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/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
*/
void resize(int newCapacity) {
Entry<K,V>[] oldTable = getTable();
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry<K,V>[] newTable = newTable(newCapacity); //重新创建新数组
boolean oldAltHashing = useAltHashing;
useAltHashing |= sun.misc.VM.isBooted() &&
(newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean rehash = oldAltHashing ^ useAltHashing; //是否重新计算 hash值
transfer(oldTable, newTable, rehash);
table = newTable;
/*
* If ignoring null elements and processing ref queue caused massive
* shrinkage, then restore old table. This should be rare, but avoids
* unbounded expansion of garbage-filled tables.
* 如果忽略null元素和处理ref队列造成了巨大的收缩，则恢复旧表。这应该很少见，但可以避免垃圾填充表的无限制扩展。
*/
if (size >= threshold / 2) {
threshold = (int)(newCapacity * loadFactor); //重新设置门槛值，
} else { //这里重新恢复旧数组，
expungeStaleEntries();
transfer(newTable, oldTable, false);
table = oldTable;
}
}
/** Transfers all entries from src to dest tables */
// 把以前数据数组复制到新数组中，调整数组下标，或者hash(key)值
private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) {
for (int j = 0; j < src.length; ++j) {
Entry<K,V> e = src[j];
src[j] = null;
while (e != null) {
Entry<K,V> next = e.next;
Object key = e.get();
if (key == null) {
e.next = null; // Help GC
e.value = null; // " "
size--;
} else {
if (rehash) { //是否重新计算hash
e.hash = hash(key);
}
int i = indexFor(e.hash, dest.length); //重新定位新数组下标
e.next = dest[i];
dest[i] = e;
}
e = next;
}
}
}
/**
* Expunges stale entries from the table.
* 从引用队列中查询gc元素，清理数组元素
*/
private void expungeStaleEntries() {
for (Object x; (x = queue.poll()) != null; ) {
synchronized (queue) {
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) x;
int i = indexFor(e.hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> p = prev;
while (p != null) {
Entry<K,V> next = p.next;
if (p == e) { //清理数组元素
if (prev == e)
table[i] = next;
else
prev.next = next;
// Must not null out e.next;
// stale entries may be in use by a HashIterator
e.value = null; // Help GC
size--;
break;
}
prev = p;
p = next;
}
}
}
}
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