java源码分析（15）-HashMap

public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
{
static final int DEFAULT_INITIAL_CAPACITY = 16;//默认容量（桶位）为16
static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量
static final float DEFAULT_LOAD_FACTOR = 0.75f;//默认加载因子为0.75，推荐的值，兼顾查询性能和扩容性能的最佳值（官方推荐）
transient Entry<K,V>[] table;//HashMap的底层为Entry<K,V>类型的数组，
transient int size;//已使用容量的大小，即有效节点的大小
int threshold;//扩容阀值
final float loadFactor;//加载因子，可以由自己设置，属性为final，一旦设定，无法修改
transient int modCount;//修改次数，所有集合类都有此属性
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
private static class Holder {
static final sun.misc.Unsafe UNSAFE;

/**
* Offset of "final" hashSeed field we must set in readObject() method.
*/
static final long HASHSEED_OFFSET;

/**
* Table capacity above which to switch to use alternative hashing.
*/
static final int ALTERNATIVE_HASHING_THRESHOLD;

static {
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));

int threshold;
try {//获取vm内置阀值
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;

// disable alternative hashing if -1
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}

if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}
ALTERNATIVE_HASHING_THRESHOLD = threshold;

try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
HASHSEED_OFFSET = UNSAFE.objectFieldOffset(
HashMap.class.getDeclaredField("hashSeed"));
} catch (NoSuchFieldException | SecurityException e) {
throw new Error("Failed to record hashSeed offset", e);
}
}
}
transient boolean useAltHashing;
transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
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);

// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)//当capacity小于传入的容量参数时，capacity 会扩容，每次扩为原来的2倍，直至大于传入的容量参数，所以HashMap的容量一定为2^n
capacity <<= 1;

this.loadFactor = loadFactor;
threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);//设定阀值，为避免capacity * loadFactor过大，阀值的最大值只能为MAXIMUM_CAPACITY + 1
table = new Entry[capacity];//初始化时即创建所有所有节点
useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
init();
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
public HashMap(Map<? extends K, ? extends V> m) {//传入的参数为集合时，必须考虑到负载因子，新建的HashMap中size必须在阀值以下
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
putAllForCreate(m);
}
void init() {
}
final int hash(Object k) {//将传入的对象转化为hash值
int h = 0;
if (useAltHashing) {
if (k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
h = hashSeed;
}

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);
}
static int indexFor(int h, int length) {
return h & (length-1);//通过&方法与length-1获得该hash值的数组下标
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public V get(Object key) {
if (key == null)
return getForNullKey();//
Entry<K,V> entry = getEntry(key);

return null == entry ? null : entry.getValue();
}
private V getForNullKey() {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
final Entry<K,V> getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];//从hash对应的下表开始遍历
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))//通过hash确定key的数组下标（开始遍历的下标），比较hash和key，只有当两者都相等时，才能确定这是要查询的key
return e;
}
return null;
}
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {//若该key已有节点，则替换旧节点中的值
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;
}
}
modCount++;
addEntry(hash, key, value, i);//该key还没有对应的节点，则在hash对应的下标节点新建包含hash，key，value的节点
return null;
}
private V putForNullKey(V value) {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {//若已有null为key对应的节点，则替换其中的value值
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(0, null, value, 0);//始终将null key放在下标为0的节点中
return null;
}
private void putForCreate(K key, V value) {//先查询，若还没有该key，则创建节点
int hash = null == key ? 0 : hash(key);
int i = indexFor(hash, table.length);
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);
}

private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}
void resize(int newCapacity) {//扩容
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {//以达最大的容量的情况下，将阀值设为Integer.MAX_VALUE
dc27

threshold = Integer.MAX_VALUE;
return;
}

Entry[] newTable = new Entry[newCapacity];
boolean oldAltHashing = useAltHashing;
useAltHashing |= sun.misc.VM.isBooted() &&
(newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean rehash = oldAltHashing ^ useAltHashing;
transfer(newTable, rehash);
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
void transfer(Entry[] newTable, boolean rehash) {//将原有节点的hash在扩容后的数组中重新确定下标，并将所有节点存入新建的数组中
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;
}
}
}
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
if (numKeysToBeAdded > threshold) {
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
newCapacity <<= 1;
if (newCapacity > table.length)
resize(newCapacity);
}

for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());
}
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
final Entry<K,V> removeEntryForKey(Object key) {
int hash = (key == null) ? 0 : hash(key);//计算hash
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;

while (e != null) {
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {//遍历查询，直到查询到所需要的key的节点
modCount++;
size--;
if (prev == e)
table[i] = next;//在删除该节点之前，先将前后有效节点连接
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}
final Entry<K,V> removeMapping(Object o) {//传入节点对象删除节点
if (!(o instanceof Map.Entry))
return null;

Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;

while (e != null) {
Entry<K,V> next = e.next;
if (e.hash == hash && e.equals(entry)) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}
public void clear() {
modCount++;
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}
public boolean containsValue(Object value) {//遍历有效节点查询是否含有value值
if (value == null)
return containsNullValue();

Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
return false;
}
private boolean containsNullValue() {//查询是否有值为null的节点
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value == null)
return true;
return false;
}
public Object clone() {//克隆该map
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
result.table = new Entry[table.length];
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);

return result;
}

static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;//用于存放下一个有效节点的地址，用于存放相同下标但key不同的值
int hash;
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}

public final K getKey() {
return key;
}

public final V getValue() {
return value;
}

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

public final boolean equals(Object o) {//比较两个节点是否相等时，只需要比较key和value
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}

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

public final String toString() {
return getKey() + "=" + getValue();
}
void recordAccess(HashMap<K,V> m) {
}
void recordRemoval(HashMap<K,V> m) {
}
}
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {//查看是否需要扩容，hashmap冲突处理是使用同下标链表存储的形式，但这不会导致hashmap中的元素超标，链表中的元素也算在hashmap中，size会增加
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) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);//若原来该下标有值，将以链表的形式在该下标位存放多个元素，并size++
size++;
}

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() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
}

public final boolean hasNext() {
return next != null;
}

final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();

if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
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;
}
}

private final class ValueIterator extends HashIterator<V> {
public V next() {
return nextEntry().value;
}
}

private final class KeyIterator extends HashIterator<K> {
public K next() {
return nextEntry().getKey();
}
}

private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}

// Subclass overrides these to alter behavior of views' iterator() method
Iterator<K> newKeyIterator()   {
return new KeyIterator();
}
Iterator<V> newValueIterator()   {
return new ValueIterator();
}
Iterator<Map.Entry<K,V>> newEntryIterator()   {
return new EntryIterator();
}

// Views

private transient Set<Map.Entry<K,V>> entrySet = null;
public Set<K> keySet() {//以set的形式返回所有key
Set<K> ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}

private final class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return newKeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return HashMap.this.removeEntryForKey(o) != null;
}
public void clear() {
HashMap.this.clear();
}
}
public Collection<V> values() {
Collection<V> vs = values;
return (vs != null ? vs : (values = new Values()));
}

private final class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return newValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
}
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();
}
}
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{//这里采用了迭代器进行遍历，因HashMap本身无序，故序列化时，直接只取键值对，无需考虑顺序
Iterator<Map.Entry<K,V>> i =
(size > 0) ? entrySet0().iterator() : null;

// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();

// Write out number of buckets
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;

/**
* Reconstitute the {@code HashMap} instance from a stream (i.e.,
* deserialize it).
*/
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 hashSeed (can only happen after VM boot)
Holder.UNSAFE.putIntVolatile(this, Holder.HASHSEED_OFFSET,
sun.misc.Hashing.randomHashSeed(this));

// Read in number of buckets and allocate the bucket array;
s.readInt(); // ignored

// Read number of mappings
int mappings = s.readInt();
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);

int initialCapacity = (int) Math.min(
// capacity chosen by number of mappings
// and desired load (if >= 0.25)
mappings * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY);
int capacity = 1;
// find smallest power of two which holds all mappings
while (capacity < initialCapacity) {
capacity <<= 1;
}

table = new Entry[capacity];
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);

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);
}
}

// These methods are used when serializing HashSets
int   capacity()     { return table.length; }
float loadFactor()   { return loadFactor;   }}