JDK容器类Map源码解读

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。

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。

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。

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。

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;
    }

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