JDK8 ConcurrentHashMap.computeIfAbsent 带来的问题
死循环问题
JDK8中的ConcurrentHashMap也不一定是安全的。
官方Bug报告： https://bugs.openjdk.java.net/browse/JDK-8062841
JDK9中变化内容： http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/6dd59c01f011
参考demo
package at.irian.misc.javabug;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class Main {
public static void main(String[] args) {
Map<String, Integer> map = new ConcurrentHashMap<>(16);
map.computeIfAbsent(
"AaAa",
key -> {
return map.computeIfAbsent(
"BBBB",
key2 -> 42);
}
);
}
}
问题在第一次调用AaAa时，会创建ReservationNode节点保存在map中
第二次调用时，因为节点存在，而且是synchronized 可重入，会调用else后代码
ReservationNode hash=-3 if elseif 都不会进入
会导致循环调用
public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
if (key == null || mappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
Node<K,V> r = new ReservationNode<K,V>();
synchronized (r) {
if (casTabAt(tab, i, null, r)) {
binCount = 1;
Node<K,V> node = null;
try {
if ((val = mappingFunction.apply(key)) != null)
node = new Node<K,V>(h, key, val, null);
} finally {
setTabAt(tab, i, node);
}
}
}
if (binCount != 0)
break;
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
boolean added = false;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek; V ev;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = e.val;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
if ((val = mappingFunction.apply(key)) != null) {
added = true;
pred.next = new Node<K,V>(h, key, val, null);
}
break;
}
}
}
else if (f instanceof TreeBin) {
binCount = 2;
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(h, key, null)) != null)
val = p.val;
else if ((val = mappingFunction.apply(key)) != null) {
added = true;
t.putTreeVal(h, key, val);
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (!added)
return val;
break;
}
}
}
if (val != null)
addCount(1L, binCount);
return val;
}
MyBatis 3.5.X遇上JDK8出现了性能问题
主要是在并发调用ConcurrentHashMap.computeIfAbsent方法时，为保证原子性，对相同的key进行修改时，会造成线程阻塞。
MyBatis 3.5.4不存在该问题，因为那是还没引进computeIfAbsent这个函数
参考DefaultReflectorFactory.java
@Override
public Reflector findForClass(Class<?> type) {
if (classCacheEnabled) {
// synchronized (type) removed see issue #461
Reflector cached = reflectorMap.get(type);
if (cached == null) {
cached = new Reflector(type);
reflectorMap.put(type, cached);
}
return cached;
} else {
return new Reflector(type);
}
}
目前MyBatis 3.5.7已修复该问题
引进了一个工具类，临时修复了JDK-8161372这个issue
JDK-8161372 issue中有并发测试computeIfAbsent的demo
public class MapUtil {
/**
* A temporary workaround for Java 8 specific performance issue JDK-8161372 .<br>
* This class should be removed once we drop Java 8 support.
*
* @see <a href="https://bugs.openjdk.java.net/browse/JDK-8161372">https://bugs.openjdk.java.net/browse/JDK-8161372</a>
*/
public static <K, V> V computeIfAbsent(Map<K, V> map, K key, Function<K, V> mappingFunction) {
V value = map.get(key);
if (value != null) {
return value;
}
return map.computeIfAbsent(key, mappingFunction::apply);
}
/**
* Map.entry(key, value) alternative for Java 8.
*/
public static <K, V> Entry<K, V> entry(K key, V value) {
return new AbstractMap.SimpleImmutableEntry<>(key, value);
}
private MapUtil() {
super();
}
}
