2023-03-13  阅读(63)
原文作者:青花鱼罐头 原文地址:https://blog.csdn.net/qq_32782279/article/details/107826317

1. 概述

在优化系统性能时,优化数据库性能是非常重要的一个环节,而添加缓存则是优化数据库时最有效的手段之一。正确、合理地使用缓存可以将一部分数据库请求拦截在缓存这一层。

MyBatis 中提供了一级缓存和二级缓存,而这两级缓存都是依赖于基础支持层中的缓 存模块实现的。这里需要读者注意的是,MyBatis 中自带的这两级缓存与 MyBatis 以及整个应用是运行在同一个 JVM 中的,共享同一块堆内存。如果这两级缓存中的数据量较大, 则可能影响系统中其他功能的运行,所以当需要缓存大量数据时,优先考虑使用 Redis、Memcache 等缓存产品

2. Cache

org.apache.ibatis.cache.Cache ,缓存容器接口。注意,它是一个容器,有点类似 HashMap ,可以往其中添加各种缓存。代码如下:

    public interface Cache {
    
        /**
         * @return The identifier of this cache 标识
         */
        String getId();
    
        /**
         * 添加指定键的值
         *
         * @param key Can be any object but usually it is a {@link CacheKey}
         * @param value The result of a select.
         */
        void putObject(Object key, Object value);
    
        /**
         * 获得指定键的值
         *
         * @param key The key
         * @return The object stored in the cache.
         */
        Object getObject(Object key);
    
        /**
         * 移除指定键的值
         *
         * As of 3.3.0 this method is only called during a rollback
         * for any previous value that was missing in the cache.
         * This lets any blocking cache to release the lock that
         * may have previously put on the key.
         * A blocking cache puts a lock when a value is null
         * and releases it when the value is back again.
         * This way other threads will wait for the value to be
         * available instead of hitting the database.
         *
         *
         * @param key The key
         * @return Not used
         */
        Object removeObject(Object key);
    
        /**
         * 清空缓存
         *
         * Clears this cache instance
         */
        void clear();
    
        /**
         * 获得容器中缓存的数量
         *
         * Optional. This method is not called by the core.
         *
         * @return The number of elements stored in the cache (not its capacity).
         */
        int getSize();
    
        /**
         * 获得读取写锁。该方法可以忽略了已经。
         *
         * Optional. As of 3.2.6 this method is no longer called by the core.
         *
         * Any locking needed by the cache must be provided internally by the cache provider.
         *
         * @return A ReadWriteLock
         */
        @Deprecated 
        ReadWriteLock getReadWriteLock();
    
    }
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Cache 基于不同的缓存过期策略、特性,有不同的实现类。下面,我们来逐个来看。可以组合多种 Cache ,实现特性的组合。这种方式,就是常见的设计模式

2.1 PerpetualCache

org.apache.ibatis.cache.impl.PerpetualCache ,实现 Cache 接口,永不过期的 Cache 实现类,基于 HashMap 实现类。代码如下:

    // PerpetualCache.java
    
    public class PerpetualCache implements Cache {
    
        /**
         * 标识
         */
        private final String id;
        /**
         * 缓存容器
         */
        private Map<Object, Object> cache = new HashMap<>();
    
        public PerpetualCache(String id) {
            this.id = id;
        }
    
        @Override
        public String getId() {
            return id;
        }
    
        @Override
        public int getSize() {
            return cache.size();
        }
    
        @Override
        public void putObject(Object key, Object value) {
            cache.put(key, value);
        }
    
        @Override
        public Object getObject(Object key) {
            return cache.get(key);
        }
    
        @Override
        public Object removeObject(Object key) {
            return cache.remove(key);
        }
    
        @Override
        public void clear() {
            cache.clear();
        }
        
        // ... 省略 equals 和 hashCode 方法
        
    }
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2.2 LoggingCache

org.apache.ibatis.cache.decorators.LoggingCache ,实现 Cache 接口,支持打印日志的 Cache 实现类。代码如下:

    public class LoggingCache implements Cache {
    
        /**
         * MyBatis Log 对象
         */
        private final Log log;
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
        /**
         * 统计请求缓存的次数
         */
        protected int requests = 0;
        /**
         * 统计命中缓存的次数
         */
        protected int hits = 0;
    
        public LoggingCache(Cache delegate) {
            this.delegate = delegate;
            this.log = LogFactory.getLog(getId());
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object object) {
            delegate.putObject(key, object);
        }
    
        @Override
        public Object getObject(Object key) {
            // 请求次数 ++
            requests++;
            // 获得缓存
            final Object value = delegate.getObject(key);
            // 如果命中缓存,则命中次数 ++
            if (value != null) {
                hits++;
            }
            if (log.isDebugEnabled()) {
                log.debug("Cache Hit Ratio [" + getId() + "]: " + getHitRatio());
            }
            return value;
        }
    
        @Override
        public Object removeObject(Object key) {
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            delegate.clear();
        }
    
        /**
         * @return 命中比率
         */
        private double getHitRatio() {
            return (double) hits / (double) requests;
        }
    
        // ... 省略 equals 和 hashCode 方法
        
    }
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2.3 BlockingCache

org.apache.ibatis.cache.decoratorsBlockingCache ,实现 Cache 接口,阻塞的 Cache 实现类。

这里的阻塞比较特殊,当线程去获取缓存值时,如果不存在,则会阻塞后续的其他线程去获取该缓存。
为什么这么有这样的设计呢?因为当线程 A 在获取不到缓存值时,一般会去设置对应的缓存值,这样就避免其他也需要该缓存的线程 B、C 等,重复添加缓存。

代码如下:

    public class BlockingCache implements Cache {
    
        /**
         * 阻塞等待超时时间
         */
        private long timeout;
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
        /**
         * 缓存键与 ReentrantLock 对象的映射
         */
        private final ConcurrentHashMap<Object, ReentrantLock> locks;
    
        public BlockingCache(Cache delegate) {
            this.delegate = delegate;
            this.locks = new ConcurrentHashMap<>();
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object value) {
            try {
                // <2.1> 添加缓存
                delegate.putObject(key, value);
            } finally {
                // <2.2> 释放锁
                releaseLock(key);
            }
        }
    
        @Override
        public Object getObject(Object key) {
            // <1.1> 获得锁
            acquireLock(key);
            // <1.2> 获得缓存值
            Object value = delegate.getObject(key);
            // <1.3> 释放锁
            if (value != null) {
                releaseLock(key);
            }
            return value;
        }
    
        @Override
        public Object removeObject(Object key) {
            // despite of its name, this method is called only to release locks
            // 释放锁
            releaseLock(key);
            return null;
        }
    
        @Override
        public void clear() {
            delegate.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        /**
         * 获得 ReentrantLock 对象。如果不存在,进行添加
         *
         * @param key 缓存键
         * @return ReentrantLock 对象
         */
        private ReentrantLock getLockForKey(Object key) {
            ReentrantLock lock = new ReentrantLock();
            ReentrantLock previous = locks.putIfAbsent(key, lock);
            return previous == null ? lock : previous;
        }
    
        private void acquireLock(Object key) {
            // 获得 ReentrantLock 对象。
            Lock lock = getLockForKey(key);
            // 获得锁,直到超时
            if (timeout > 0) {
                try {
                    boolean acquired = lock.tryLock(timeout, TimeUnit.MILLISECONDS);
                    if (!acquired) {
                        throw new CacheException("Couldn't get a lock in " + timeout + " for the key " + key + " at the cache " + delegate.getId());
                    }
                } catch (InterruptedException e) {
                    throw new CacheException("Got interrupted while trying to acquire lock for key " + key, e);
                }
            } else {
                // 释放锁
                lock.lock();
            }
        }
    
        private void releaseLock(Object key) {
            // 获得 ReentrantLock 对象
            ReentrantLock lock = locks.get(key);
            // 如果当前线程持有,进行释放
            if (lock.isHeldByCurrentThread()) {
                lock.unlock();
            }
        }
    
        public long getTimeout() {
            return timeout;
        }
    
        public void setTimeout(long timeout) {
            this.timeout = timeout;
        }
    
    }
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2.4 SynchronizedCache

org.apache.ibatis.cache.decorators.SynchronizedCache ,实现 Cache 接口,同步的 Cache 实现类。代码如下:

    public class SynchronizedCache implements Cache {
    
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
    
        public SynchronizedCache(Cache delegate) {
            this.delegate = delegate;
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override // 同步
        public synchronized int getSize() {
            return delegate.getSize();
        }
    
        @Override // 同步
        public synchronized void putObject(Object key, Object object) {
            delegate.putObject(key, object);
        }
    
        @Override // 同步
        public synchronized Object getObject(Object key) {
            return delegate.getObject(key);
        }
    
        @Override // 同步
        public synchronized Object removeObject(Object key) {
            return delegate.removeObject(key);
        }
    
        @Override // 同步
        public synchronized void clear() {
            delegate.clear();
        }
    
        // ... 省略 equals 和 hashCode 方法
    
    }
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2.5 SerializedCache

org.apache.ibatis.cache.decorators.SerializedCache ,实现 Cache 接口,支持序列化值的 Cache 实现类。代码如下:

    public class SerializedCache implements Cache {
    
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
    
        public SerializedCache(Cache delegate) {
            this.delegate = delegate;
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object object) {
            if (object == null || object instanceof Serializable) {
                delegate.putObject(key, serialize((Serializable) object)); // 序列化
            } else {
                throw new CacheException("SharedCache failed to make a copy of a non-serializable object: " + object);
            }
        }
    
        @Override
        public Object getObject(Object key) {
            Object object = delegate.getObject(key);
            return object == null ? null : deserialize((byte[]) object); // 反序列化
        }
    
        @Override
        public Object removeObject(Object key) {
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            delegate.clear();
        }
    
        // ... 省略 equals 和 hashCode 方法
    
        private byte[] serialize(Serializable value) {
            try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
                 ObjectOutputStream oos = new ObjectOutputStream(bos)) {
                oos.writeObject(value);
                oos.flush();
                return bos.toByteArray();
            } catch (Exception e) {
                throw new CacheException("Error serializing object.  Cause: " + e, e);
            }
        }
    
        private Serializable deserialize(byte[] value) {
            Serializable result;
            try (ByteArrayInputStream bis = new ByteArrayInputStream(value);
                 ObjectInputStream ois = new CustomObjectInputStream(bis)) {
                result = (Serializable) ois.readObject();
            } catch (Exception e) {
                throw new CacheException("Error deserializing object.  Cause: " + e, e);
            }
            return result;
        }
    
        public static class CustomObjectInputStream extends ObjectInputStream {
    
            public CustomObjectInputStream(InputStream in) throws IOException {
                super(in);
            }
    
            @Override
            protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
                return Resources.classForName(desc.getName()); // 解析类
            }
    
        }
    
    }
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2.6 ScheduledCache

org.apache.ibatis.cache.decorators.ScheduledCache ,实现 Cache 接口,定时清空整个容器的 Cache 实现类。代码如下:

    public class ScheduledCache implements Cache {
    
        /**
         * 被装饰的 Cache 对象
         */
        private final Cache delegate;
        /**
         * 清空间隔,单位:毫秒
         */
        protected long clearInterval;
        /**
         * 最后清空时间,单位:毫秒
         */
        protected long lastClear;
    
        public ScheduledCache(Cache delegate) {
            this.delegate = delegate;
            this.clearInterval = 60 * 60 * 1000; // 1 hour
            this.lastClear = System.currentTimeMillis();
        }
    
        public void setClearInterval(long clearInterval) {
            this.clearInterval = clearInterval;
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            // 判断是否要全部清空
            clearWhenStale();
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object object) {
            // 判断是否要全部清空
            clearWhenStale();
            delegate.putObject(key, object);
        }
    
        @Override
        public Object getObject(Object key) {
            // 判断是否要全部清空
            return clearWhenStale() ? null : delegate.getObject(key); // 获得值
        }
    
        @Override
        public Object removeObject(Object key) {
            // 判断是否要全部清空
            clearWhenStale();
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            // 记录清空时间
            lastClear = System.currentTimeMillis();
            // 全部清空
            delegate.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        @Override
        public int hashCode() {
            return delegate.hashCode();
        }
    
        @Override
        public boolean equals(Object obj) {
            return delegate.equals(obj);
        }
    
        /**
         * 判断是否要全部清空
         *
         * @return 是否全部清空
         */
        private boolean clearWhenStale() {
            // 判断是否要全部清空
            if (System.currentTimeMillis() - lastClear > clearInterval) {
                // 清空
                clear();
                return true;
            }
            return false;
        }
    
    }
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2.7 FifoCache

org.apache.ibatis.cache.decorators.FifoCache ,实现 Cache 接口,基于先进先出的淘汰机制的 Cache 实现类。代码如下:

    // FifoCache.java
    
    public class FifoCache implements Cache {
    
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
        /**
         * 双端队列,记录缓存键的添加
         */
        private final Deque<Object> keyList;
        /**
         * 队列上限
         */
        private int size;
    
        public FifoCache(Cache delegate) {
            this.delegate = delegate;
            this.keyList = new LinkedList<>(); // 使用了 LinkedList
            this.size = 1024; // 默认为 1024
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            return delegate.getSize();
        }
    
        public void setSize(int size) {
            this.size = size;
        }
    
        @Override
        public void putObject(Object key, Object value) {
            // 循环 keyList
            cycleKeyList(key);
            delegate.putObject(key, value);
        }
    
        @Override
        public Object getObject(Object key) {
            return delegate.getObject(key);
        }
    
        @Override
        public Object removeObject(Object key) {
            // <2> 此处,理论应该也要移除 keyList 呀
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            delegate.clear();
            keyList.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        private void cycleKeyList(Object key) {
            // <1> 添加到 keyList 对位
            keyList.addLast(key);
            // 超过上限,将队首位移除
            if (keyList.size() > size) {
                Object oldestKey = keyList.removeFirst();
                delegate.removeObject(oldestKey);
            }
        }
    
    }
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2.8 LruCache

org.apache.ibatis.cache.decorators.LruCache ,实现 Cache 接口,基于最少使用的淘汰机制的 Cache 实现类。代码如下:

    public class LruCache implements Cache {
    
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
        /**
         * 基于 LinkedHashMap 实现淘汰机制
         */
        private Map<Object, Object> keyMap;
        /**
         * 最老的键,即要被淘汰的
         */
        private Object eldestKey;
    
        public LruCache(Cache delegate) {
            this.delegate = delegate;
            // 初始化 keyMap 对象
            setSize(1024);
        }
    
        @Override
        public String getId() {
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
            return delegate.getSize();
        }
    
        public void setSize(final int size) {
            // LinkedHashMap的一个构造函数,当参数accessOrder为true时,即会按照访问顺序排序,最近访问的放在最前,最早访问的放在后面
            keyMap = new LinkedHashMap<Object, Object>(size, .75F, true) { //
    
                private static final long serialVersionUID = 4267176411845948333L;
    
                // LinkedHashMap自带的判断是否删除最老的元素方法,默认返回false,即不删除老数据
                // 我们要做的就是重写这个方法,当满足一定条件时删除老数据
                @Override
                protected boolean removeEldestEntry(Map.Entry<Object, Object> eldest) {
                    boolean tooBig = size() > size;
                    if (tooBig) {
                        eldestKey = eldest.getKey();
                    }
                    return tooBig;
                }
    
            };
        }
    
        @Override
        public void putObject(Object key, Object value) {
            // 添加到缓存
            delegate.putObject(key, value);
            // 循环 keyMap
            cycleKeyList(key);
        }
    
        @Override
        public Object getObject(Object key) {
            // 刷新 keyMap 的访问顺序
            keyMap.get(key); // touch
            // 获得缓存值
            return delegate.getObject(key);
        }
    
        @Override
        public Object removeObject(Object key) {
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            delegate.clear();
            keyMap.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        private void cycleKeyList(Object key) {
            // 添加到 keyMap 中
            keyMap.put(key, key);
            // 如果超过上限,则从 delegate 中,移除最少使用的那个
            if (eldestKey != null) {
                delegate.removeObject(eldestKey);
                eldestKey = null; // 置空
            }
        }
    
    }
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2.9 WeakCache

org.apache.ibatis.cache.decorators.WeakCache ,实现 Cache 接口,基于 java.lang.ref.WeakReference 的 Cache 实现类。代码如下:

    public class WeakCache implements Cache {
    
        /**
         * 强引用的键的队列
         */
        private final Deque<Object> hardLinksToAvoidGarbageCollection;
        /**
         * {@link #hardLinksToAvoidGarbageCollection} 的大小
         */
        private int numberOfHardLinks;
        /**
         * 被 GC 回收的 WeakEntry 集合,避免被 GC。
         */
        private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
    
        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() {
            // 移除已经被 GC 回收的 WeakEntry
            removeGarbageCollectedItems();
            return delegate.getSize();
        }
    
        public void setSize(int size) {
            this.numberOfHardLinks = size;
        }
    
        @Override
        public void putObject(Object key, Object value) {
            // 移除已经被 GC 回收的 WeakEntry
            removeGarbageCollectedItems();
            // 添加到 delegate 中
            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
            // 获得值的 WeakReference 对象
            WeakReference<Object> weakReference = (WeakReference<Object>) delegate.getObject(key);
            if (weakReference != null) {
                // 获得值
                result = weakReference.get();
                // 为空,从 delegate 中移除 。为空的原因是,意味着已经被 GC 回收
                if (result == null) {
                    delegate.removeObject(key);
                // 非空,添加到 hardLinksToAvoidGarbageCollection 中,避免被 GC
                } else {
                    // 添加到 hardLinksToAvoidGarbageCollection 的队头
                    hardLinksToAvoidGarbageCollection.addFirst(result);
                    // 超过上限,移除 hardLinksToAvoidGarbageCollection 的队尾
                    if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
                        hardLinksToAvoidGarbageCollection.removeLast();
                    }
                }
            }
            return result;
        }
    
        @Override
        public Object removeObject(Object key) {
            // 移除已经被 GC 回收的 WeakEntry
            removeGarbageCollectedItems();
            // 移除出 delegate
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            // 清空 hardLinksToAvoidGarbageCollection
            hardLinksToAvoidGarbageCollection.clear();
            // 移除已经被 GC 回收的 WeakEntry
            removeGarbageCollectedItems();
            // 清空 delegate
            delegate.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        /**
         * 移除已经被 GC 回收的键
         */
        private void removeGarbageCollectedItems() {
            WeakEntry sv;
            while ((sv = (WeakEntry) queueOfGarbageCollectedEntries.poll()) != null) {
                delegate.removeObject(sv.key);
            }
        }
    
        private static class WeakEntry extends WeakReference<Object> {
    
            /**
             * 键
             */
            private final Object key;
    
            private WeakEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
                super(value, garbageCollectionQueue);
                this.key = key;
            }
    
        }
    
    }
复制代码
2.10 SoftCache

org.apache.ibatis.cache.decorators.SoftCache ,实现 Cache 接口,基于 java.lang.ref.SoftReference 的 Cache 实现类。代码如下:

    public class SoftCache implements Cache {
    
        /**
         * 强引用的键的队列
         */
        private final Deque<Object> hardLinksToAvoidGarbageCollection;
        /**
         * {@link #hardLinksToAvoidGarbageCollection} 的大小
         */
        private int numberOfHardLinks;
        /**
         * 被 GC 回收的 WeakEntry 集合,避免被 GC。
         */
        private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
        /**
         * 装饰的 Cache 对象
         */
        private final Cache delegate;
    
    
        public SoftCache(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) {
            // 移除已经被 GC 回收的 SoftEntry
            removeGarbageCollectedItems();
            delegate.putObject(key, new SoftEntry(key, value, queueOfGarbageCollectedEntries));
        }
    
        @Override
        public Object getObject(Object key) {
            Object result = null;
            @SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache
            // 获得值的 WeakReference 对象
            SoftReference<Object> softReference = (SoftReference<Object>) delegate.getObject(key);
            if (softReference != null) {
                // 获得值
                result = softReference.get();
                // 为空,从 delegate 中移除 。为空的原因是,意味着已经被 GC 回收
                if (result == null) {
                    delegate.removeObject(key);
                // 非空,添加到 hardLinksToAvoidGarbageCollection 中,避免被 GC
                } else {
                    // See #586 (and #335) modifications need more than a read lock
                    // 添加到 hardLinksToAvoidGarbageCollection 的队头
                    synchronized (hardLinksToAvoidGarbageCollection) {
                        hardLinksToAvoidGarbageCollection.addFirst(result);
                        // 超过上限,移除 hardLinksToAvoidGarbageCollection 的队尾
                        if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
                            hardLinksToAvoidGarbageCollection.removeLast();
                        }
                    }
                }
            }
            return result;
        }
    
        @Override
        public Object removeObject(Object key) {
            // 移除已经被 GC 回收的 SoftEntry
            removeGarbageCollectedItems();
            // 移除出 delegate
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
            // 清空 hardLinksToAvoidGarbageCollection
            synchronized (hardLinksToAvoidGarbageCollection) {
                hardLinksToAvoidGarbageCollection.clear();
            }
            // 移除已经被 GC 回收的 WeakEntry
            removeGarbageCollectedItems();
            // 清空 delegate
            delegate.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
            return null;
        }
    
        private void removeGarbageCollectedItems() {
            SoftEntry sv;
            while ((sv = (SoftEntry) queueOfGarbageCollectedEntries.poll()) != null) {
                delegate.removeObject(sv.key);
            }
        }
    
        private static class SoftEntry extends SoftReference<Object> {
    
            /**
             * 键
             */
            private final Object key;
    
            SoftEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
                super(value, garbageCollectionQueue);
                this.key = key;
            }
        }
    
    }
复制代码

3. CacheKey

org.apache.ibatis.cache.CacheKey ,实现 Cloneable、Serializable 接口,缓存键。

因为 MyBatis 中的缓存键不是一个简单的 String ,而是通过多个对象组成。所以 CacheKey 可以理解成将多个对象放在一起,计算其缓存键。


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