基于哈希表的 Map 接口的实现。此实现提供所有可选的映射操作，并允许使用 null 值和 null 键。（除了非同步和允许使用 null 之外，HashMap 类与 Hashtable 大致相同。）
HashMap的数据成员
/**
* The default initial capacity - MUST be a power of two.
*/
// 默认的初始容量大小，必须为2的整数倍
static final int DEFAULT_INITIAL_CAPACITY = 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.
*/
// 最大容量大小
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
// 默认的加载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
// 用来存储entry项的数组（即哈希桶）
transient Entry[] table;
/**
* The number of key-value mappings contained in this map.
*/
// 存放的entry个数
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
// 阈值 = 容器 * 加载因子，达到该值时进行resize扩容操作
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
// 加载因子
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).
*/
// 累计HashMap结构修改的次数，用来迭代器在判断并发修改时快速失败
transient int modCount;
HashMap的默认构造函数
public HashMap() {
// 采用默认值进行构造
this.loadFactor = DEFAULT_LOAD_FACTOR;
threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];
init();
}
// 用来给子类在构造后进行拓展的方法
void init() {
}
hash方法
/**
* Applies a supplemental hash function to a given hashCode, 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.
*/
static int hash(int h) {
// 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.
*/
// 用来定位把key 放到哪个hash桶中
static int indexFor(int h, int length) {
// 等价于 h % (length -1)
return h & (length-1);
}
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) {
// key 为null
if (key == null)
return putForNullKey(value);
// 获取key的hash值
int hash = hash(key.hashCode());
// 根据hash值计算当前key 对应的hash桶index
int i = indexFor(hash, table.length);
// 遍历hash桶的链表
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
// 判断hash值相等，并且 (是同一个key 或 key.equals(k) ）
// 这也就是为什么把对象放进HashMap时，需要实现 hashCode 与 equals方法
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
// 该key已存在，则更新value，并返回记录oldValue
V oldValue = e.value;
e.value = value;
e.recordAccess(this);// 记录该项被访问过
return oldValue;
}
}
// 修改数加1
modCount++; // modCount不是volatile修饰的，且自增操作无法保证线程安全！！！
// key不存在Map中，则添加该项
addEntry(hash, key, value, i);
return null;
}
/**
* Offloaded version of put for null keys
*/
private V putForNullKey(V value) {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(0, null, value, 0);
return null;
}
addEntry方法
// 添加一个Entry
void addEntry(int hash, K key, V value, int bucketIndex) {
// 获取相应的hash桶
Entry<K,V> e = table[bucketIndex];
// 修改hash桶的head头指针为新建的Entry
// 旧的head则作为构造函数传入Entry,新建Entry的next将指向旧的head.
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
// 如果大小大于或等于阈值，则进行resize重新hash操作
resize(2 * table.length);// 增长为原来大小的2倍
// 如果恶意制造大量相同hash桶index的值，则会将HashMap退化为链表
// 从而产生HashMap碰撞攻击
}
Entry相当于链表中的一个节点（Node）
Entry类的成员与构建函数:
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
final int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
resize方法
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
// 如果已经达到最大容量上限，则调整threshold为整数的最大值，然后返回
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
// 创建一个新容器大小的hash桶
Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
// 重新计算阈值
threshold = (int)(newCapacity * loadFactor);
}
/**
* Transfers all entries from current table to newTable.
*/
// 迁移所有的旧hash桶的entry到新的hash桶内
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
// 遍历每个hash桶
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
// hash链表不为空
if (e != null) {
src[j] = null;// 将src的hash桶的头指针置空
do {// 遍历hash链表
Entry<K,V> next = e.next;
// 计算在新hash桶中的桶索引
int i = indexFor(e.hash, newCapacity);
// 与addEntry一样，修改Entry的next
e.next = newTable[i];
// 使hash桶的头指针指向entry
newTable[i] = e;
e = next;// 遍历下一项
} while (e != null);
}
}
}
迭代器的实现
public Set<Map.Entry<K,V>> entrySet() {
return entrySet0();
}
private Set<Map.Entry<K,V>> entrySet0() {
Set<Map.Entry<K,V>> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}
private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public Iterator<Map.Entry<K,V>> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<K,V> e = (Map.Entry<K,V>) o;
Entry<K,V> candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);
}
public boolean remove(Object o) {
return removeMapping(o) != null;
}
public int size() {
return size;
}
public void clear() {
HashMap.this.clear();
}
}
Iterator<Map.Entry<K,V>> newEntryIterator() {
return new EntryIterator();
}
private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}
private abstract class HashIterator<E> implements Iterator<E> {
Entry<K,V> next; // next entry to return
int expectedModCount; // For fast-fail
int index; // current slot
Entry<K,V> current; // current entry
HashIterator() {
// 初始化为保存当前的modCount
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
// 遍历hash桶，获取第一个不为空的hash桶
while (index < t.length && (next = t[index++]) == null)
;
}
}
public final boolean hasNext() {
return next != null;
}
final Entry<K,V> nextEntry() {
// 如果expectedModCount 与 modCount 不一致，则说明其它地方对Map进行了修改
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();
// 如果当前项的next为空，则找到一个不为空的项，如果没有则为null
if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
// 返回当前项
current = e;
return e;
}
public void remove() {
if (current == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Object k = current.key;
current = null;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
}
}
注意事项
- HashMap不是线程安全的。
- 迭代器的快速失败行为不能得到保证。
一般来说，存在非同步的并发修改时，不可能作出任何坚决的保证。快速失败迭代器尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的做法是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。 - 如果你有一个已知大小的HashMap,初始化时最好带上容量参数，以避免频繁进行resize操作。
- HashMap碰撞拒绝服务漏洞
Apache的方案是在Tomcat中增加一个新的选项maxParameterCount，用来限制单个请求中的最大参数量。参数默认值设为10000，确保既不会对应用程序造成影响（对多数应用来说已经足够），也足以减轻DoS攻击的压力。
####参考资料