【转】Java HashMap 源码解析（好文章）

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public class HashMap<K,V>

extends AbstractMap<K,V>

implements Map<K,V>, Cloneable, Serializable

public abstract class AbstractMap<K,V> implements Map<K,V>

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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;

    threshold = initialCapacity;

    init();

}

public HashMap(int initialCapacity) {

    this(initialCapacity, DEFAULT_LOAD_FACTOR);

}

public HashMap() {

    this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);

}

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/**

 * The default initial capacity - MUST be a power of two.

 */

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

/**

 * The maximum capacity, used if a higher value is implicitly specified

 * by either of the constructors with arguments.

 * MUST be a power of two <= 1<<30.

 */

static final int MAXIMUM_CAPACITY = 1 << 30;

/**

 * The load factor used when none specified in constructor.

 */

static final float DEFAULT_LOAD_FACTOR = 0.75f;

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/**

  * Retrieve object hash code and applies a supplemental hash function to the

  * result hash, which defends against poor quality hash functions.  This is

  * critical because HashMap uses power-of-two length hash tables, that

  * otherwise encounter collisions for hashCodes that do not differ

  * in lower bits. Note: Null keys always map to hash 0, thus index 0.

  */

 final int hash(Object k) {

     int h = hashSeed;

     if (0 != h && k instanceof String) {

         return sun.misc.Hashing.stringHash32((String) k);

     }

     h ^= k.hashCode();

     // This function ensures that hashCodes that differ only by

     // constant multiples at each bit position have a bounded

     // number of collisions (approximately 8 at default load factor).

     h ^= (h >>> 20) ^ (h >>> 12);

     return h ^ (h >>> 7) ^ (h >>> 4);

 }

 /**

  * Returns index for hash code h.

  */

 static int indexFor(int h, int length) {

     // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";

     return h & (length-1);

 }

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static class Entry<K,V> implements Map.Entry<K,V> {

    final K key;

    V value;

    Entry<K,V> next;

    int hash;

    Entry(int h, K k, V v, Entry<K,V> n) {

        value = v;

        next = n;

        key = k;

        hash = h;

    }

    // setter, getter, equals, toString 方法省略

    public final int hashCode() {

        //用key的hash值与上value的hash值作为Entry的hash值

        return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());

    }

    /**

     * This method is invoked whenever the value in an entry is

     * overwritten by an invocation of put(k,v) for a key k that‘s already

     * in the HashMap.

     */

    void recordAccess(HashMap<K,V> m) {

    }

    /**

     * This method is invoked whenever the entry is

     * removed from the table.

     */

    void recordRemoval(HashMap<K,V> m) {

    }

}

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/**

 * The table, resized as necessary. Length MUST Always be a power of two.

 */

//HashMap内部维护了一个为数组类型的Entry变量table，用来保存添加进来的Entry对象

transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

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public V get(Object key) {

    //单独处理key为null的情况

    if (key == null)

        return getForNullKey();

    Entry<K,V> entry = getEntry(key);

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

}

private V getForNullKey() {

    if (size == 0) {

        return null;

    }

    //key为null的Entry用于放在table[0]中，但是在table[0]冲突链中的Entry的key不一定为null

    //所以需要遍历冲突链，查找key是否存在

    for (Entry<K,V> e = table[0]; e != null; e = e.next) {

        if (e.key == null)

            return e.value;

    }

    return null;

}

final Entry<K,V> getEntry(Object key) {

    if (size == 0) {

        return null;

    }

    int hash = (key == null) ? 0 : hash(key);

    //首先定位到索引在table中的位置

    //然后遍历冲突链，查找key是否存在

    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;

}

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private void inflateTable(int toSize) {

    //辅助函数，用于填充HashMap到指定的capacity

    // Find a power of 2 >= toSize

    int capacity = roundUpToPowerOf2(toSize);

    //threshold为resize的阈值，超过后HashMap会进行resize，内容的entry会进行rehash

    threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);

    table = new Entry[capacity];

    initHashSeedAsNeeded(capacity);

}

/**

 * 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.

 */

public V put(K key, V value) {

    if (table == EMPTY_TABLE) {

        inflateTable(threshold);

    }

    if (key == null)

        return putForNullKey(value);

    int hash = hash(key);

    int i = indexFor(hash, table.length);

    //这里的循环是关键

    //当新增的key所对应的索引i，对应table[i]中已经有值时，进入循环体

    for (Entry<K,V> e = table[i]; e != null; e = e.next) {

        Object k;

        //判断是否存在本次插入的key，如果存在用本次的value替换之前oldValue，相当于update操作

        //并返回之前的oldValue

        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

            V oldValue = e.value;

            e.value = value;

            e.recordAccess(this);

            return oldValue;

        }

    }

    //如果本次新增key之前不存在于HashMap中，modCount加1，说明结构改变了

    modCount++;

    addEntry(hash, key, value, i);

    return null;

}

void addEntry(int hash, K key, V value, int bucketIndex) {

    //如果增加一个元素会后，HashMap的大小超过阈值，需要resize

    if ((size >= threshold) && (null != table[bucketIndex])) {

        //增加的幅度是之前的1倍

        resize(2 * table.length);

        hash = (null != key) ? hash(key) : 0;

        bucketIndex = indexFor(hash, table.length);

    }

    createEntry(hash, key, value, bucketIndex);

}

void createEntry(int hash, K key, V value, int bucketIndex) {

    //首先得到该索引处的冲突链Entries，有可能为null，不为null

    Entry<K,V> e = table[bucketIndex];

    //然后把新的Entry添加到冲突链的开头，也就是说，后插入的反而在前面（第一次还真没看明白）

    //需要注意的是table[bucketIndex]本身并不存储节点信息，

    //它就相当于是单向链表的头指针，数据都存放在冲突链中。

    table[bucketIndex] = new Entry<>(hash, key, value, e);

    size++;

}

//下面看看HashMap是如何进行resize，庐山真面目就要揭晓了

void resize(int newCapacity) {

    Entry[] oldTable = table;

    int oldCapacity = oldTable.length;

    //如果已经达到最大容量，那么就直接返回

    if (oldCapacity == MAXIMUM_CAPACITY) {

        threshold = Integer.MAX_VALUE;

        return;

    }

    Entry[] newTable = new Entry[newCapacity];

    //initHashSeedAsNeeded(newCapacity)的返回值决定了是否需要重新计算Entry的hash值

    transfer(newTable, initHashSeedAsNeeded(newCapacity));

    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;

    //遍历当前的table，将里面的元素添加到新的newTable中

    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];

            //最后这两句用了与put放过相同的技巧

            //将后插入的反而在前面

            newTable[i] = e;

            e = next;

        }

    }

}

/**

 * Initialize the hashing mask value. We defer initialization until we

 * really need it.

 */

final boolean initHashSeedAsNeeded(int capacity) {

    boolean currentAltHashing = hashSeed != 0;

    boolean useAltHashing = sun.misc.VM.isBooted() &&

            (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);

    //这里说明了，在hashSeed不为0或满足useAltHash时，会重算Entry的hash值

    //至于useAltHashing的作用可以参考下面的链接

    // http://stackoverflow.com/questions/29918624/what-is-the-use-of-holder-class-in-hashmap

    boolean switching = currentAltHashing ^ useAltHashing;

    if (switching) {

        hashSeed = useAltHashing

            ? sun.misc.Hashing.randomHashSeed(this)

            : 0;

    }

    return switching;

}

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public V remove(Object key) {

    Entry<K,V> e = removeEntryForKey(key);

    //可以看到删除的key如果存在，就返回其所对应的value

    return (e == null ? null : e.value);

}

final Entry<K,V> removeEntryForKey(Object key) {

    if (size == 0) {

        return null;

    }

    int hash = (key == null) ? 0 : hash(key);

    int i = indexFor(hash, table.length);

    //这里用了两个Entry对象，相当于两个指针，为的是防治冲突链发生断裂的情况

    //这里的思路就是一般的单向链表的删除思路

    Entry<K,V> prev = table[i];

    Entry<K,V> e = prev;

    //当table[i]中存在冲突链时，开始遍历里面的元素

    while (e != null) {

        Entry<K,V> next = e.next;

        Object k;

        if (e.hash == hash &&

            ((k = e.key) == key || (key != null && key.equals(k)))) {

            modCount++;

            size--;

            if (prev == e) //当冲突链只有一个Entry时

                table[i] = next;

            else

                prev.next = next;

            e.recordRemoval(this);

            return e;

        }

        prev = e;

        e = next;

    }

    return e;

}

transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

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private void writeObject(java.io.ObjectOutputStream s)

    throws IOException

{

    // Write out the threshold, loadfactor, and any hidden stuff

    s.defaultWriteObject();

    // Write out number of buckets

    if (table==EMPTY_TABLE) {

        s.writeInt(roundUpToPowerOf2(threshold));

    } else {

       s.writeInt(table.length);

    }

    // Write out size (number of Mappings)

    s.writeInt(size);

    // Write out keys and values (alternating)

    if (size > 0) {

        for(Map.Entry<K,V> e : entrySet0()) {

            s.writeObject(e.getKey());

            s.writeObject(e.getValue());

        }

    }

}

private static final long serialVersionUID = 362498820763181265L;

private void readObject(java.io.ObjectInputStream s)

     throws IOException, ClassNotFoundException

{

    // Read in the threshold (ignored), loadfactor, and any hidden stuff

    s.defaultReadObject();

    if (loadFactor <= 0 || Float.isNaN(loadFactor)) {

        throw new InvalidObjectException("Illegal load factor: " +

                                           loadFactor);

    }

    // set other fields that need values

    table = (Entry<K,V>[]) EMPTY_TABLE;

    // Read in number of buckets

    s.readInt(); // ignored.

    // Read number of mappings

    int mappings = s.readInt();

    if (mappings < 0)

        throw new InvalidObjectException("Illegal mappings count: " +

                                           mappings);

    // capacity chosen by number of mappings and desired load (if >= 0.25)

    int capacity = (int) Math.min(

                mappings * Math.min(1 / loadFactor, 4.0f),

                // we have limits...

                HashMap.MAXIMUM_CAPACITY);

    // allocate the bucket array;

    if (mappings > 0) {

        inflateTable(capacity);

    } else {

        threshold = capacity;

    }

    init();  // Give subclass a chance to do its thing.

    // Read the keys and values, and put the mappings in the HashMap

    for (int i = 0; i < mappings; i++) {

        K key = (K) s.readObject();

        V value = (V) s.readObject();

        putForCreate(key, value);

    }

}

private void putForCreate(K key, V value) {

    int hash = null == key ? 0 : hash(key);

    int i = indexFor(hash, table.length);

    /**

     * Look for preexisting entry for key.  This will never happen for

     * clone or deserialize.  It will only happen for construction if the

     * input Map is a sorted map whose ordering is inconsistent w/ equals.

     */

    for (Entry<K,V> e = table[i]; e != null; e = e.next) {

        Object k;

        if (e.hash == hash &&

            ((k = e.key) == key || (key != null && key.equals(k)))) {

            e.value = value;

            return;

        }

    }

    createEntry(hash, key, value, i);

}