Java中的HashMap源码记录以及并发环境的几个问题

HashMap源码简单分析：

1 一切需要从HashMap属性字段说起：

    /** The default initial capacity - MUST be a power of two. 初始容量 */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.  最大容量
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor. 
　　　* 默认的负载因子，当map的size>=负载因子*capacity时候并且插入元素时候的table[i]!=null进行扩容
     * 扩容判断逻辑：java.util.HashMap#addEntry函数中
     *
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * An empty table instance to share when the table is not inflated.
     */
    static final Entry<?,?>[] EMPTY_TABLE = {};

    /**
     * The table, resized as necessary. Length MUST Always be a power of two. 哈希表
     */
    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

    /**
     * The number of key-value mappings contained in this map. map的大小
     */
    transient int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    // If table == EMPTY_TABLE then this is the initial capacity at which the
    // table will be created when inflated.  扩容的阈值 = capacity * 负载因子
    int threshold;

    /**
     * The load factor for the hash table. 负载因子，默认是0.75，可以在创建HashMap时候通过构造函数指定
     *
     * @serial
     */
    final float loadFactor;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     * 修改次数：例如进行rehash或者返回hashMap视图时候如果发生修改可以fast-fail
     */
    transient int modCount;

    /**
     * The default threshold of map capacity above which alternative hashing is
     * used for String keys. Alternative hashing reduces the incidence of
     * collisions due to weak hash code calculation for String keys.
     * <p/>
     * This value may be overridden by defining the system property
     * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
     * forces alternative hashing to be used at all times whereas
     * {@code -1} value ensures that alternative hashing is never used.
　　　* rehash时候判断的一个阈值
     */
    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

2: 接下来查看一下HashMap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) {//初始化哈希表
            inflateTable(threshold);
        }
        if (key == null) //如果key 为null 存储到table[0]位置
            return putForNullKey(value);
        int hash = hash(key); //计算hash值
        int i = indexFor(hash, table.length);//计算entry在table中的位置
       //for循环逻辑用于修改key对应的value的
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;//如果是更新返回旧值
            }
        }
        //修改次数++
        modCount++;
        //添加元素到哈希表中
        addEntry(hash, key, value, i);
        // 如果是添加元素则返回null
        return null;
    }

3 put中调用的inflateTable方法：

    /**
     * Inflates the table.
     */
    private void inflateTable(int toSize) {
        // Find a power of 2 >= toSize
        //计算大于等于toSize的最小的2的整数次幂的值
        int capacity = roundUpToPowerOf2(toSize);
        //计算扩容阈值
        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
        //初始化哈希表
        table = new Entry[capacity];
        //更新一下rehash的判断条件，便于以后判断是否rehash
        initHashSeedAsNeeded(capacity);
    }

4 put方法中调用的indexFor方法：

  /**
     * Returns index for hash code h. 返回哈希值对应的哈希表索引
     */
    static int indexFor(int h, int length) {
        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
      //使用&操作，而不使用取余原因：均匀分布在哈希表中  。length-1目的是：由于table的长度都是2的整数次幂进行扩容，length-1的二进制全是1，计算效率高
        return h & (length-1);
    }

5 put方法中调用的addEntry方法：

/**
     * 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) {
     //判断是否扩容，只有size大于等于阈值而且当前插入table[i]!=null(就是able[i]已经被占用则扩容)
      if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
            //如果需要扩容的话则需要更新再次重新计算哈希表位置
            bucketIndex = indexFor(hash, table.length);
        }
        //将值插入到哈希表中
        createEntry(hash, key, value, bucketIndex);
    }

6 addEntry方法中调用的createEntry方法：

 /**
     * 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) {
       // 获取到哈希表指定位置
        Entry<K,V> e = table[bucketIndex];
       // 链表的头插入方式进行插入，插入逻辑在Entry的构造器中。然后将新节点存储到 table[bucketIndex]中
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;//更新size即可
    }

Entry构造器：

    /**
     *
     * @param h hash值
     * @param k key
     * @param v value
     * @param n 原始链表
     */
    Entry(int h, K k, V v, Entry<K,V> n) {
        value = v;
        //将原始链表接该节点后面
        next = n;
        key = k;
        hash = h;
    }

7 接下来看一下java.util.HashMap#addEntry扩容机制：

当进行扩容时候需要重新计算哈希值和在哈希表中的位置。

    void addEntry(int hash, K key, V value, int bucketIndex) {
        //满足扩容条件进行扩容
        if ((size >= threshold) && (null != table[bucketIndex])) {
            //扩容，2倍进行扩容
            resize(2 * table.length);
            //重新计算哈数值
            hash = (null != key) ? hash(key) : 0;
            //重新计算哈希表中的位置
            bucketIndex = indexFor(hash, table.length);
        }

        createEntry(hash, key, value, bucketIndex);
    }

接下来看一下java.util.HashMap#resize方法：

   /**
     * 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) {//判断当前old容量是否最最大容量，是的话更新阈值
            threshold = Integer.MAX_VALUE;
            return;
        }
        //创建新的表
        Entry[] newTable = new Entry[newCapacity];
       //元素转移，根据initHashSeedAsNeeded结果判断是否进行rehash
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        // 新表赋给table
        table = newTable;
       //更新阈值
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

关于HashMap在并发情况下的常见问题，其实在多线程环境下使用HashMap本来就是有风险错误的，但是一般面试却喜欢这么问，下面列举一下自己印象中的常见问题：

1：在进行扩容时候，其他线程是否可以进行进行插入操作（多线程环境下可能会导致HashMap进入死循环，此处暂不考虑）？

答：首先HashMap就不是一个线程安全的容器，所以在多线程环境下使用就是错误的。其次在扩容时候可以进行插入的，但是不安全。例如：

当主线程在调用transfer方法进行复制元素：

    /**
     * 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);
                }
                int i = indexFor(e.hash, newCapacity);
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

此时另一个线程在添加新元素是可以的，新元素添加到table中。如果子线程需要扩容的话可以进行扩容，然后将新容器赋给table。而此时主线程转移元素的工作就是将table中元素转移到newTable中。注意main线程的transfer方法：

如果main线程刚进入transfer方法时候newTable大小是32的话，由于子线程的添加操作导致table此时元素如果有128的话。则128个元素就会存储到大小为32的newTable中（此处不会扩容）。这就会导致HashMap性能下降！！！

可以使用多线程环境进行debug查看即可确定（推荐Idea的debug，的确强大，尤其是Evaluate Expression功能）。

2：进行扩容时候元素是否需要重新Hash？

这个需要具体情况判断，调用initHashSeedAsNeeded方法判断（判断逻辑这里先不介绍）。

    /**
     * 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) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
       //initHashSeedAsNeeded 判断是否需要重新Hash
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }    

然后进行转移元素：

    /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        //多线程环境下，如果其他线程导致table快速扩大。newTable在此处无法扩容会导致性能下降。但是如果后面有再次调用put方法的话可以再次触发resize。
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) { //判断是否需要重新Hash
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

3：如何判断是否需要重新Hash？

    /**
     * Initialize the hashing mask value. We defer initialization until we
     * really need it.
     */
    final boolean initHashSeedAsNeeded(int capacity) {

        // hashSeed降低hash碰撞的hash种子，初始值为0
        boolean currentAltHashing = hashSeed != 0;
        //ALTERNATIVE_HASHING_THRESHOLD： 当map的capacity容量大于这个值的时候并满足其他条件时候进行重新hash
        boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        //TODO 异或操作，二者满足一个条件即可rehash
        boolean switching = currentAltHashing ^ useAltHashing;
        if (switching) {
            // 更新hashseed的值
            hashSeed = useAltHashing ? sun.misc.Hashing.randomHashSeed(this) : 0;
        }
        return switching;
    }

4：HashMap在多线程环境下进行put操作如何导致的死循环？

死循环产生时机：

当两个线程同时需要进行扩容，而且对哈希表同一个桶（table[i]）进行扩容时候，一个线程刚好确定e和next元素之后，线程被挂起。此时另一个线程得到cpu并顺利对该桶完成转移（需要要求被转移之后的线程1中的e和next指的元素在新哈希表的同一个桶中，此时e和next被逆序了）。接着线程从挂起恢复回来时候就会陷入死循环中。参考：https://coolshell.cn/articles/9606.html

产生原因：主要由于并发操作，对用一个桶的两个节点构成了环，导致对环进行无法转移完毕元素陷入死循环。