java-blog

有一个很耗时的运算需要缓存其计算结果，采用备忘录模式实现，如：

public interface Computable<A, V> {
V compute(A arg) throws InterruptedException;
}
public class ExpensiveFunction
implements Computable<String, BigInteger> {
public BigInteger compute(String arg) {
// 假设这里非常耗时...
return new BigInteger(arg);
}
}

备忘录缓存方案一：

public class Memoizer1<A, V> implements Computable<A, V> {
@GuardedBy("this")
private final Map<A, V> cache = new HashMap<A, V>();
private final Computable<A, V> c;
public Memoizer1(Computable<A, V> c) {
this.c = c;
}
public synchronized V compute(A arg) throws InterruptedException {
V result = cache.get(arg);
if (result == null) {
result = c.compute(arg);
cache.put(arg, result);
}
return result;
}
}

这里备忘录的整个compute过程被同步，相当于我最原始的低效方案，


备忘录缓存方案二：

public class Memoizer2<A, V> implements Computable<A, V> {
private final Map<A, V> cache = new ConcurrentHashMap<A, V>();
private final Computable<A, V> c;
public Memoizer2(Computable<A, V> c) { this.c = c; }
public V compute(A arg) throws InterruptedException {
V result = cache.get(arg);
if (result == null) {
result = c.compute(arg);
cache.put(arg, result);
}
return result;
}
}

将线程安全性委给cache，注：这个方案的cache用了ConcurrentHashMap，它是线程安全的。

备忘录缓存方案三：

public 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; }
public V compute(final A arg) throws InterruptedException {
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 = ft;
cache.put(arg, ft);
ft.run(); // call to c.compute happens here
}
try {
return f.get();
} catch (ExecutionException e) {
throw launderThrowable(e.getCause());
}
}
}

使用Future(相当上一帖的Entry)封装，因为这里资源获取过程不固定(通用方案)，所以使用了Callable进行回调资源获取过程(求值)，
这个方案的 if (f == null) 不安全，特殊性可能会进行多次运算，下面的方案四解决这个问题。

备忘录缓存方案四（最终方案）：