java.util.concurrent包含两个线程安全的Map，即ConcurrentHashMap类和ConcurrentSkipListMap类。这两个类都是线程安全的和高性能的。但是由于读取修改写入竞争条件，因此使用它们容易出错。Lambda表达式帮助我们优雅地避免了这些竞争条件。
当我们从ConcurrentHashMap中读取元素，修改该元素并将该元素写回到Map中时，多线程操作就会发生竞争，请参考：原子操作组合与线程安全。如以下示例所示：
package com.fun;
import org.junit.Test;
import java.util.concurrent.ConcurrentHashMap;
import static org.junit.Assert.assertEquals;
public class TestFun {
public void update(ConcurrentHashMap<Integer, Integer> map) {
Integer result = map.get(1);
if (result == null) {
map.put(1, 1);
} else {
map.put(1, result + 1);
}
}
@Test
public void testUpdate() throws InterruptedException {
final ConcurrentHashMap<Integer, Integer> map =
new ConcurrentHashMap<Integer, Integer>();
Thread first = new Thread(() -> {
update(map);
update(map);
update(map);
update(map);
update(map);
});
Thread second = new Thread(() -> {
update(map);
update(map);
update(map);
update(map);
update(map);
});
Thread third = new Thread(() -> {
update(map);
update(map);
update(map);
update(map);
update(map);
});
first.start();
second.start();
third.start();
first.join();
second.join();
third.join();
assertEquals(15, map.get(1).intValue());
}
}
在这里，我们实现了每个按键计数器。在update方法中，如果不存在映射，则将计数初始化为1，否则将计数加1。为了重现竞争条件，我们从三个不同的线程更新了ConcurrentHashMap。在线程都停止之后，我们检查该值是否跟方法的调用次数一致。
java.lang.AssertionError:
Expected :15
Actual :10
<Click to see difference>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:834)
at org.junit.Assert.assertEquals(Assert.java:645)
at org.junit.Assert.assertEquals(Assert.java:631)
at com.fun.TestFun.testUpdate(TestFun.java:51)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Process finished with exit code 255
为了避免这种竞争情况，我们需要一种方法来执行所有三个操作，即读取，修改和写入单个原子方法调用。该方法compute使用lambda表达式来做到这一点：
public void update(
ConcurrentHashMap<Integer,Integer> map ) {
map.compute(1, (key, value) -> {
if (value == null) {
return 1;
}
return value + 1;
});
}
现在，读取，修改和写入操作以一种原子方法发生，并且竞争消失了。
ConcurrentHashMap中的lambda表达式应该在节点的同步锁下执行。因此，其他线程不得调用此ConcurrentHashMap对应节点实例的其他写入操作。
对于的compute的源码如下：
public V compute(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
if (key == null || remappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int delta = 0;
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 = remappingFunction.apply(key, null)) != null) {
delta = 1;
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 {
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f, pred = null;; ++binCount) {
K ek;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = remappingFunction.apply(key, e.val);
if (val != null)
e.val = val;
else {
delta = -1;
Node<K,V> en = e.next;
if (pred != null)
pred.next = en;
else
setTabAt(tab, i, en);
}
break;
}
pred = e;
if ((e = e.next) == null) {
val = remappingFunction.apply(key, null);
if (val != null) {
delta = 1;
pred.next =
new Node<K,V>(h, key, val, null);
}
break;
}
}
}
else if (f instanceof TreeBin) {
binCount = 1;
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> r, p;
if ((r = t.root) != null)
p = r.findTreeNode(h, key, null);
else
p = null;
V pv = (p == null) ? null : p.val;
val = remappingFunction.apply(key, pv);
if (val != null) {
if (p != null)
p.val = val;
else {
delta = 1;
t.putTreeVal(h, key, val);
}
}
else if (p != null) {
delta = -1;
if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
break;
}
}
}
if (delta != 0)
addCount((long)delta, binCount);
return val;
}
可以看到其中多次用的了synchronized关键词完成线程同步，每次锁住对节点对象，操作完成之后释放锁。
Fhaohaizi@163.com。