MyBatis缓存模块之SoftCache

一 SoftCache

  SoftCache 和 WeakCache的类图以下

  

看一下SoftCache的源码

package org.apache.ibatis.cache.decorators;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.Deque;
import java.util.LinkedList;
import java.util.concurrent.locks.ReadWriteLock;

import org.apache.ibatis.cache.Cache;

/**
 * Soft Reference cache decorator
 * Thanks to Dr. Heinz Kabutz for his guidance here.
 *
 * @author Clinton Begin
 */
public class SoftCache implements Cache {
   // 在SoftCache 中，最近使用的一部分缓存项不会被GC回收，这就是经过将其value添加到
   // hardLinksToAvoidGarbageCollection集合中实现的
  private final Deque<Object> hardLinksToAvoidGarbageCollection;
   // 引用队列，用于记录GC回收的缓存项所对应的SoftEntry对象
  private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
  // 底层被修饰的Cache 对象
  private final Cache delegate;
   // 强连接的个数，默认是256
  private int numberOfHardLinks;

  public SoftCache(Cache delegate) {
    this.delegate = delegate;
    this.numberOfHardLinks = 256;
    this.hardLinksToAvoidGarbageCollection = new LinkedList<Object>();
    this.queueOfGarbageCollectedEntries = new ReferenceQueue<Object>();
  }

  @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) {
    // 清除被GC回收的缓存项
    removeGarbageCollectedItems();
    // 向缓存中添加缓存项
    delegate.putObject(key, new SoftEntry(key, value, queueOfGarbageCollectedEntries));
  }

  @Override
  public Object getObject(Object key) {
    Object result = null;
    // 查找对应的缓存项
    @SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache
    SoftReference<Object> softReference = (SoftReference<Object>) delegate.getObject(key);
    if (softReference != null) {
      result = softReference.get();
      // 已经被GC 回收
      if (result == null) {
        // 从缓存中清除对应的缓存项
        delegate.removeObject(key);
      } else {
        // See #586 (and #335) modifications need more than a read lock 
        synchronized (hardLinksToAvoidGarbageCollection) {
          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() {
    synchronized (hardLinksToAvoidGarbageCollection) {
      // 清理强引用集合
      hardLinksToAvoidGarbageCollection.clear();
    }
    // 清理被GC回收的缓存项
    removeGarbageCollectedItems();
    delegate.clear();
  }

  @Override
  public ReadWriteLock getReadWriteLock() {
    return null;
  }

  private void removeGarbageCollectedItems() {
    SoftEntry sv;
    // 遍历集合
    while ((sv = (SoftEntry) queueOfGarbageCollectedEntries.poll()) != null) {
      // 将value对象清除
      delegate.removeObject(sv.key);
    }
  }
  
  /**
  **SoftCache 中缓存项的value是SoftEntry对象，SoftEnty对象继承了SoftReference
  **其中指向key的引用是强引用，而指向value的引用是若引用
  **/
  private static class SoftEntry extends SoftReference<Object> {
    private final Object key;

    SoftEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
      super(value, garbageCollectionQueue);
      this.key = key;
    }
  }

}