mybatis之缓存模块

向右看齐 2022-09-01 12:55 196阅读 0赞

# mybatis之缓存模块 #

mybatis中存在一级(默认开启)、二级缓存。缓存可以加快查询速度，减少连接db的次数。

cache模块属于mybatis的基础支持层。位于org.apache.ibatis.cache包下。

--------org.apache.ibatis.cache
    -------------------------------decorators
    -----------------------------------------BlockingCache
    -----------------------------------------FifoCache
    -----------------------------------------LoggingCache
    -----------------------------------------LruCache
    -----------------------------------------ScheduledCache
    -----------------------------------------SerializedCache
    -----------------------------------------SoftCache
    -----------------------------------------SynchronizedCache
    -----------------------------------------TransactionalCache
    -----------------------------------------WeakCache
    -------------------------------impl
    -----------------------------------PerpetualCache
    ------------------------------Cache

CachePerpetualCacheBlockingCacheFifoCacheLoggingCacheLruCacheScheduledCacheSerializedCacheSoftCacheSynchronizedCacheTransactionalCacheWeakCache

## Cache ##

缓存的顶级接口

public interface Cache { 
        // 得到缓存的id
        String getId();
        // 插入缓存
        void putObject(Object key, Object value);
        // 查询到缓存
        Object getObject(Object key);
        // 删除缓存
        Object removeObject(Object key);
        //清理这个缓存实例
        void clear();
        // 这个方法可选，不是核心调用方法，查询缓存key的个数
        int getSize();
        //
        ReadWriteLock getReadWriteLock();
    }

## PerpetualCache ##

缓存的基础实现(实现了基本的缓存功能，底层数据结构使用HashMap)

一级缓存是会话级别的缓存(SqlSession)。

public class PerpetualCache implements Cache { 
    
        private final String id;
        // 存储缓存底层数据结构使用hashMap
        private Map<Object, Object> cache = new HashMap<Object, Object>();
        // 传入
        public PerpetualCache(String id) { 
            this.id = id;
        }
    
        @Override
        public String getId() { 
            return id;
        }
    
    
        @Override
        public int getSize() { 
            return cache.size();
        }
    
        // 插入缓存
        @Override
        public void putObject(Object key, Object value) { 
            cache.put(key, value);
        }
    
        //查询缓存
        @Override
        public Object getObject(Object key) { 
            return cache.get(key);
        }
    
        // 移除缓存
        @Override
        public Object removeObject(Object key) { 
            return cache.remove(key);
        }
    
        @Override
        public void clear() { 
            cache.clear();
        }
    
          @Override
      public ReadWriteLock getReadWriteLock() { 
        return null;
      }
    
      @Override
      public boolean equals(Object o) { 
        if (getId() == null) { 
          throw new CacheException("Cache instances require an ID.");
        }
        if (this == o) { 
          return true;
        }
        if (!(o instanceof Cache)) { 
          return false;
        }
    
        Cache otherCache = (Cache) o;
        return getId().equals(otherCache.getId());
      }
    
      @Override
      public int hashCode() { 
        if (getId() == null) { 
          throw new CacheException("Cache instances require an ID.");
        }
        return getId().hashCode();
      }
    }

## BlockingCache ##

BlockingCache是带有阻塞功能的缓存装饰器。

public class BlockingCache implements Cache { 
    
        //超时时间
        private long timeout;
        // 被装饰的缓存
        private final Cache delegate;
        // 每一个Key拥有一把可重入锁
        private final ConcurrentHashMap<Object, ReentrantLock> locks;
        //构造器，装饰其他cache对象
        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();
        }
    
        // 插入Key的缓存，插入完毕同时释放锁
        @Override
        public void putObject(Object key, Object value) { 
          try { 
            delegate.putObject(key, value);
          } finally { 
            releaseLock(key);
          }
        }
    
      // 查询key的缓存
      @Override
      public Object getObject(Object key) { 
        //获取到key的锁
        acquireLock(key);
        // 查询key缓存的值
        Object value = delegate.getObject(key);
        //如果value不为null,则释放锁，返回值，否则一直持有锁，知道key有值插入到缓存
        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;
      }
      
      //查询Key持有的锁，如果没有则创建新锁
      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;
      } 
    
    
    }

线程A第一次查询key，结果不存在缓存，A持有key的锁; 同时线程B查询key，此时线程B被阻塞，A去数据查询获取到数据缓存起来释放key的锁，B被唤醒继续从缓存查询。

BlockingCache会装饰PerpetualCache缓存。

## 装饰者模式 ##

从上面缓存的包结构上我们可以看到，Cache是顶层接口，PerpetualCache是基本实现，decorators下的各个类是不同功能的缓存器，通过装饰者模式来实现不同功能的组合。

//顶测接口
    public interface  Cache{ 
    
      Object getObject(Object key);
    
    }
    
    //基本实现类
    public class PerpetualCache implements  Cache { 
        
        @Override
        public Object getObject(Object key) { 
          return cache.get(key);
        }
    }
    
    //装饰者
    public class BlockingCache implements Cache { 
      
      @Override
      public Object getObject(Object key) { 
          acquireLock(key);
          Object value = delegate.getObject(key);
          if (value != null) { 
            releaseLock(key);
          }        
          return value;
      }
    
    }
    
    //示例
    public class Example { 
    
        public static void main() { 
            Cache cache = new PerpetualCache();
            //装饰缓存，增加锁的功能
            cache = new BlockingCache(cache);
            CacheKey cacheKey = new CacheKey();
            cache.getOject(cacheKey); 
        }
    }

## CacheKey ##

由于mybati支持动态sql，因此不能通过一个简单的string来构造缓存的key,CacheKey就是Cache中的key。

//重写了Equals与hashCode方法
    public class CacheKey implements Cloneable, Serializable { 
    private static final long serialVersionUID = 1146682552656046210L;
    
      public static final CacheKey NULL_CACHE_KEY = new NullCacheKey();
      //multiplier的默认值
      private static final int DEFAULT_MULTIPLYER = 37;
      //hashcode的默认值
      private static final int DEFAULT_HASHCODE = 17;
      //用来计算hashcode值
      private final int multiplier;
      private int hashcode;
      private long checksum;
      //对象的个数
      private int count;
      //存储构成CacheKey的对象
      private transient List<Object> updateList;
    
      public CacheKey() { 
        this.hashcode = DEFAULT_HASHCODE;
        this.multiplier = DEFAULT_MULTIPLYER;
        this.count = 0;
        this.updateList = new ArrayList<Object>();
      }
    
      public CacheKey(Object[] objects) { 
        this();
        updateAll(objects);
      }
    
      public int getUpdateCount() { 
        return updateList.size();
      }
    
      //存储构成CacheKey的对象,以及重新计算hashcode
      public void update(Object object) { 
        int baseHashCode = object == null ? 1 : ArrayUtil.hashCode(object); 
    
        count++;
        checksum += baseHashCode;
        baseHashCode *= count;
        //重新计算hashcode
        hashcode = multiplier * hashcode + baseHashCode;
        //存储对象
        updateList.add(object);
      }
    
      public void updateAll(Object[] objects) { 
        for (Object o : objects) { 
          update(o);
        }
      }
    
      @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;
        }
    
        for (int i = 0; i < updateList.size(); i++) { 
          Object thisObject = updateList.get(i);
          Object thatObject = cacheKey.updateList.get(i);
          if (!ArrayUtil.equals(thisObject, thatObject)) { 
            return false;
          }
        }
        return true;
      }
    
      @Override
      public int hashCode() { 
        return hashcode;
      }
    
      @Override
      public String toString() { 
        StringBuilder returnValue = new StringBuilder().append(hashcode).append(':').append(checksum);
        for (Object object : updateList) { 
          returnValue.append(':').append(ArrayUtil.toString(object));
        }
        return returnValue.toString();
      }
    
      @Override
      public CacheKey clone() throws CloneNotSupportedException { 
        CacheKey clonedCacheKey = (CacheKey) super.clone();
        clonedCacheKey.updateList = new ArrayList<Object>(updateList);
        return clonedCacheKey;
      }
    
    }

CacheKey是由以下5个部分构成的

1.MappedStatement的id  
2.指定查询结果的范围也就是rowBounds.Offset,rowBounds.limit  
3.查询所使用的包含？的动态sql  
4.用户传递的实际参数  
5.配置的环境id，如果mybatis-config.xml有配置环境信息的话。

public abstract class BaseExecutor implements Executor { 
    //创建CacheKey
    @Override
      public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) { 
        if (closed) { 
          throw new ExecutorException("Executor was closed.");
        }
        CacheKey cacheKey = new CacheKey();
        //MappedStatement的id
        cacheKey.update(ms.getId());
        // 指定查询结果的范围也就是rowBounds.Offset,rowBounds.limit
        cacheKey.update(rowBounds.getOffset());
        cacheKey.update(rowBounds.getLimit());
        // 查询所使用的包含？的动态sql
        cacheKey.update(boundSql.getSql());
        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
        TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
        // mimic DefaultParameterHandler logic
        for (ParameterMapping parameterMapping : parameterMappings) { 
          if (parameterMapping.getMode() != ParameterMode.OUT) { 
            Object value;
            String propertyName = parameterMapping.getProperty();
            if (boundSql.hasAdditionalParameter(propertyName)) { 
              value = boundSql.getAdditionalParameter(propertyName);
            } else if (parameterObject == null) { 
              value = null;
            } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) { 
              value = parameterObject;
            } else { 
              MetaObject metaObject = configuration.newMetaObject(parameterObject);
              value = metaObject.getValue(propertyName);
            }
            // 用户传递的实际参数
            cacheKey.update(value);
          }
        }
        if (configuration.getEnvironment() != null) { 
          // issue #176
          // 配置的环境id，如果mybatis-config.xml有配置环境信息的话。
          cacheKey.update(configuration.getEnvironment().getId());
        }
        return cacheKey;
      }
    }

## 总结 ##

1.mybatis中存在一级(默认开启)、二级缓存。缓存可以加快查询速度，减少连接db的次数。  
2.cache模块属于mybatis的基础支持层。位于org.apache.ibatis.cache包下。  
3.PerpetualCache缓存的基础实现(实现了基本的缓存功能，底层数据结构使用HashMap)  
4.BlockingCache是带有阻塞功能的缓存装饰器。  
5.缓存模块的设计采用了装饰者模式  
6.Cache是顶层接口，PerpetualCache是基本实现，decorators下的各个类是不同功能的缓存器，通过装饰者模式来实现不同功能的组合。  
7.CacheKey是由5个部分构成的：mappedStatement的id、指定查询结果的范围、查询所使用的包含？的动态sql、用户传递的实际参数、配置的环境id