Mybatis一级缓存源码详解

ゞ浴缸里的玫瑰 2022-05-26 10:11 270阅读 0赞

解析基于Mybatis 3.4.4 Release，近几个版本应该都适用。

Mybatis一级缓存默认是开启的，缓存数据时存储在Executor中，而Executor则被SqlSession持有。

我们先来看生成SqlSession和Executor实例的过程：

public class DefaultSqlSessionFactory implements SqlSessionFactory { 
    
        //通过连接池生成SqlSession对象
        private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          //生成执行器Executor
          final Executor executor = configuration.newExecutor(tx, execType);         
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          closeTransaction(tx); // may have fetched a connection so lets call close()
          throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }
    
    }
    
    
    public class Configuration { 
    
        // 若未显式指定，defaultExecutorType 默认值为 ExecutorType.SIMPLE
        protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
    
        public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
            executorType = executorType == null ? defaultExecutorType : executorType;    
            executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
            Executor executor;
            if (ExecutorType.BATCH == executorType) {
                executor = new BatchExecutor(this, transaction);
            } else if (ExecutorType.REUSE == executorType) {
                executor = new ReuseExecutor(this, transaction);
            } else {
                executor = new SimpleExecutor(this, transaction);
            }
            // 二级缓存也是默认开启的，这里下篇博客再讲
            if (cacheEnabled) {
                executor = new CachingExecutor(executor);
            }
            executor = (Executor) interceptorChain.pluginAll(executor);
            return executor;
        }
    
    }

ReuseExecutor，SimpleExecutor 都直接继承自BaseExecutor，他们在实例化时都会调用BaseExecutor的唯一构造方法：

public abstract class BaseExecutor implements Executor { 
    
      ......
      protected Transaction transaction;
      protected Executor wrapper;
    
      protected PerpetualCache localCache;
      protected Configuration configuration;
    
    
      protected BaseExecutor(Configuration configuration, Transaction transaction) {
        this.transaction = transaction;
        this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
        // 初始化localCache 属性,生成一个PerpetualCache对象
        this.localCache = new PerpetualCache("LocalCache");
        this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
        this.closed = false;
        this.configuration = configuration;
        this.wrapper = this;
      }
    
    }
    
    public class PerpetualCache implements Cache { 
    
      private String id;
    
      private Map<Object, Object> cache = new HashMap<Object, Object>();
    
      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();
      }
    
      @Override
      public ReadWriteLock getReadWriteLock() {
        return null;
      }
    
      ......
    
    }

PerpetualCache 持有一个HashMap类型的属性，可作为一个缓存实现。

接下来我们看DefaultSqlSession在增删改查时的缓存操作。

public class DefaultSqlSession implements SqlSession { 
    
        public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
            try {
                MappedStatement ms = configuration.getMappedStatement(statement);
                // 查询操作借助executor对象来执行query实现
                return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
            } catch (Exception e) {
                throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
            } finally {
                ErrorContext.instance().reset();
            }
        }
    }
    
    
    public abstract class BaseExecutor implements Executor { 
    
        public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
            BoundSql boundSql = ms.getBoundSql(parameter);
            // 创建一个CacheKey对象,用于缓存的Key
            CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
            return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    
        // 创建CacheKey,将查询时的参数，SQL字符串，分页数据等都填充进去
        public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
            if (closed) {
                throw new ExecutorException("Executor was closed.");
            }
            CacheKey cacheKey = new CacheKey();
            cacheKey.update(ms.getId());
            cacheKey.update(rowBounds.getOffset());
            cacheKey.update(rowBounds.getLimit());
            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);
                }
                cacheKey.update(value);
              }
            }
            if (configuration.getEnvironment() != null) {
              // issue #176
              cacheKey.update(configuration.getEnvironment().getId());
            }
            return cacheKey;
        }
    
    }

CacheKey 创建时会关联Mapper的id，也就是方法名称，Offset，Limit，SQL语句和SQL的参数。

CacheKey 重写了equals和hashCode方法，当两个CacheKey 这些数据完全相同时，就能 以 后一个CacheKey 为Key获取到前一个CacheKey 存入的Value。

public abstract class BaseExecutor implements Executor { 
    
         public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
            ......
            List<E> list;
            try {
              queryStack++;
              // 查询前先勇CacheKey 从localCache获取Value,如果有值就不用再查数据库了
              list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
              if (list != null) {
                handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
              } else {
                list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
              }
            } finally {
              queryStack--;
            }
            ......
        return list;
      }
    
    }

当DefaultSqlSession Insert，Update，Delete时，会调用Eexcutor的Update方法：

public abstract class BaseExecutor implements Executor { 
    
        public int update(MappedStatement ms, Object parameter) throws SQLException {
            ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
            if (closed) {
                throw new ExecutorException("Executor was closed.");
            }
            clearLocalCache();
            return doUpdate(ms, parameter);
        }
    
        public void clearLocalCache() {
            if (!closed) {
              // 清空Cache
              localCache.clear();
              localOutputParameterCache.clear();
            }
        }
    
    }

当DefaultSqlSession Commit或者Rollback时，会调用Executor相应的Commit和Rollback方法：

public abstract class BaseExecutor implements Executor { 
    
        @Override
        public void commit(boolean required) throws SQLException {
            if (closed) {
                throw new ExecutorException("Cannot commit, transaction is already closed");
            }
            clearLocalCache();   // 清空Cache
            flushStatements();
            if (required) {
                transaction.commit();
            }
        }
    
      @Override
      public void rollback(boolean required) throws SQLException {
        if (!closed) {
          try {
            clearLocalCache();   // 清空Cache
            flushStatements(true);
          } finally {
            if (required) {
              transaction.rollback();
            }
          }
        }
      }
    
    }

他们都会清空缓存。

总结：

1.  **Mybatis一级缓存默认开启，缓存数据存储在Executor中，实际就是BaseExecutor对象的localCache属性，属性类型是PerpetualCache。**
2.  **缓存的Key类型是CacheKey，Mybatis重写了其hashcode和equals方法，需要Mapper方法名称一致，SQL字符串一致，Limit，Offset一致，SQL参数一致才能在Map中有相同的映射关系。也就是两个CacheKey的这些数据相同，后一个CacheKey才能获取到前一个存入的Value。**
3.  **Insert，Update，Delete操作会清空缓存**
4.  **Mybatis的SqlSession在Commit和Rollback时会清空缓存。如果当前数据库操作不在事务中，那么每个操作完成后都是默认提交或者回滚的，都会清空缓存，那么缓存就没有效果了。**
5.  **一级缓存的使用前提很苛刻，首先需要在同一个事务中，且多次查询操作完全相同，且多次操作之间不能夹杂 增，删，改 操作。**