Mybatis源码分析（六）：SQL执行器Executor与一二级缓存的实现

妖狐艹你老母 2022-03-16 13:48 327阅读 0赞

### SQL执行器Executor ###

*  由上篇文章分析可知，mybatis在内部主要通过executor包的Executor来调用JDBC的相关API来完成SQL语句的执行。Executor通常绑定到SqlSession中，即Executor作为SqlSession对象的一个内部属性，故Executor的对象创建是与SqlSession的对象创建一起的。
 *  SqlSession对象是由SqlSessionFactory来创建的，所以SqlSessionFactory在创建SqlSession对象时，也会创建对应的Executor对象。DefaultSqlSessionFactory的具体如下：
    
        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);
              
              // 创建SQL执行器executor
              final Executor executor = configuration.newExecutor(tx, execType);
              
              // 将配置configuration，SQL执行器executor作为参数SqlSession对象
              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();
            }
        }
 *  Configuration创建Executor对象的newExecutor方法实现如下：如果在mybatisConfig.xml配置了cacheEnabled为true，则使用Executor接口的CachingExecutor实现类，其中CachingExecutor为使用二级缓存的SQL执行器。而一级缓存是默认开启的。
    
        // SQL执行器，主要负责解析参数，调用底层的JDBC接口来执行SQL
        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);
            }
        
            // 如果是开启了二级缓存，则使用CachingExecutor来包装该executor
            if (cacheEnabled) { 
              executor = new CachingExecutor(executor);
            }
            executor = (Executor) interceptorChain.pluginAll(executor);
            return executor;
        }
 *  缓存主要是在查询select操作中使用。

### 一级缓存实现 ###

*  一级缓存是默认开启的，且不能通过配置来显式关闭一级缓存（不过可以设置一级缓存级别为STATEMENT来关闭），所以是在顶层抽象基类BaseExecutor中定义，核心实现如下：其中一级缓存为使用HashMap的包装类caching包的PerpetualCache来实现。
    
        public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { 
            BoundSql boundSql = ms.getBoundSql(parameter);
            // 缓存key的生成
            CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
            return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
        
        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++;
              list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
              if (list != null) { 
                // 从缓存获取指定SQL的结果，而不用去数据库查询
                handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
              } else { 
                // 去数据库查询
                list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
              }
            } finally { 
              queryStack--;
            }
        
            // 由于SqlSession不是线程安全的，故任何时候只存在一个线程操作，故queryStack总是0的
            if (queryStack == 0) { 
              for (DeferredLoad deferredLoad : deferredLoads) { 
                deferredLoad.load();
              }
              // issue #601
              deferredLoads.clear();
        
              // 如果是STATEMENT，则每次执行完查询都清空缓存，故其实是没有缓存
              if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { 
                // issue #482
                clearLocalCache();
              }
            }
            return list;
        }
 *  缓存key的生成算法如下：
    
        // 缓存key
        @Override
        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;
        }

### 二级缓存实现 ###

*  由以上分析，如果开启了二级缓存，则SqlSessionFactory在创建SqlSession对象时，同步创建的是类型为CachingExecutor的SQL执行器，故后续通过该SqlSession执行的查询操作都是通过该CachingExecutor对象来执行SQL调用，CachingExecutor的查询方法query定义如下：
    
        // 在Executor级别实现二级缓存，这是一个包装类，由所有executor共享，
        // 所以所有executor，即所有SqlSession共享这个Executor提供的缓存
        public class CachingExecutor implements Executor { 
        
          // 被包装的实际执行的executor
          private final Executor delegate;
        
          // 二级缓存管理器，内部维护了各个namespace对应的二级缓存
          private final TransactionalCacheManager tcm = new TransactionalCacheManager();
        
          ...
        
          @Override
          public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
              throws SQLException { 
            // 获取该语句对应的namespace关联的二级缓存
            Cache cache = ms.getCache();
        
            if (cache != null) { 
              flushCacheIfRequired(ms);
              if (ms.isUseCache() && resultHandler == null) { 
                ensureNoOutParams(ms, boundSql);
                @SuppressWarnings("unchecked")
                List<E> list = (List<E>) tcm.getObject(cache, key);
        
                // 二级缓存没有，则交给被包装的executor去查询，即可能是从一级缓存获取，如果一级缓存也没有，则查询数据库
                if (list == null) { 
                  list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                  tcm.putObject(cache, key, list); // issue #578 and #116
                }
                return list;
              }
            }
            
            // 如果缓存没有，则执行数据库查询获取
            return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          }
          
          ...
            
        }