mybatis源码分析三之SqlSession

SqlSession

1. SqlSession是一个会话，相当于jdbc的Connection对象，生命周期应该是请求数据库处理事务的过程中。它是非线程安全的，在每次创建SqlSession都必须及时关闭它，它长期存在就会使数据库连接池的活动资源减少。

SqlSession下的四大对象

1. Mapper执行的过程是通过Executor、StatementHandler、ParameterHandler和ResultHandler来完成数据库操作和结果返回的
   
   • Executor表示执行器，调度相关的handler来执行对应的sql(StatementHandler、ParameterHandler、ResultHandler)
   • StatementHandler的作用是使用数据库的Statement(PreparedStatement)执行操作
   • ParameterHandler用于SQL对参数的处理
   • ResultHandler是进行最后数据集(ResultSet)的封装返回处理的

Executor

1. Executor是一个真正执行java和数据库交互的东西
   
   • SIMPLE
   • REUSE
   • BATCH

2. 创建

   在DefaultSqlSessionFactory中创建DefaultSqlSession之前创建Executor(通过Configuration创建)，以下是Configuration中创建Executor的代码。根据配置类型去创建三种执行器中的一种，在创建之后将Executor放到interceptorChain之中
    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);
       }
       if (cacheEnabled) {
         executor = new CachingExecutor(executor);
       }
       executor = (Executor) interceptorChain.pluginAll(executor);
       return executor;
     }
   

3. SimpleExecutor

    public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
       Statement stmt = null;
       try {
         Configuration configuration = ms.getConfiguration();
         //通过Configuration创建StatementHandler
         StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, rowBounds, resultHandler, boundSql);
         //预编译sql，并对参数进行初始化操作
         stmt = prepareStatement(handler, ms.getStatementLog());
         //resultHandler 组装返回结果
         return handler.query(stmt, resultHandler);
       } finally {
         closeStatement(stmt);
       }
     }
   
     private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
       Statement stmt;
       Connection connection = getConnection(statementLog);
       stmt = handler.prepare(connection);
       //设置参数并执行(预编译sql)
       handler.parameterize(stmt);
       return stmt;
     }
   

StatementHandler

1. 创建

       /***
       在Executor创建StatementHandler，StatementHandler是在Configuration中创建，代码如下
       **/
         public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
           /**
           RoutingStatementHandler不是真实的服务对象，而是通过适配器找到对应的StatementHandler(对象的适配器模式）
               SimpleStatementHandler
               PreparedStatementHandler
               CallableStatementHandler
           */
           StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
           statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
           return statementHandler;
         }
   

   

2. SimpleStatementHandler

   
         public  List query(Statement statement, ResultHandler resultHandler)
             throws SQLException {
           String sql = boundSql.getSql();
           statement.execute(sql);
           //使用ResultSetHandler封装结果返回给调用者
           return resultSetHandler.handleResultSets(statement);
         }   

3. PreparedStatementHandler

     public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
       //sql编译已经在之前都做好了，所以这里直接执行就可以了
       PreparedStatement ps = (PreparedStatement) statement;
       ps.execute();
       return resultSetHandler. handleResultSets(ps);
     }
   
       public void parameterize(Statement statement) throws SQLException {
       KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
       ErrorContext.instance().store();
       keyGenerator.processBefore(executor, mappedStatement, statement, boundSql.getParameterObject());
       ErrorContext.instance().recall();
       rebindGeneratedKey();
       parameterHandler.setParameters((PreparedStatement) statement);
     }

paramsHandler

1. 代码

   public interface ParameterHandler {
          
    //返回参数对象   
     Object getParameterObject();
    //设置预编译sql语句的参数
     void setParameters(PreparedStatement ps)
         throws SQLException;
   
   }

   mybatis提供一个默认的DefaultParameterHandler
   public void setParameters(PreparedStatement ps)
         throws SQLException {
       ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
       List parameterMappings = boundSql.getParameterMappings();
       if (parameterMappings != null) {
         //从parameterObject对象中获取参数，然后使用typeHandler进行参数处理
         MetaObject metaObject = parameterObject == null ? null : configuration.newMetaObject(parameterObject);
         for (int i = 0; i < parameterMappings.size(); i++) {
           ParameterMapping parameterMapping = parameterMappings.get(i);
           if (parameterMapping.getMode() != ParameterMode.OUT) {
             Object value;
             String propertyName = parameterMapping.getProperty();
             PropertyTokenizer prop = new PropertyTokenizer(propertyName);
             if (parameterObject == null) {
               value = null;
             } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
               value = parameterObject;
             } else if (boundSql.hasAdditionalParameter(propertyName)) {
               value = boundSql.getAdditionalParameter(propertyName);
             } else if (propertyName.startsWith(ForEachSqlNode.ITEM_PREFIX)
                 && boundSql.hasAdditionalParameter(prop.getName())) {
               value = boundSql.getAdditionalParameter(prop.getName());
               if (value != null) {
                 value = configuration.newMetaObject(value).getValue(propertyName.substring(prop.getName().length()));
               }
             } else {
               value = metaObject == null ? null : metaObject.getValue(propertyName);
             }
             TypeHandler typeHandler = parameterMapping.getTypeHandler();
             if (typeHandler == null) {
               throw new ExecutorException("There was no TypeHandler found for parameter " + propertyName + " of statement " + mappedStatement.getId());
             }
             JdbcType jdbcType = parameterMapping.getJdbcType();
             if (value == null && jdbcType == null) jdbcType = configuration.getJdbcTypeForNull();
             typeHandler.setParameter(ps, i + 1, value, jdbcType);
           }
         }
       }
     }
   

resultSetHandler

1. 结果集处理器

    public interface ResultSetHandler {
          
    //包装结果集
      List handleResultSets(Statement stmt) throws SQLException;
      //处理存储过程输出参数的
     void handleOutputParameters(CallableStatement cs) throws SQLException;
   
   }