十、Mybatis 缓存系统解析

布满荆棘的人生 2023-01-23 11:49 34阅读 0赞

**本文从以下几个方面介绍：**

*  1、如何开启 Mybatis 的缓存
 *  2、缓存的核心接口以及底层实现
 *  3、一级缓存的实现过程
 *  4、二级缓存的实现过程
 *  5、缓存的装饰器

## 前言 ##

为了提高查询速度，减少数据库压力；Mybatis 提供了缓存功能，它分为一级缓存和二级缓存。

Mybatis 缓存系统的实现使用了 **模板方法模式** 和 **装饰器模式**

## 1、如何开启 Mybatis 的缓存 ##

### 1.1、一级缓存的作用 ###

Mybatis 的一级缓存是会话级别的缓存，Mybatis 每创建一个 SqlSession 对象，就表示打开一次数据库会话，在一次会话中，应用程序很可能在短时间内反复执行相同的查询语句，如果不对数据进行缓存，则每查询一次就要执行一次数据库查询，这就造成数据库资源的浪费。又因为通过 SqlSession 执行的操作，实际上由 Executor 来完成数据库操作的，所以在 Executor 中会建立一个简单的缓存，即一级缓存；将每次的查询结果缓存起来，再次执行查询的时候，会先查询一级缓存，如果命中，则直接返回，否则再去查询数据库并放入缓存中。

### 1.2、一级缓存的开启以及生命周期 ###

一级缓存的生命周期与 SqlSession 的生命周期相同，当调用 Executor.close 方法的时候，缓存变得不可用。**一级缓存是默认开启的**，一般情况下不需要特殊的配置，如果需要特殊配置，则可以通过插件的形式来实现

### 1.3、二级缓存的开启以及生命周期 ###

Mybatis 提供的二级缓存是应用级别的缓存，它的生命周期和应用程序的生命周期相同，且与二级缓存相关的配置有以下 3 个

*  1）、 mybatis-config.xml 配置文件中的 cacheEnabled 配置，它是二级缓存的总开关，只有该配置为 true ，后面的缓存配置才会生效。默认为 true，即二级缓存默认是开启的。

*  2）、Mapper.xml 配置文件中配置的 和 标签，如果 Mapper.xml 配置文件中配置了这两个标签中的任何一个，则表示开启了二级缓存的功能，如果配置了 标签，则在解析配置文件的时候，会为该配置文件指定的 namespace 创建相应的 Cache 对象作为其二级缓存（默认为 PerpetualCache 对象），如果配置了 节点，则通过 ref 属性的namespace值引用别的Cache对象作为其二级缓存。通过 和 标签来管理其在namespace中二级缓存功能的开启和关闭
 *  3）、 节点中的 useCache 属性也可以开启二级缓存，该属性表示查询的结果是否要存入到二级缓存中，该属性默认为 true，也就是说 标签默认会把查询结果放入到二级缓存中

![img][]

## 2、缓存的核心接口 ##

![preview][]

### 2.1、Cache ###

Mybatis 使用 Cache 来表示缓存，它是一个接口，定义了缓存需要的一些方法

public interface Cache {
        
     //获取缓存的id，即 namespace
     String getId();
     // 添加缓存
     void putObject(Object key, Object value);
     //根据key来获取缓存对应的值
     Object getObject(Object key);
     // 删除key对应的缓存
     Object removeObject(Object key);
     // 清空缓存  
     void clear();
     // 获取缓存中数据的大小
     int getSize();
     //取得读写锁, 从3.2.6开始没用了
     ReadWriteLock getReadWriteLock();
    }

### 2.2、PerpetualCache ###

Mybatis 为 Cache 接口提供的真正的实现类是 PerpetualCache，其他的只是应用装饰器模式，提供能额外功能，

如线程安全缓存 SynchronizedCache，它的底层存储还是 PerpetualCache。

PerpetualCache 的实现就是一个简单的 HashMap

public class PerpetualCache implements Cache {
        
     // id,一般对应mapper.xml 的namespace 的值
     private String id;
     
     // 用来存放数据，即缓存底层就是使用 map 来实现的
     private Map<Object, Object> cache = new HashMap<Object, Object>();
     
     public PerpetualCache(String id) {
        
     	this.id = id;
      }
     //......其他的getter方法.....
     // 添加缓存
     @Override
     public void putObject(Object key, Object value) {
        
        cache.put(key, value);
      }
     // 获取缓存
     @Override
     public Object getObject(Object key) {
        
     	return cache.get(key);
      }
     // 删除缓存
     @Override
     public Object removeObject(Object key) {
        
     	return cache.remove(key);
      }
     // 清空缓存
     @Override
     public void clear() {
        
        cache.clear();
      }
    }

### 2.3、CacheKey ###

Mybatis 的缓存使用了 key-value 的形式存入到 HashMap 中，而 key 的话，Mybatis 使用了 CacheKey 来表示 key，

它的生成规则为：mappedStementId + offset + limit + SQL + queryParams + environment生成一个哈希码.

public class CacheKey implements Cloneable, Serializable {
        
    
        private static final int DEFAULT_MULTIPLYER = 37;
        private static final int DEFAULT_HASHCODE = 17;
    
        // 参与计算hashcode，默认值为37
        private int multiplier;
        // CacheKey 对象的 hashcode ，默认值 17
        private int hashcode;
        // 检验和 
        private long checksum;
        // updateList 集合的个数
        private int count;
        // 由该集合中的所有对象来共同决定两个 CacheKey 是否相等
        private List<Object> updateList;
    
        public int getUpdateCount() {
        
            return updateList.size();
        }
    
        // 调用该方法，向 updateList 集合添加对应的对象
        public void update(Object object) {
        
            if (object != null && object.getClass().isArray()) {
        
                // 如果是数组，则循环处理每一项
                int length = Array.getLength(object);
                for (int i = 0; i < length; i++) {
        
                    Object element = Array.get(object, i);
                    doUpdate(element);
                }
            } else {
        
                doUpdate(object);
            }
        }
    
        // 计算 count checksum hashcode 和把对象添加到 updateList 集合中
        private void doUpdate(Object object) {
        
            int baseHashCode = object == null ? 1 : object.hashCode();
            count++;
            checksum += baseHashCode;
            baseHashCode *= count;
            hashcode = multiplier * hashcode + baseHashCode;
    
            updateList.add(object);
        }
    
        // 判断两个 CacheKey 是否相等
        @Override
        public boolean equals(Object object) {
        
            if (this == object) {
        
                return true;
            }
            if (!(object instanceof CacheKey)) {
        
                return false;
            }
    
            final CacheKey cacheKey = (CacheKey) object;
    
            if (hashcode != cacheKey.hashcode) {
        
                return false;
            }
            if (checksum != cacheKey.checksum) {
        
                return false;
            }
            if (count != cacheKey.count) {
        
                return false;
            }
            // 如果前几项都不满足，则循环遍历 updateList 集合，判断每一项是否相等，如果有一项不相等则这两个CacheKey不相等
            for (int i = 0; i < updateList.size(); i++) {
        
                Object thisObject = updateList.get(i);
                Object thatObject = cacheKey.updateList.get(i);
                if (thisObject == null) {
        
                    if (thatObject != null) {
        
                        return false;
                    }
                } else {
        
                    if (!thisObject.equals(thatObject)) {
        
                        return false;
                    }
                }
            }
            return true;
        }
    
        @Override
        public int hashCode() {
        
            return hashcode;
        }
    }

**CacheKey 的创建**

public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        
            //cacheKey 对象 
            CacheKey cacheKey = new CacheKey();
            // 向 updateList 存入id
            cacheKey.update(ms.getId());
            // 存入offset
            cacheKey.update(rowBounds.getOffset());
            // 存入limit
            cacheKey.update(rowBounds.getLimit());
            // 存入sql
            cacheKey.update(boundSql.getSql());
            List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            for (ParameterMapping parameterMapping : parameterMappings) {
        
                if (parameterMapping.getMode() != ParameterMode.OUT) {
        
                    String propertyName = parameterMapping.getProperty();
                    MetaObject metaObject = configuration.newMetaObject(parameterObject);
                    Object value = metaObject.getValue(propertyName);
                    // 存入每一个参数
                    cacheKey.update(value);
                }
            }
            if (configuration.getEnvironment() != null) {
        
                // 存入 environmentId
                cacheKey.update(configuration.getEnvironment().getId());
            }
            return cacheKey;
        }

## 3、一级缓存的实现 ##

Mybatis 的一级缓存是在 BaseExecutor 中实现的

### 3.1、Executor ###

Executor 接口定义了操作数据库的基本方法，我们用 SqlSession 来执行 sql时，其实是操作了 Executor 的相关方法

public interface Executor {
        
    
        ResultHandler NO_RESULT_HANDLER = null;
    
        // insert | update | delete 的操作方法
        int update(MappedStatement ms, Object parameter) throws SQLException;
    
        // 查询，带分页，带缓存  
        <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
    
        // 查询，带分页 
        <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
    
        // 查询存储过程
        <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
    
        //刷新批处理语句
        List<BatchResult> flushStatements() throws SQLException;
    
        // 事务提交
        void commit(boolean required) throws SQLException;
    
        // 事务回滚
        void rollback(boolean required) throws SQLException;
    
        // 创建缓存的key
        CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
    
        // 是否缓存
        boolean isCached(MappedStatement ms, CacheKey key);
    
        // 清空缓存
        void clearLocalCache();
    
        // 延迟加载
        void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
    
        // 获取事务
        Transaction getTransaction();
    }

### 3.2、BaseExecutor ###

BaseExecutor 是一个抽象类，实现了 Executor 接口的所有方法，使用了**模板模式**和**策略模式**，抽象了 （doUpdate, doQuery, doQueryCursor, doFlushStatement）4个核心方法用来执行sql，它们由不同的子类实现

Mybatis 的一级缓存就是在该类中实现的。

具体代码实现如下

@Override
        public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        
            ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
            if (closed) {
        
                throw new ExecutorException("Executor was closed.");
            }
            if (queryStack == 0 && ms.isFlushCacheRequired()) {
        
                clearLocalCache();
            }
            List<E> list;
            try {
        
                queryStack++;
                // 从缓存中获取结果
                list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
                if (list != null) {
        
                    handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
        
                    // 缓存中获取不到，则调用queryFromDatabase（）方法从数据库中查询
                    list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
        
                queryStack--;
            }
            if (queryStack == 0) {
        
                for (DeferredLoad deferredLoad : deferredLoads) {
        
                    deferredLoad.load();
                }
                // issue #601
                deferredLoads.clear();
                if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        
                    // issue #482
                    clearLocalCache();
                }
            }
            return list;
        }
    
     	private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        
            List<E> list;
            localCache.putObject(key, EXECUTION_PLACEHOLDER);
            try {
        
                // 调用doQuery（）方法查询
                list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
            } finally {
        
                localCache.removeObject(key);
            }
            // 缓存查询结果
            localCache.putObject(key, list);
            if (ms.getStatementType() == StatementType.CALLABLE) {
        
                localOutputParameterCache.putObject(key, parameter);
            }
            return list;
        }

## 4、二级缓存的实现 ##

Mybatis 的二级缓存是用 CachingExecutor 来实现的，它是 Executor 的一个装饰器类。为 Executor 对象添加了缓存的功能。

在介绍 CachingExecutor 之前，先来看看 CachingExecutor 依赖的两个类，TransactionalCacheManager 和 TransactionalCache。

### 4.1、TransactionalCache ###

TransactionalCache 实现了 Cache 接口，主要用于保存在某个 SqlSession 的某个事务中需要向某个二级缓存中添加的数据，代码如下：

public class TransactionalCache implements Cache {
        
        // 底层封装的二级缓存对应的Cache对象
        private Cache delegate;
        // 为true时，表示当前的 TransactionalCache 不可查询，且提交事务时会清空缓存
        private boolean clearOnCommit;
        // 存放需要添加到二级缓存中的数据
        private Map<Object, Object> entriesToAddOnCommit;
        // 存放未命中缓存的 CacheKey 对象
        private Set<Object> entriesMissedInCache;
    
        public TransactionalCache(Cache delegate) {
        
            this.delegate = delegate;
            this.clearOnCommit = false;
            this.entriesToAddOnCommit = new HashMap<Object, Object>();
            this.entriesMissedInCache = new HashSet<Object>();
        }
    
        // 添加缓存数据的时候，先暂时放到 entriesToAddOnCommit 集合中，在事务提交的时候，再把数据放入到二级缓存中，避免脏数据
        @Override
        public void putObject(Object key, Object object) {
        
            entriesToAddOnCommit.put(key, object);
        }
        // 提交事务，
        public void commit() {
        
            if (clearOnCommit) {
        
                delegate.clear();
            }
            // 把 entriesToAddOnCommit  集合中的数据放入到二级缓存中
            flushPendingEntries();
            reset();
        }
        // 把 entriesToAddOnCommit  集合中的数据放入到二级缓存中
        private void flushPendingEntries() {
        
            for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
        
                // 放入到二级缓存中
                delegate.putObject(entry.getKey(), entry.getValue());
            }
            for (Object entry : entriesMissedInCache) {
        
                if (!entriesToAddOnCommit.containsKey(entry)) {
        
                    delegate.putObject(entry, null);
                }
            }
        }
        // 事务回滚
        public void rollback() {
        
            // 把未命中缓存的数据清除掉
            unlockMissedEntries();
            reset();
        }
        private void unlockMissedEntries() {
        
            for (Object entry : entriesMissedInCache) {
        
                delegate.removeObject(entry);
            }
        }

### 4.2、TransactionalCacheManager ###

TransactionalCacheManager 用于管理 CachingExecutor 使用的二级缓存：

public class TransactionalCacheManager {
        
     
     //用来管理 CachingExecutor 使用的二级缓存
     // key 为对应的CachingExecutor 使用的二级缓存
     // value 为对应的 TransactionalCache 对象
     private Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();
     
     public void clear(Cache cache) {
        
        getTransactionalCache(cache).clear();
      }
     public Object getObject(Cache cache, CacheKey key) {
        
     return getTransactionalCache(cache).getObject(key);
      }  
     public void putObject(Cache cache, CacheKey key, Object value) {
        
        getTransactionalCache(cache).putObject(key, value);
      }
     public void commit() {
        
     for (TransactionalCache txCache : transactionalCaches.values()) {
        
          txCache.commit();
        }
      }
     public void rollback() {
        
     for (TransactionalCache txCache : transactionalCaches.values()) {
        
          txCache.rollback();
        }
      }
     // 所有的调用都会调用 TransactionalCache 的方法来实现
     private TransactionalCache getTransactionalCache(Cache cache) {
        
        TransactionalCache txCache = transactionalCaches.get(cache);
     if (txCache == null) {
        
          txCache = new TransactionalCache(cache);
          transactionalCaches.put(cache, txCache);
        }
     return txCache;
      }
     
    }

### 4.3、CachingExecutor ###

接下来看下 二级缓存的实现 CachingExecutor ：主要是通过 TransactionalCache 来操作二级缓存的。

public class CachingExecutor implements Executor {
        
        // 底层的 Executor
        private Executor delegate;
        private TransactionalCacheManager tcm = new TransactionalCacheManager();
    
        // 查询方法
        @Override
        public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
        
            // 获取 SQL
            BoundSql boundSql = ms.getBoundSql(parameterObject);
            // 创建缓存key，在CacheKey中已经分析过创建过程
            CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
            return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
        }
    
        // 查询
        public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
                throws SQLException {
        
            // 获取查询语句所在namespace对应的二级缓存
            Cache cache = ms.getCache();
            // 是否开启了二级缓存
            if (cache != null) {
        
                // 根据 <select> 的属性 useCache 的配置，决定是否需要清空二级缓存
                flushCacheIfRequired(ms);
                if (ms.isUseCache() && resultHandler == null) {
        
                    // 二级缓存不能保存输出参数，否则抛异常
                    ensureNoOutParams(ms, parameterObject, boundSql);
                    // 从二级缓存中查询对应的值
                    List<E> list = (List<E>) tcm.getObject(cache, key);
                    if (list == null) {
        
                        // 如果二级缓存没有命中，则调用底层的 Executor 查询，其中会先查询一级缓存，一级缓存也未命中，才会去查询数据库
                        list = delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                        // 查询到的数据放入到二级缓存中去
                        tcm.putObject(cache, key, list); // issue #578 and #116
                    }
                    return list;
                }
            }
            // 如果没有开启二级缓存，则直接调用底层的 Executor 查询，还是会先查一级缓存
            return delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
        }

![img][img 1]

## 5、缓存的装饰器 ##

Cache 接口由很多的装饰器类，共 10 个，添加了不同的功能，如下所示：后面列了几个，具体详情请参考源码

![img][img 2]

### 5.1、SynchronizedCache ###

*  线程安全的缓存，使用关键字 synchronized 来保证线程安全

public class SynchronizedCache implements Cache {
        
     
     private Cache delegate;
     
     public SynchronizedCache(Cache delegate) {
        
     this.delegate = delegate;
      }
     @Override
     public synchronized int getSize() {
        
     return delegate.getSize();
      }
     @Override
     public synchronized void putObject(Object key, Object object) {
        
        delegate.putObject(key, object);
      }
     @Override
     public synchronized Object getObject(Object key) {
        
     return delegate.getObject(key);
      }
     
     @Override
     public synchronized Object removeObject(Object key) {
        
     return delegate.removeObject(key);
      }
     // ............
    }

### 5.2、LruCache ###

*  最近最少使用缓存：最近使用最少的缓存优先删除

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    
    import java.util.LinkedHashMap;
    import java.util.Map;
    import java.util.concurrent.locks.ReadWriteLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 21:01
     * @description：
     * Lru (least recently used) cache decorator
     * 最近最少使用缓存
     * 最近没怎么使用的的缓存优先删除
     */
    public class LruCache implements Cache {
        
    
        private final Cache delegate;
        private Map<Object, Object> keyMap;
        private Object eldestKey;
    
        public LruCache(Cache delegate) {
        
            this.delegate = delegate;
            setSize(1024);
        }
    
        @Override
        public String getId() {
        
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
        
            return delegate.getSize();
        }
    
        public void setSize(final int size) {
        
            keyMap = new LinkedHashMap<Object, Object>(size, .75F, true) {
        
                private static final long serialVersionUID = 4267176411845948333L;
    
                @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) {
        
            delegate.putObject(key, value);
            cycleKeyList(key);
        }
    
        @Override
        public Object getObject(Object key) {
        
            keyMap.get(key); //touch
            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) {
        
            keyMap.put(key, key);
            if (eldestKey != null) {
        
                delegate.removeObject(eldestKey);
                eldestKey = null;
            }
        }
    }

### 5.3、FifoCache ###

*  先进先出 先添加的缓存优先删除

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    
    import java.util.Deque;
    import java.util.LinkedList;
    import java.util.concurrent.locks.ReadWriteLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 21:23
     * @description： 先进先出 先添加的缓存优先删除
     * FIFO (first in, first out) cache decorator
     */
    public class FifoCache implements Cache {
        
    
        private final Cache delegate;
        private final Deque<Object> keyList;
        private int size;
    
        public FifoCache(Cache delegate) {
        
            this.delegate = delegate;
            this.keyList = new LinkedList<Object>();
            this.size = 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) {
        
            cycleKeyList(key);
            delegate.putObject(key, value);
        }
    
        @Override
        public Object getObject(Object key) {
        
            return delegate.getObject(key);
        }
    
        @Override
        public Object removeObject(Object key) {
        
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
        
            delegate.clear();
            keyList.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
        
            return null;
        }
    
        private void cycleKeyList(Object key) {
        
            keyList.addLast(key);
            if (keyList.size() > size) {
        
                Object oldestKey = keyList.removeFirst();
                delegate.removeObject(oldestKey);
            }
        }
    }

### 5.4、SoftCache ###

*  软引用缓存：jvm 堆内存不够时，自动删除
 *  **软引用**：用来描述一些还有用但并非必须的对象。对于软引用关联着的对象，在系统将要发生内存溢出异常之前，将会把这些对象列进回收范围进行第二次回收。如果这次回收还没有足够的内存，才会抛出内存溢出异常。

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    
    import java.lang.ref.ReferenceQueue;
    import java.lang.ref.SoftReference;
    import java.util.Deque;
    import java.util.LinkedList;
    import java.util.concurrent.locks.ReadWriteLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 21:27
     * @description：软引用缓存装饰器
     * Soft Reference cache decorator
     * Thanks to Dr. Heinz Kabutz for his guidance（指导） here.
     */
    public class SoftCache implements Cache {
        
        private final Deque<Object> hardLinksToAvoidGarbageCollection;
        private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
        private final Cache delegate;
        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) {
        
            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
            SoftReference<Object> softReference = (SoftReference<Object>) delegate.getObject(key);
            if (softReference != null) {
        
                result = softReference.get();
                if (result == null) {
        
                    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) {
        
                hardLinksToAvoidGarbageCollection.clear();
            }
            removeGarbageCollectedItems();
            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> {
        
            private final Object key;
    
            SoftEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
        
                super(value, garbageCollectionQueue);
                this.key = key;
            }
        }
    
    }

### 5.5、WeakCache ###

*  弱引用缓存
 *  弱引用：用来描述非必须对象的，他的强度比软引用更弱一些，被弱引用关联的对象，在垃圾回收时，如果这个对象只被弱引用关联（没有任何强引用关联他），那么这个对象就会被回收。

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    
    import java.lang.ref.ReferenceQueue;
    import java.lang.ref.WeakReference;
    import java.util.Deque;
    import java.util.LinkedList;
    import java.util.concurrent.locks.ReadWriteLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 22:02
     * @description： 软引用缓存装饰器
     * Weak Reference cache decorator.
     * Thanks to Dr. Heinz Kabutz for his guidance here.
     */
    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<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) {
        
            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();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
        
            return null;
        }
    
        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;
            }
        }
    
    }

### 5.6、ScheduledCache ###

*  超时自动失效的缓存：自动失效的时间点不是时间到了，而是下次查询缓存时，如果判断时间过期才会失效，所以并不是真正的超时自动失效的缓存

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    
    import java.util.concurrent.locks.ReadWriteLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 21:26
     * @description：
     */
    public class ScheduledCache implements Cache {
        
    
        private final Cache delegate;
        protected long clearInterval;
        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() {
        
            clearWhenStale();
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object object) {
        
            clearWhenStale();
            delegate.putObject(key, object);
        }
    
        @Override
        public Object getObject(Object key) {
        
            return clearWhenStale() ? null : delegate.getObject(key);
        }
    
        @Override
        public Object removeObject(Object key) {
        
            clearWhenStale();
            return delegate.removeObject(key);
        }
    
        @Override
        public void clear() {
        
            lastClear = System.currentTimeMillis();
            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);
        }
    
        private boolean clearWhenStale() {
        
            if (System.currentTimeMillis() - lastClear > clearInterval) {
        
                clear();
                return true;
            }
            return false;
        }
    
    }

### 5.7、BlockingCache ###

*  阻塞缓存：当缓存不存在阻塞，直到有缓存才返回

package com.luo.ibatis.cache.decorators;
    
    import com.luo.ibatis.cache.Cache;
    import com.luo.ibatis.cache.CacheException;
    
    import java.util.concurrent.ConcurrentHashMap;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReadWriteLock;
    import java.util.concurrent.locks.ReentrantLock;
    
    /**
     * @author ：archer
     * @date ：Created in 2021/6/30 21:22
     * @description： 简单阻塞，缓存中找不到会阻塞其他线程（效率低下）
     * Simple blocking decorator
     *
     * Simple and inefficient version of EhCache's BlockingCache decorator.
     * It sets a lock over a cache key when the element is not found in cache.
     * This way, other threads will wait until this element is filled instead of hitting the database.
     */
    public class BlockingCache implements Cache {
        
    
        private long timeout;
        private final Cache delegate;
        private final ConcurrentHashMap<Object, ReentrantLock> locks;
    
        public BlockingCache(Cache delegate) {
        
            this.delegate = delegate;
            this.locks = new ConcurrentHashMap<Object, ReentrantLock>();
        }
    
        @Override
        public String getId() {
        
            return delegate.getId();
        }
    
        @Override
        public int getSize() {
        
            return delegate.getSize();
        }
    
        @Override
        public void putObject(Object key, Object value) {
        
            try {
        
                delegate.putObject(key, value);
            } finally {
        
                releaseLock(key);
            }
        }
    
        @Override
        public Object getObject(Object key) {
        
            acquireLock(key);
            Object value = delegate.getObject(key);
            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
            releaseLock(key);
            return null;
        }
    
        @Override
        public void clear() {
        
            delegate.clear();
        }
    
        @Override
        public ReadWriteLock getReadWriteLock() {
        
            return null;
        }
    
        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) {
        
            Lock lock = getLockForKey(key);
            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 {
        
                lock.lock();
            }
        }
    
        private void releaseLock(Object key) {
        
            ReentrantLock lock = locks.get(key);
            if (lock.isHeldByCurrentThread()) {
        
                lock.unlock();
            }
        }
    
        public long getTimeout() {
        
            return timeout;
        }
    
        public void setTimeout(long timeout) {
        
            this.timeout = timeout;
        }
    }

[img]: /images/20221004/ab333cdacf5d4d579c9cc6801f9c8ca1.png
[preview]: /images/20221004/57178c93e74c44f792a79541821754cf.png
[img 1]: /images/20221004/4f1c8f5949d54333b170c8d588631a12.png
[img 2]: /images/20221004/5ff022eca7c948a7bc4e8519c2149c63.png