JAVA并发编程实践
第85页

环境 JDK1.6(貌似使用1.7编译会有问题)

第一版 synchronized 控制并发

ExpensiveFunction 模拟一个很费时间的操作.比如数据库读取。

这个版本问题很明显,就是synchronized 限制了并发.

第二版 ConcurrentHashMap替换HashMap

这个版本的问题是,在并发很高的情况下,一旦缓存失效,大量的请求会拥塞数据库的所有线程.造成数据库的间歇性问题.

第三版 Future

import java.math.BigInteger;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
interface Computable<A, V> {
V compute(final A arg) throws InterruptedException, ExecutionException;
}
class ExpensiveFunction implements Computable<String, BigInteger> {
@Override
public BigInteger compute(String arg) throws InterruptedException, ExecutionException {
return new BigInteger(arg);
}
}
class Memoizer3<A, V> implements Computable<A, V> {
private final Map<A, Future<V>> cache = new ConcurrentHashMap<A, Future<V>>();
private final Computable<A, V> c;
public Memoizer3(Computable<A, V> c) {
this.c = c;
}
@Override
public V compute(final A arg) throws InterruptedException, ExecutionException {
Future<V> f = cache.get(arg);
if (f == null) {
Callable<V> eval = new Callable<V>() {
@Override
public V call() throws ExecutionException, InterruptedException {
return c.compute(arg);
}
};
FutureTask<V> ft = new FutureTask<V>(eval);
f = ft;
cache.put(arg, ft);
ft.run();
}
return f.get();
}
}

这个版本的问题是
if(f==null)是非原子的"先检查后执行",可能会有多个线程同时进入该段代码.但是相对于第二版,概率已经大大降低.

最终版本

import java.math.BigInteger;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
interface Computable<A, V> {
V compute(final A arg) throws InterruptedException;
}
class ExpensiveFunction implements Computable<String, BigInteger> {
@Override
public BigInteger compute(String arg) throws InterruptedException {
return new BigInteger(arg);
}
}
class Memoizer3<A, V> implements Computable<A, V> {
private final ConcurrentHashMap<A, Future<V>> cache = new ConcurrentHashMap<A, Future<V>>();
private final Computable<A, V> c;
public Memoizer3(Computable<A, V> c) {
this.c = c;
}
@Override
public V compute(final A arg) throws InterruptedException {
while (true) {
Future<V> f = cache.get(arg);
if (f == null) {
Callable<V> eval = new Callable<V>() {
public V call() throws InterruptedException {
return c.compute(arg);
}
};
FutureTask<V> ft = new FutureTask<V>(eval);
f = cache.putIfAbsent(arg, ft);
if (f == null) {
f = ft;
ft.run();
}
}
try {
return f.get();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
}

这个版本还是有一些问题，比如没有缓存失效机制,并且异常的时候没有将Future从Map中移除.
不过已经说明了很多的用法.

这个while循环感觉很精妙
一旦 return f.get()发生ExecutionException异常,代码不会退出,而是会再次执行.

ITPub博客

构建高效且可伸缩的结果缓存