public class HashMapDeadLoopTest extends Thread {
private static Map<Integer, Integer> sharedMap = new HashMap<>(2);
private static AtomicInteger keyCounter = new AtomicInteger(1);
private static LoopPointerObjectDump table[] = new LoopPointerObjectDump[3];
private static int size = 0;
public static void main(String[] args) throws InterruptedException, IOException {
// 开400个线程同时跑（过小的线程数不易模拟出结果），死循环命中率很高，但是机器可能被卡死，小心运行
int i, runNum = 400;
HashMapDeadLoopTest[] tests = new HashMapDeadLoopTest[runNum];
for( i = 0; i < runNum; i++) {
tests[i] = new HashMapDeadLoopTest();
tests[i].start();
}
// 自己模拟单个线程出现死循环情况，说明
// HashMapDeadLoopTest test = new HashMapDeadLoopTest();
// LoopPointerObjectDump next1 = new LoopPointerObjectDump();
// LoopPointerObjectDump next2 = new LoopPointerObjectDump(next1);
//
// test.put(next2);
// LoopPointerObjectDump[] newTable = new LoopPointerObjectDump[3];
// newTable[0] = next2;
// test.transfer(newTable, true);
System.in.read();
}
void put(LoopPointerObjectDump e) {
table[size++] = e;
}
void transfer(LoopPointerObjectDump[] newTable, boolean rehash) {
int newCapacity = newTable.length;
int loopId = 0;
for (LoopPointerObjectDump e : table) {
while(null != e) {
LoopPointerObjectDump next = e.next;
// 假设线程一看不到线程最后的这个变更，next已经被赋旧值，打开注释慢慢查看死循环效果
// if(++loopId == 1) {
// next = e;
// }
// if(loopId > 100000) {
// break;
// }
int i = 0;
e.next = newTable[i];
newTable[i] = e;
e = next;
}
System.out.println("loopId:" + loopId);
}
}
@Override
public void run() {
while (keyCounter.get() < 1000000)
{
sharedMap.put(keyCounter.get(), keyCounter.get());
keyCounter.addAndGet(2);
System.out.println(Thread.currentThread().getName() + " : put a-> " + keyCounter.get());
}
}
private static class LoopPointerObjectDump {
private LoopPointerObjectDump next;
public LoopPointerObjectDump() {
next = null;
}
public LoopPointerObjectDump(LoopPointerObjectDump next) {
this.next = next;
}
public void setNext(LoopPointerObjectDump next) {
this.next = next;
}
}
}
不管怎么样，原理咱们还是来理解下的！
// java.utils.HashMap.put()
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
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++;
// 新元素，插入
addEntry(hash, key, value, i);
return null;
}
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
// 容量达到阀值，扩容
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity];
// 转移老容器内的元素到新容器内，重新平衡分布元素
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
// 注意，只要达到 e != null 条件，死循环就生成了
while(null != e) {
// 获取末尾元素
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
// 此处可能形成循环链 select * from com.test.HashMapDeadLoopTest, 查看 循环链
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
jdk8 中的 HashMap, 是这样的！
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
// 容量为空时重新赋值
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 元素不存在，则直接插入数组即可，线程安全性？
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 原值已存在，直接替换
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 如果是 LinkedHashMap 实现的话，会使用红黑树作为数据结构，调用其 putTreeVal()
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
// 找到最后一个 next 不会 null 的位置，插入元素
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 如果树的深度大于阀值-1, 则重新调整，平衡二叉树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 找到元素存在，直接进入后续更新
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 当元素存在时，更新，并返回旧值
if (e != null) { // existing mapping for key
V oldValue = e.value;
// 存在才添加判定
if (!onlyIfAbsent || oldValue == null)
e.value = value;
// LinkedHashMap 预留
afterNodeAccess(e);
return oldValue;
}
}
// 修改+1
++modCount;
// 容量超过阀值，扩容
if (++size > threshold)
resize();
// LinkedHashMap 预留
afterNodeInsertion(evict);
return null;
}
/**
* Replaces all linked nodes in bin at index for given hash unless
* table is too small, in which case resizes instead.
*/
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
// 对于小于容量 64 的情况，直接扩容解决
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
// 否则将数据结构升级为 TreeNode
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
// 二叉树建立 tl -> p ->
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
// 如果结构发生变更，则做一次平衡操作
hd.treeify(tab);
}
}
/**
* Forms tree of the nodes linked from this node.
* @return root of tree
*/
final void treeify(Node<K,V>[] tab) {
TreeNode<K,V> root = null;
for (TreeNode<K,V> x = this, next; x != null; x = next) {
next = (TreeNode<K,V>)x.next;
x.left = x.right = null;
if (root == null) {
x.parent = null;
x.red = false;
root = x;
}
else {
K k = x.key;
int h = x.hash;
Class<?> kc = null;
for (TreeNode<K,V> p = root;;) {
int dir, ph;
K pk = p.key;
// 根据hash值决定 node 放在左子树还有右子树
if ((ph = p.hash) > h)
dir = -1;
else if (ph < h)
dir = 1;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0)
dir = tieBreakOrder(k, pk);
TreeNode<K,V> xp = p;
// 找到叶子节点为空的位置，插入 x , 并做平衡操作
if ((p = (dir <= 0) ? p.left : p.right) == null) {
x.parent = xp;
if (dir <= 0)
xp.left = x;
else
xp.right = x;
root = balanceInsertion(root, x);
break;
}
}
}
}
moveRootToFront(tab, root);
}
// 平衡二叉树的节点插入
static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
TreeNode<K,V> x) {
x.red = true;
for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {
if ((xp = x.parent) == null) {
x.red = false;
return x;
}
else if (!xp.red || (xpp = xp.parent) == null)
return root;
if (xp == (xppl = xpp.left)) {
if ((xppr = xpp.right) != null && xppr.red) {
xppr.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {
if (x == xp.right) {
root = rotateLeft(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
if (xp != null) {
xp.red = false;
if (xpp != null) {
xpp.red = true;
root = rotateRight(root, xpp);
}
}
}
}
else {
if (xppl != null && xppl.red) {
xppl.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {
if (x == xp.left) {
root = rotateRight(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
if (xp != null) {
xp.red = false;
if (xpp != null) {
xpp.red = true;
root = rotateLeft(root, xpp);
}
}
}
}
}
}
/**
* Ensures that the given root is the first node of its bin.
*/
static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
int n;
if (root != null && tab != null && (n = tab.length) > 0) {
int index = (n - 1) & root.hash;
TreeNode<K,V> first = (TreeNode<K,V>)tab[index];
if (root != first) {
Node<K,V> rn;
tab[index] = root;
TreeNode<K,V> rp = root.prev;
if ((rn = root.next) != null)
((TreeNode<K,V>)rn).prev = rp;
if (rp != null)
rp.next = rn;
if (first != null)
first.prev = root;
root.next = first;
root.prev = null;
}
assert checkInvariants(root);
}
}
// 扩容
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
// 超过最大容量限制后，不再扩容，而是直接返回原值
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
// 判定扩容(*2)后不超过最大限制，且原有容量需大于最小容量，后更新扩容阀值，否则走后续逻辑扩容
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
