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。

巴特西

Java中的HashMap源码记录以及并发环境的几个问题

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