Android HashMap源码分析

类前注释:

/**
 * HashMap is an implementation of {@link Map}. All optional operations are supported.
 *
 * <p>All elements are permitted as keys or values, including null.
 *
 * <p>Note that the iteration order for HashMap is non-deterministic. If you want
 * deterministic iteration, use {@link LinkedHashMap}.
 *
 * <p>Note: the implementation of {@code HashMap} is not synchronized.
 * If one thread of several threads accessing an instance modifies the map
 * structurally, access to the map needs to be synchronized. A structural
 * modification is an operation that adds or removes an entry. Changes in
 * the value of an entry are not structural changes.
 *
 * <p>The {@code Iterator} created by calling the {@code iterator} method
 * may throw a {@code ConcurrentModificationException} if the map is structurally
 * changed while an iterator is used to iterate over the elements. Only the
 * {@code remove} method that is provided by the iterator allows for removal of
 * elements during iteration. It is not possible to guarantee that this
 * mechanism works in all cases of unsynchronized concurrent modification. It
 * should only be used for debugging purposes.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */

HashMap是Map的一种常用实现。HashMap中的元素排列无序,如果需要有序排列,请用LinkedHashMap。

    /**
     * Min capacity (other than zero) for a HashMap. Must be a power of two
     * greater than 1 (and less than 1 << 30).
     */
    private static final int MINIMUM_CAPACITY = 4;

JDK中HashMap的初始容量是16,而此处为4。

    /**
     * An empty table shared by all zero-capacity maps (typically from default
     * constructor). It is never written to, and replaced on first put. Its size
     * is set to half the minimum, so that the first resize will create a
     * minimum-sized table.
     */
    private static final Entry[] EMPTY_TABLE
            = new HashMapEntry[MINIMUM_CAPACITY >>> 1];

空HashMap持有的空表,大小为2(扩容后变成4)。

    /**
     * The default load factor. Note that this implementation ignores the
     * load factor, but cannot do away with it entirely because it's
     * mentioned in the API.
     *
     * <p>Note that this constant has no impact on the behavior of the program,
     * but it is emitted as part of the serialized form. The load factor of
     * .75 is hardwired into the program, which uses cheap shifts in place of
     * expensive division.
     */
    static final float DEFAULT_LOAD_FACTOR = .75F;

加载因子固定为0.75。当实际大小超过当前容量*加载因子时进行扩容。实际上是由阈值threshold控制。

    /**
     * The hash table. If this hash map contains a mapping for null, it is
     * not represented this hash table.
     */
    transient HashMapEntry<K, V>[] table;

保存Entry的table数组。注意这里也用了transient 修饰符,原理同ArrayList(见前一篇)。

    /**
     * The entry representing the null key, or null if there's no such mapping.
     */
    transient HashMapEntry<K, V> entryForNullKey;

保存key为空的Entry。

    /**
     * The table is rehashed when its size exceeds this threshold.
     * The value of this field is generally .75 * capacity, except when
     * the capacity is zero, as described in the EMPTY_TABLE declaration
     * above.
     */
    private transient int threshold;

阈值。当实际大小超过阈值时重新进行哈希。

    /**
     * Constructs a new empty {@code HashMap} instance.
     */
    @SuppressWarnings("unchecked")
    public HashMap() {
        table = (HashMapEntry<K, V>[]) EMPTY_TABLE;
        threshold = -1; // Forces first put invocation to replace EMPTY_TABLE
    }

    /**
     * Constructs a new {@code HashMap} instance with the specified capacity.
     *
     * @param capacity
     *            the initial capacity of this hash map.
     * @throws IllegalArgumentException
     *                when the capacity is less than zero.
     */
    public HashMap(int capacity) {
        if (capacity < 0) {
            throw new IllegalArgumentException("Capacity: " + capacity);
        }

        if (capacity == 0) {
            @SuppressWarnings("unchecked")
            HashMapEntry<K, V>[] tab = (HashMapEntry<K, V>[]) EMPTY_TABLE;
            table = tab;
            threshold = -1; // Forces first put() to replace EMPTY_TABLE
            return;
        }

        if (capacity < MINIMUM_CAPACITY) {
            capacity = MINIMUM_CAPACITY;
        } else if (capacity > MAXIMUM_CAPACITY) {
            capacity = MAXIMUM_CAPACITY;
        } else {
            capacity = Collections.roundUpToPowerOfTwo(capacity);
        }
        makeTable(capacity);
    }

HashMap的几个构造函数。不指定容量的情况下table指向EMPTY_TABLE;指定容量的情况下容量将被调整为大于4的2的某次方值。

    static class HashMapEntry<K, V> implements Entry<K, V> {
        final K key;
        V value;
        final int hash;
        HashMapEntry<K, V> next;

        HashMapEntry(K key, V value, int hash, HashMapEntry<K, V> next) {
            this.key = key;
            this.value = value;
            this.hash = hash;
            this.next = next;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return value;
        }

        public final V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        @Override public final boolean equals(Object o) {
            if (!(o instanceof Entry)) {
                return false;
            }
            Entry<?, ?> e = (Entry<?, ?>) o;
            return Objects.equal(e.getKey(), key)
                    && Objects.equal(e.getValue(), value);
        }

        @Override public final int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^
                    (value == null ? 0 : value.hashCode());
        }

        @Override public final String toString() {
            return key + "=" + value;
        }
    }

Entry类,HashMap内部的数据结构。table是HashMapEntry数组,同时HashMapEntry有一个HashMapEntry类型的next指针。当数组哈希位置已经存在元素时将新元素加入到该位置的链表末位,这也就是哈希冲突处理方法中的拉链法。

    /**
     * Maps the specified key to the specified value.
     *
     * @param key
     *            the key.
     * @param value
     *            the value.
     * @return the value of any previous mapping with the specified key or
     *         {@code null} if there was no such mapping.
     */
    @Override public V put(K key, V value) {
        if (key == null) {
            return putValueForNullKey(value);
        }

        int hash = Collections.secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index]; e != null; e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                preModify(e);
                V oldValue = e.value;
                e.value = value;
                return oldValue;
            }
        }

        // No entry for (non-null) key is present; create one
        modCount++;
        if (size++ > threshold) {
            tab = doubleCapacity();
            index = hash & (tab.length - 1);
        }
        addNewEntry(key, value, hash, index);
        return null;
    }

    /**
     * Creates a new entry for the given key, value, hash, and index and
     * inserts it into the hash table. This method is called by put
     * (and indirectly, putAll), and overridden by LinkedHashMap. The hash
     * must incorporate the secondary hash function.
     */
    void addNewEntry(K key, V value, int hash, int index) {
        table[index] = new HashMapEntry<K, V>(key, value, hash, table[index]);
    }

增。对key二次哈希,与运算(相当于hash%length但是效率更高)得到index,然后遍历index对应的链表查找key相同的Entry,若有则替换之,若无则先判断是否需要扩容,再添加新Entry,将新Entry作为链表头并指向原来的链表头。如上文所说,这是拉链法的应用。

    /**
     * Doubles the capacity of the hash table. Existing entries are placed in
     * the correct bucket on the enlarged table. If the current capacity is,
     * MAXIMUM_CAPACITY, this method is a no-op. Returns the table, which
     * will be new unless we were already at MAXIMUM_CAPACITY.
     */
    private HashMapEntry<K, V>[] doubleCapacity() {
        HashMapEntry<K, V>[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            return oldTable;
        }
        int newCapacity = oldCapacity * 2;
        HashMapEntry<K, V>[] newTable = makeTable(newCapacity);
        if (size == 0) {
            return newTable;
        }

        for (int j = 0; j < oldCapacity; j++) {
            /*
             * Rehash the bucket using the minimum number of field writes.
             * This is the most subtle and delicate code in the class.
             */
            HashMapEntry<K, V> e = oldTable[j];
            if (e == null) {
                continue;
            }
            int highBit = e.hash & oldCapacity;
            HashMapEntry<K, V> broken = null;
            newTable[j | highBit] = e;
            for (HashMapEntry<K, V> n = e.next; n != null; e = n, n = n.next) {
                int nextHighBit = n.hash & oldCapacity;
                if (nextHighBit != highBit) {
                    if (broken == null)
                        newTable[j | nextHighBit] = n;
                    else
                        broken.next = n;
                    broken = e;
                    highBit = nextHighBit;
                }
            }
            if (broken != null)
                broken.next = null;
        }
        return newTable;
    }

扩容。扩容大小是固定的2倍。难点是扩容之后重新哈希的这段颇为风骚的代码。倒是在行间注释里特地写一句“This is the most subtle and delicate code in the class”是几个意思www。这里是以(hash & oldCapacity) | j求出index,该结果和hash & newCapacity - 1的运算结果相等,可以作出证明:
(hash & oldCapacity) | j
= (hash & oldCapacity) | (hash & (oldCapacity - 1))
= hash & (oldCapacity | (oldCapacity - 1))
因为容量capacity为2的某次幂,如4(0100),8(1000),形为除了第n位为1其他位为0;capacity - 1形为从第1位到第n - 1位为1其他位为0;则capacity | (capacity - 1)形为从第1位到第n位为1其他位为0,即capacity * 2 - 1也就是newCapacity - 1。
运算如此巧妙以至于让人初见如坠五里云雾。然而为什么要如此这番而不是直接使用hash & newCapacity - 1呢?就运算步骤来说反而是多了一步或运算。关于这个问题,我依旧不得其解。
将链表头移动到新表后遍历链表,判断每个Entry是否需要移动,判断的依据是高位是否相等,相等说明该Entry与上一Entry位于同一链表。
broken的作用是移动Entry之后将原链表上被移走的部分截掉。

    /**
     * Removes the mapping with the specified key from this map.
     *
     * @param key
     *            the key of the mapping to remove.
     * @return the value of the removed mapping or {@code null} if no mapping
     *         for the specified key was found.
     */
    @Override public V remove(Object key) {
        if (key == null) {
            return removeNullKey();
        }
        int hash = Collections.secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        int index = hash & (tab.length - 1);
        for (HashMapEntry<K, V> e = tab[index], prev = null;
                e != null; prev = e, e = e.next) {
            if (e.hash == hash && key.equals(e.key)) {
                if (prev == null) {
                    tab[index] = e.next;
                } else {
                    prev.next = e.next;
                }
                modCount++;
                size--;
                postRemove(e);
                return e.value;
            }
        }
        return null;
    }

删。原理和put类似。若删除的Entry是链表头则将下一Entry作为链表头,否则将上一Entry的next指向删除Entry的next。

    /**
     * Returns the value of the mapping with the specified key.
     *
     * @param key
     *            the key.
     * @return the value of the mapping with the specified key, or {@code null}
     *         if no mapping for the specified key is found.
     */
    public V get(Object key) {
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            return e == null ? null : e.value;
        }

        int hash = Collections.secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
                e != null; e = e.next) {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                return e.value;
            }
        }
        return null;
    }

查。与上面类似的原理。

谷歌对Android环境下的HashMap进行了一定的优化,同时也推出了在一些场景下性能更好的替代品SparseArray和ArrayMap。另外若在多线程环境中,应使用ConcurrentHashMap而不是HashMap。

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