到JDK12,主流的JDK版本是JDK8, 但是如果阅读HashMap的源码建议先看JDK7的源码。JDK7和JDK8的源码中HashMap的实现原理大体相同,只不过是在JDK8中做了部分优化。但是JDK8的源码可读性非常差。

HashMap 是一个存储键值对(key-value)映射的散列表,继承于AbstractMap,实现了Map、Cloneable、java.io.Serializable接口,HashMap是线程不安全的,它存储的映射也是无序的。

HashMap的底层主要是基于数组和链表来实现的(JDK8之后又引入了红黑树),数据存储时会通过对key进行哈希操作取到哈希值,然后将哈希值对数组长度取模,得到的值就是该键值对在数组中的索引index值,如果数组该位置没有值则直接将该键值对放在该位置,如果该位置已经有值则将其插入相应链表的位置,JDK8开始为优化链表长度过长导致的性能问题从而引入了红黑树,当链表的长度大于8时会自动将链表转成红黑树。

JDK7中HashMap的源码解读

JDK7中HashMap采用Entry数组来存储键值对,每一个键值对组成了一个Entry实体,Entry类实际上是一个单向的链表结构,它具有Next指针,可以连接下一个Entry实体组成链表。

《JDK容器类Map源码解读》

JDK7中HashMap源码中的主要字段

// 数组默认的大小
// 1 << 4,表示1,左移4位,变成10000,即16,以二进制形式运行,效率更高
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

// 数组最大值
static final int MAXIMUM_CAPACITY = 1 << 30; 

// 默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 真正存放数据的数组
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

HashMap中默认的数组容量为 16,负载因子为 0.75。Map 在使用过程中不断的往里面存放数据,当数量达到了 16 * 0.75 = 12 就需要将当前 16 的容量进行扩容,而扩容这个过程涉及到 rehash、复制数据等操作,所以非常消耗性能。因此通常建议能提前预估 HashMap 的大小最好,尽量的减少扩容带来的性能损耗。

JDK7中HashMap源码中的构造器


    /**  默认的初始化容量、默认的加载因子
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {    //16  0.75
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

    /**
     * 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.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**   做了两件事:1、为threshold、loadFactor赋值   2、调用init()
     * 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
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)     //限制最大容量
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))     //检查 loadFactor
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        //真正在做的,只是记录下loadFactor、initialCpacity的值
        this.loadFactor = loadFactor;       //记录下loadFactor
        threshold = initialCapacity;        //初始的 阈值threshold=initialCapacity=16
        init();
    }

    /**
     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified <tt>Map</tt>.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
        inflateTable(threshold);

        putAllForCreate(m);
    }

JDK7中HashMap源码中的put方法


    /**
     * 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.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);    //初始化表 (初始化、扩容 合并为了一个方法)
        }
        if (key == null)        //对key为null做特殊处理
            return putForNullKey(value);
        int hash = hash(key);           //计算hash值
        int i = indexFor(hash, table.length);   //根据hash值计算出index下标
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {  //遍历下标为i处的链表
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {  //如果key值相同,覆盖旧值,返回新值
                V oldValue = e.value;
                e.value = value;    //新值 覆盖 旧值
                e.recordAccess(this);   //do nothing
                return oldValue;    //返回旧值
            }
        }

        modCount++;         //修改次数+1,类似于一个version number
        addEntry(hash, key, value, i);
        return null;
    }


    /**
     * Adds a new entry with the specified key, value and hash code to
     * the specified bucket.  It is the responsibility of this
     * method to resize the table if appropriate.
     *
     * Subclass overrides this to alter the behavior of put method.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {  //如果size大于threshold && table在下标为index的地方已经有entry了
            resize(2 * table.length);       //扩容,将数组长度变为原来两倍
            hash = (null != key) ? hash(key) : 0;       //重新计算 hash 值
            bucketIndex = indexFor(hash, table.length); //重新计算下标
        }

        createEntry(hash, key, value, bucketIndex);     //创建entry
    }

    /**
     * Rehashes the contents of this map into a new array with a
     * larger capacity.  This method is called automatically when the
     * number of keys in this map reaches its threshold.
     *
     * If current capacity is MAXIMUM_CAPACITY, this method does not
     * resize the map, but sets threshold to Integer.MAX_VALUE.
     * This has the effect of preventing future calls.
     *
     * @param newCapacity the new capacity, MUST be a power of two;
     *        must be greater than current capacity unless current
     *        capacity is MAXIMUM_CAPACITY (in which case value
     *        is irrelevant).
     */
    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {  //状态检查
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];      //实例化新的table
        transfer(newTable, initHashSeedAsNeeded(newCapacity));  //赋值数组元素到新的数组
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

    /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);       //对key进行hash
                }
                int i = indexFor(e.hash, newCapacity);      //用新的index来取模
                e.next = newTable[i];
                newTable[i] = e;            //把元素存入新table新的新的index处
                e = next;
            }
        }
    }

    /**
     * Like addEntry except that this version is used when creating entries
     * as part of Map construction or "pseudo-construction" (cloning,
     * deserialization).  This version needn't worry about resizing the table.
     *
     * Subclass overrides this to alter the behavior of HashMap(Map),
     * clone, and readObject.
     */
    void createEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];      //获取table中存的entry
        table[bucketIndex] = new Entry<>(hash, key, value, e);   //将新的entry放到数组中,next指向旧的table[i]
        size++;         //修改map中元素个数
    }

JDK7中HashMap源码中的put方法


    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);

        return null == entry ? null : entry.getValue();
    }


    /**
     * Returns the entry associated with the specified key in the
     * HashMap.  Returns null if the HashMap contains no mapping
     * for the key.
     */
    final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }

JDK8中HashMap的源码解读

JDK8中HashMap采用Node数组来存储键值对,Node其实就是JDK7中的Entry,只不过是换了一个名字,同样每一个键值对组成了一个Node实体,然后组成链表。当 Hash 冲突严重时,链表会变的越来越长,这样在查询时的效率就会越来越低,JDK8所做的优化就是,当链表的长度达到8的时候会转变成红黑树TreeNode。

《JDK容器类Map源码解读》

JDK8中HashMap源码中的主要字段

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

static final int MAXIMUM_CAPACITY = 1 << 30;

static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 用于判断是否需要将链表转换为红黑树的阈值
static final int TREEIFY_THRESHOLD = 8;

// 用于判断是否需要将红黑树转换为链表的阈值
static final int UNTREEIFY_THRESHOLD = 6;

static final int MIN_TREEIFY_CAPACITY = 64;

// 存放数据的数组
transient Node<K,V>[] table;

JDK8中HashMap源码中的构造器

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    /**
     * 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.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * 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
     */
    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;
        this.threshold = tableSizeFor(initialCapacity);
    }

    /**
     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified <tt>Map</tt>.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

JDK8中HashMap源码中的put方法


    /**
     * 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.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods.  添加元素
     *
     * @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;
        if ((tab = table) == null || (n = tab.length) == 0)     //若table为null
            n = (tab = resize()).length;                        //resize
        if ((p = tab[i = (n - 1) & hash]) == null)              //计算下标i,取出i处的元素为p,如果p为null
            tab[i] = newNode(hash, key, value, null);       //创建新的node,放到数组中
        else {                  //若 p!=null
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))     //若key相同
                e = p;      //直接覆盖
            else if (p instanceof TreeNode)     //如果为 树节点
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);     //放到树中
            else {                                          //如果key不相同,也不是treeNode
                for (int binCount = 0; ; ++binCount) {      //遍历i处的链表
                    if ((e = p.next) == null) {             //找到尾部
                        p.next = newNode(hash, key, value, null);       //在末尾添加一个node
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st    //如果链表长度  >= 8
                            treeifyBin(tab, hash);             //将链表转成共黑树
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))     //若果key相同,直接退出循环
                        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;
    }

    /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        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);
        }
    }

   /**
     * 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;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;  // 如果 旧数组为null就讲旧的容量看做是0,否则用旧的table长度当做容量
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            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];     //创建新的数组
        table = newTab;                                         //赋值给table
        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;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        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);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

JDK8中HashMap源码中的get方法


    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> 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<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) {
            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<K,V>)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;
    }

ConcurrentHashMap源码解读

ConcurrentHashMap是一个线程安全的HashMap实现,ConcurrentHashMap在JDK7和JDK8中的实现差别比较大,JDK7中ConcurrentHashMap是使用Segment数组来存放数据,一个Segment就相当于一个HashMap的数据结构,每个Segment使用一个锁。JDK8之后Segment虽保留,但仅是为了兼容旧版本,已经不再使用,JDK8中ConcurrentHashMap使用和HashMap一样的数据结构Node数组来存储数据,每个数组位置使用一个锁。

JDK7中的ConcurrentHashMap源码解读

JDK7中ConcurrentHashMap的底层Segment组,而Segment其实就是特殊的HashMap,Segment的数据结构跟HashMap一样,同时它继承了ReentrantLock,通过ReentrantLock提供的锁实现了线程的安全。ConcurrentHashMap使用分段锁技术,将数据分成一段一段的存储,每个Segment就是一段,然后给每一段数据配一把锁,当一个线程占用锁访问其中一个段数据的时候,其他段的数据也能被其他线程访问,能够实现并发访问,Segment数组的长度就是ConcurrentHashMap的线程并行级别,Segment数组默认的长度为16,也就是说最多同时可以有16个线程去访问ConcurrentHashMap。segment 数组不能扩容,而是对 segment 数组某个位置的segmen内部的数组HashEntry[] 进行扩容,扩容后容量为原来的 2 倍,该方法没有考虑并发,因为执行该方法之前已经获取了锁。

《JDK容器类Map源码解读》

JDK7中的ConcurrentHashMap源码中的主要字段

// 数组默认大小
static final int DEFAULT_INITIAL_CAPACITY = 16;

// 默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 默认线程并发度
static final int DEFAULT_CONCURRENCY_LEVEL = 16;

static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

static final int MAX_SEGMENTS = 1 << 16;

// 数组最大大小
static final int MAXIMUM_CAPACITY = 1 << 30;

static final int MAXIMUM_CAPACITY = 1 << 30;

static final int RETRIES_BEFORE_LOCK = 2;

JDK7中的ConcurrentHashMap源码中的构造器

    /**
     * Creates a new, empty map with a default initial capacity (16),
     * load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity,
     * and with default load factor (0.75) and concurrencyLevel (16).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative.
     */
    public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity
     * and load factor and with the default concurrencyLevel (16).
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative or the load factor is nonpositive
     *
     * @since 1.6
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial
     * capacity, load factor and concurrency level.
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently
     * updating threads. The implementation performs internal sizing
     * to try to accommodate this many threads.
     * @throws IllegalArgumentException if the initial capacity is
     * negative or the load factor or concurrencyLevel are
     * nonpositive.
     */
    @SuppressWarnings("unchecked")
    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)  //参数检查
            throw new IllegalArgumentException();
        if (concurrencyLevel > MAX_SEGMENTS)    //ConcurrentcyLevel实际上就是最大并发数
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);     //创建一个segment
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];         //创建一个segment数组
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]     //将s0设置为ss的第一个元素
        this.segments = ss;             //将ss作为segments
    }

JDK7中的ConcurrentHashMap源码中put方法


    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p> The value can be retrieved by calling the <tt>get</tt> method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V put(K key, V value) {
        Segment<K,V> s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);       // 计算Hash值
        int j = (hash >>> segmentShift) & segmentMask;      //计算下标j
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);       //若j处有segment就返回,若没有就创建并返回
        return s.put(key, hash, value, false);  //将值put到segment中去
    }

        // Segment 中put数据的方法
        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);        //如果tryLock成功,就返回null,否则。。。
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;        //根据table数组的长度 和 hash值计算index小标
                HashEntry<K,V> first = entryAt(tab, index); //找到table数组在 index处链表的头部
                for (HashEntry<K,V> e = first;;) {      //从first开始遍历链表
                    if (e != null) {                    //若e!=null
                        K k;
                        if ((k = e.key) == key ||
                            (e.hash == hash && key.equals(k))) {        //如果key相同
                            oldValue = e.value;                 //获取旧值
                            if (!onlyIfAbsent) {                //若absent=false
                                e.value = value;                //覆盖旧值
                                ++modCount;                     //
                            }
                            break;      //若已经找到,就退出链表遍历
                        }
                        e = e.next;     //若key不相同,继续遍历
                    }
                    else {              //直到e为null
                        if (node != null)   //将元素放到链表头部
                            node.setNext(first);
                        else
                            node = new HashEntry<K,V>(hash, key, value, first); //创建新的Entry
                        int c = count + 1;      //count 用来记录元素个数
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)     //如果hashmap元素个数超过threshold,并且table长度小于最大容量
                            rehash(node);       //rehash跟resize的功能差不多,将table的长度变为原来的两倍,重新打包entries,并将给定的node添加到新的table
                        else        //如果还有容量
                            setEntryAt(tab, index, node);   //就在index处添加链表节点
                        ++modCount;     //修改操作数
                        count = c;      //将count+1
                        oldValue = null;    //
                        break;
                    }
                }
            } finally {
                unlock();           //执行完操作后,释放锁
            }
            return oldValue;        //返回oldValue
        }

  /**  将table的长度变为原来的两倍,重新打包entries,并将给定的node添加到新的table
         * Doubles size of table and repacks entries, also adding the
         * given node to new table
         */
        @SuppressWarnings("unchecked")
        private void rehash(HashEntry<K,V> node) {
            /*
             * Reclassify nodes in each list to new table.  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. We eliminate unnecessary node
             * creation by catching cases where old nodes can be
             * reused because their next fields won't change.
             * Statistically, at the default threshold, only about
             * one-sixth of them need cloning when a table
             * doubles. The nodes they replace will be garbage
             * collectable as soon as they are no longer referenced by
             * any reader thread that may be in the midst of
             * concurrently traversing table. Entry accesses use plain
             * array indexing because they are followed by volatile
             * table write.
             */
            HashEntry<K,V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            int newCapacity = oldCapacity << 1;
            threshold = (int)(newCapacity * loadFactor);
            HashEntry<K,V>[] newTable =
                (HashEntry<K,V>[]) new HashEntry[newCapacity];
            int sizeMask = newCapacity - 1;
            for (int i = 0; i < oldCapacity ; i++) {
                HashEntry<K,V> e = oldTable[i];
                if (e != null) {
                    HashEntry<K,V> next = e.next;
                    int idx = e.hash & sizeMask;
                    if (next == null)   //  Single node on list
                        newTable[idx] = e;
                    else { // Reuse consecutive sequence at same slot
                        HashEntry<K,V> lastRun = e;
                        int lastIdx = idx;
                        for (HashEntry<K,V> last = next;
                             last != null;
                             last = last.next) {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;
                        // Clone remaining nodes
                        for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
                            V v = p.value;
                            int h = p.hash;
                            int k = h & sizeMask;
                            HashEntry<K,V> n = newTable[k];
                            newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
                        }
                    }
                }
            }
            int nodeIndex = node.hash & sizeMask; // add the new node
            node.setNext(newTable[nodeIndex]);
            newTable[nodeIndex] = node;
            table = newTable;
        }

JDK7中的ConcurrentHashMap源码中get方法


    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        HashEntry<K,V>[] tab;
        int h = hash(key);
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return e.value;
            }
        }
        return null;
    }

JDK8中的ConcurrentHashMap源码解读

JDK8中的ConcurrentHashMap取消了基于 Segment 的分段锁思想,改用 CAS + synchronized 控制并发操作,锁的粒度变得更小,并发度更高。并且追随JDK8的HashMap底层实现,使用数组+链表+红黑树进行数据存储。

《JDK容器类Map源码解读》

JDK8中的ConcurrentHashMap源码中的主要字段

private static final int MAXIMUM_CAPACITY = 1 << 30;

private static final int DEFAULT_CAPACITY = 16;

private static final float LOAD_FACTOR = 0.75f;

static final int TREEIFY_THRESHOLD = 8;

static final int UNTREEIFY_THRESHOLD = 6;

static final int MIN_TREEIFY_CAPACITY = 64;

private static final int MIN_TRANSFER_STRIDE = 16;

static final int MOVED     = -1; // hash for forwarding nodes       //转发节点的hash值
static final int TREEBIN   = -2; // hash for roots of trees     //树的根节点的hash值
static final int RESERVED  = -3; // hash for transient reservations     //临时节点的 hash值
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash //正常节点的hash值

JDK8中的ConcurrentHashMap源码中构造器


    /**
     * Creates a new, empty map with the default initial table size (16).
     */
    public ConcurrentHashMap() {
    }

    /**
     * Creates a new, empty map with an initial table size
     * accommodating the specified number of elements without the need
     * to dynamically resize.
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative
     */
    public ConcurrentHashMap(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException();
        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
                   MAXIMUM_CAPACITY :
                   tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
        this.sizeCtl = cap;
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}) and
     * initial table density ({@code loadFactor}).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements,
     * given the specified load factor.
     * @param loadFactor the load factor (table density) for
     * establishing the initial table size
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative or the load factor is nonpositive
     *
     * @since 1.6
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, 1);
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}), table
     * density ({@code loadFactor}), and number of concurrently
     * updating threads ({@code concurrencyLevel}).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements,
     * given the specified load factor.
     * @param loadFactor the load factor (table density) for
     * establishing the initial table size
     * @param concurrencyLevel the estimated number of concurrently
     * updating threads. The implementation may use this value as
     * a sizing hint.
     * @throws IllegalArgumentException if the initial capacity is
     * negative or the load factor or concurrencyLevel are
     * nonpositive
     */
    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        long size = (long)(1.0 + (long)initialCapacity / loadFactor);
        int cap = (size >= (long)MAXIMUM_CAPACITY) ?
            MAXIMUM_CAPACITY : tableSizeFor((int)size);
        this.sizeCtl = cap;
    }

JDK8中的ConcurrentHashMap源码中的put方法


    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p>The value can be retrieved by calling the {@code get} method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        return putVal(key, value, false);
    }

    /** Implementation for put and putIfAbsent */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        int hash = spread(key.hashCode());      //计算hash值
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {   //自旋
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)       //table==null || table.length==0
                tab = initTable();                          //就initTable
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {    //若下标 i 处的元素为null
                if (casTabAt(tab, i, null,                           //直接用CAS操作,i处的元素
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin   想emptybin中假如元素的时候,不需要加锁
            }
            else if ((fh = f.hash) == MOVED)    //若下标 i 处的元素不为null,且f.hash==MOVED MOVED为常量值-1
                tab = helpTransfer(tab, f);     //
            else {                              //如果是一般的节点
                V oldVal = null;
                synchronized (f) {              //当头部元素不为null,且不需要转换成树时,需要进行同步操作
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {          //若 链表头部hash值 >=0
                            binCount = 1;
                            for (Node<K,V> e = f;; ++binCount) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {     //如果key相同
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)      //且不为absent
                                        e.val = value;      //旧值覆盖新值
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null), {     //如果链表遍历完成,还没退出,说明没有相同的key存在,在尾部添加节点
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {        //如果f是Tree的节点
                            Node<K,V> p;
                            binCount = 2;
                            if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                           value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent)
                                    p.val = value;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

    /**
     * Initializes table, using the size recorded in sizeCtl.
     *///通过CAS抢sizeCtl,来抢占initTable的资格,其他线程自旋等待,直到table不为null
    private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; int sc;
        while ((tab = table) == null || tab.length == 0) {
            if ((sc = sizeCtl) < 0)
                Thread.yield(); // lost initialization race; just spin  //线程让步,让其他线程优先执行
            else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                try {
                    if ((tab = table) == null || tab.length == 0) {
                        int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                        @SuppressWarnings("unchecked")
                        Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; //初始化数组
                        table = tab = nt;               //将nt赋值给table
                        sc = n - (n >>> 2);
                    }
                } finally {
                    sizeCtl = sc;
                }
                break;
            }
        }
        return tab;
    }

JDK8中的ConcurrentHashMap源码中的get方法

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null) {
            if ((eh = e.hash) == h) {
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                    return e.val;
            }
            else if (eh < 0)
                return (p = e.find(h, key)) != null ? p.val : null;
            while ((e = e.next) != null) {
                if (e.hash == h &&
                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

《JDK容器类Map源码解读》

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