WeakHashMap 和 ReentrantReadWriteLock

Question

我使用 WeakHashMap 和 ReentrantReadWriteLock 实现了缓存，我的代码是这样的：

class Demo<T, K> {

    private final ReentrantReadWriteLock LOCK = new ReentrantReadWriteLock();

    private final Map<T, K> CACHE = new WeakHashMap<>();

    public K get(T t) {
        ReentrantReadWriteLock.ReadLock readLock = LOCK.readLock();
        ReentrantReadWriteLock.WriteLock writeLock = LOCK.writeLock();

        readLock.lock();
        if(CACHE.containsKey(t)){

            //-- question point --

            K result = CACHE.get(t);
            readLock.unlock();
            return result;
        }
        readLock.unlock();

        K result = // find from db;

        writeLock.lock();
        CACHE.put(t,result);
        writeLock.unlock();

        return result;
    }
}

我的问题是，在 if(CACHE.containsKey(t)) 之后但在 K result = CACHE.get(t); 之前执行 gc 是否会发生读取锁定并导致 if(CACHE.containsKey(t)) 为真但 K result = CACHE.get(t); 得到空。

Answer 1

那么您的代码将 return 为空。

如果这不是您想要的，只需执行 get() 调用并检查是否得到非空结果。如果您担心 returning null，那么像您一样在此处调用 containsKey() 没有任何好处。

Answer 2

您的 ReentrantReadWriteLock 无法控制 WeakHashMap 关于垃圾收集器的行为。

WeakHashMap 状态的 class javadoc

The behavior of the WeakHashMap class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class. Because the garbage collector may discard keys at any time, a WeakHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a WeakHashMap instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value collection, and the entry set to yield successively smaller numbers of elements.

换句话说，是的，您的 containsKey 调用 return true 和 return [=20] 之后的 get 是可能的=]，如果垃圾收集器在两次调用之间起作用（并且您没有对相应键的其他强引用）。

您可以使用像

这样的小程序来验证此行为

public class Example {
    public static void main(String[] args) throws Exception {
        WeakHashMap<Example, Integer> CACHE = new WeakHashMap<>();
        CACHE.put(new Example(), 2);
        if (CACHE.containsKey(new Example())) {
            System.gc();
            System.out.println("missing? " + CACHE.get(new Example()));
        }
    }

    @Override
    public int hashCode() {
        return 42;
    }

    @Override
    public boolean equals(Object obj) {
        return true;
    }
}

打印

missing? null

WeakHashMap 和 ReentrantReadWriteLock

WeakHashMap and ReentrantReadWriteLock

java

garbage-collection

jvm

reentrantreadwritelock