~~~ package org.apache.ibatis.cache.decorators; import java.lang.ref.ReferenceQueue; import java.lang.ref.WeakReference; import java.util.Deque; import java.util.LinkedList; import org.apache.ibatis.cache.Cache; /** * 虚引用缓存 * Weak Reference cache decorator. * Thanks to Dr. Heinz Kabutz for his guidance here. * * @author Clinton Begin */ public class WeakCache implements Cache { private final Deque<Object> hardLinksToAvoidGarbageCollection; private final ReferenceQueue<Object> queueOfGarbageCollectedEntries; private final Cache delegate; private int numberOfHardLinks; public WeakCache(Cache delegate) { this.delegate = delegate; this.numberOfHardLinks = 256; this.hardLinksToAvoidGarbageCollection = new LinkedList<>(); this.queueOfGarbageCollectedEntries = new ReferenceQueue<>(); } @Override public String getId() { return delegate.getId(); } @Override public int getSize() { removeGarbageCollectedItems(); return delegate.getSize(); } public void setSize(int size) { this.numberOfHardLinks = size; } @Override public void putObject(Object key, Object value) { removeGarbageCollectedItems(); delegate.putObject(key, new WeakEntry(key, value, queueOfGarbageCollectedEntries)); } @Override public Object getObject(Object key) { Object result = null; @SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache WeakReference<Object> weakReference = (WeakReference<Object>) delegate.getObject(key); if (weakReference != null) { result = weakReference.get(); if (result == null) { delegate.removeObject(key); } else { hardLinksToAvoidGarbageCollection.addFirst(result); if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) { hardLinksToAvoidGarbageCollection.removeLast(); } } } return result; } @Override public Object removeObject(Object key) { removeGarbageCollectedItems(); return delegate.removeObject(key); } @Override public void clear() { hardLinksToAvoidGarbageCollection.clear(); removeGarbageCollectedItems(); delegate.clear(); } private void removeGarbageCollectedItems() { WeakEntry sv; while ((sv = (WeakEntry) queueOfGarbageCollectedEntries.poll()) != null) { delegate.removeObject(sv.key); } } private static class WeakEntry extends WeakReference<Object> { private final Object key; private WeakEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) { super(value, garbageCollectionQueue); this.key = key; } } } ~~~