剖析Java中HashMap数据结构的源码及其性能优化

//默认初始容量
static final int DEFAULT_INITIAL_CAPACITY = 16;
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认装载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//哈希表
transient Entry<K,V>[] table;
//键值对的数量
transient int size;
//扩容的阈值
int threshold;
//哈希数组的装载因子
final float loadFactor;

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) //计算出大于指定容量的最小的2的幂
    capacity <<= 1;

  this.loadFactor = loadFactor;
  threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
  table = new Entry[capacity];  //给哈希表分配空间
  useAltHashing = sun.misc.VM.isBooted() &&
      (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
  init();
}

final int hash(Object k) {
  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);
}

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;
  }

  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) {
    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) {
  }
}

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) {
    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);
  return null;
}

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;
}

void addEntry(int hash, K key, V value, int bucketIndex) {
  if ((size >= threshold) && (null != table[bucketIndex])) {
    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);
  size++;
}

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;
}
final Entry<K,V> getEntry(Object key) {
  int hash = (key == null) ? 0 : hash(key);
  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;
}

class Key implements Comparable<Key> {
private final int value;
Key(int value) {
this.value = value;
}
@Override
public int compareTo(Key o) {
return Integer.compare(this.value, o.value);
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass())
return false;
Key key = (Key) o;
return value == key.value;
}
@Override
public int hashCode() {
return value;
}
}

class Key implements Comparable<Key> {
public class Keys {
public static final int MAX_KEY = 10_000_000;
private static final Key[] KEYS_CACHE = new Key[MAX_KEY];
static {
for (int i = 0; i < MAX_KEY; ++i) {
KEYS_CACHE[i] = new Key(i);
}
}
public static Key of(int value) {
return KEYS_CACHE[value];
}

import com.google.caliper.Param;
import com.google.caliper.Runner;
import com.google.caliper.SimpleBenchmark;
public class MapBenchmark extends SimpleBenchmark {
private HashMap<Key, Integer> map;
@Param
private int mapSize;
@Override
protected void setUp() throws Exception {
map = new HashMap<>(mapSize);
for (int i = 0; i < mapSize; ++i) {
map.put(Keys.of(i), i);
}
}
public void timeMapGet(int reps) {
for (int i = 0; i < reps; i++) {
map.get(Keys.of(i % mapSize));
}
}
}

class Key implements Comparable<Key> {
//...
@Override
public int hashCode() {
return 0;
}
}