Java:与垃圾收集器竞争 [英] Java: Racing against the garbage collector

问题描述

我已经实现了Object缓存，如下所示:

I've implemented an Object cache like so:

// Dictionary with weak keys & values
private Map<Object, WeakReference<Object>> cache = new WeakHashMap<>();

private Object checkCache(Object obj) {

    // If it's in the cache, returned the cached copy.
    if (cache.containsKey(obj)) return cache.get(obj).get();

    // Store it in, and return it.
    cache.put(obj, new WeakReference<>(obj));
    return obj;
}

生动描述以下竞赛条件场景:

Picture the following race-condition scenario:

1. cache.containsKey(obj)返回true
2. 垃圾收集器启动并获取缓存中的对象.
3. null返回.

1. cache.containsKey(obj) returns true
2. The garbage collector kicks in and reaps the object in the cache.
3. null is returned.

问题是:

• 这真的可以发生吗? AFAIK GC可以在任何时间启动.
• 可以为单个方法调用禁用Java GC吗? synchronized似乎并没有阻止GC.
• 有没有解决方法?

• Can this really happen? AFAIK the GC can kick in at any time.
• Can Java GC be disabled for a single method call? synchronized doesn't seem to prevent GC.
• Are there any work-arounds?

提前谢谢！

推荐答案

这真的可以发生吗?

Can this really happen?

是的

可以为单个方法调用禁用Java GC吗?

Can Java GC be disabled for a single method call?

否.

有没有解决方法?

Are there any work-arounds?

是:您尝试从缓存中检索对象(从而在对象仍位于缓存中时建立一个强引用)，如果该引用为null，则将其添加到缓存中:

Yes: you attempt to retrieve the object from the cache (thereby establishing a strong reference if it's still in the cache), and add to the cache if that reference is null:

WeakReference<Object> ref = cache.get(obj);
Object cached = (ref != null) ? ref.get() : null;
if (cached != null) {
    return cached;
}
else {
    cache.put(obj, new WeakReference(obj));
    return obj;
}

您仍然需要同步该方法，否则您可能需要两个线程同时更新缓存(以及生成的

You still need to synchronize the method, otherwise you could have two threads updating the cache at the same time (and the resulting update race would be the least of your worries).

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