SqlSession

1. SqlSession是一個會話，相當於jdbc的Connection物件，生命週期應該是請求資料庫處理事務的過程中。它是非執行緒安全的，在每次建立SqlSession都必須及時關閉它，它長期存在就會使資料庫連線池的活動資源減少。

SqlSession下的四大物件

1. Mapper執行的過程是通過Executor、StatementHandler、ParameterHandler和ResultHandler來完成資料庫操作和結果返回的
   
   • Executor表示執行器，排程相關的handler來執行對應的sql(StatementHandler、ParameterHandler、ResultHandler)
   • StatementHandler的作用是使用資料庫的Statement(PreparedStatement)執行操作
   • ParameterHandler用於SQL對引數的處理
   • ResultHandler是進行最後資料集(ResultSet)的封裝返回處理的

Executor

1. Executor是一個真正執行java和資料庫互動的東西
   
   • SIMPLE
   • REUSE
   • BATCH

2. 建立

   在DefaultSqlSessionFactory中建立DefaultSqlSession之前建立Executor(通過Configuration建立)，以下是Configuration中建立Executor的程式碼。根據配置型別去建立三種執行器中的一種，在建立之後將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 <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
       Statement stmt = null;
       try {
         Configuration configuration = ms.getConfiguration();
         //通過Configuration建立StatementHandler
         StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, rowBounds, resultHandler, boundSql);
         //預編譯sql，並對引數進行初始化操作
         stmt = prepareStatement(handler, ms.getStatementLog());
         //resultHandler 組裝返回結果
         return handler.<E>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);
       //設定引數並執行(預編譯sql)
       handler.parameterize(stmt);
       return stmt;
     }
   

StatementHandler

1. 建立

       /***
       在Executor建立StatementHandler，StatementHandler是在Configuration中建立，程式碼如下
       **/
         public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
           /**
           RoutingStatementHandler不是真實的服務物件，而是通過介面卡找到對應的StatementHandler(物件的介面卡模式）
               SimpleStatementHandler
               PreparedStatementHandler
               CallableStatementHandler
           */
           StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
           statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
           return statementHandler;
         }
   

   

2. SimpleStatementHandler

   
         public <E> List<E> query(Statement statement, ResultHandler resultHandler)
             throws SQLException {
           String sql = boundSql.getSql();
           statement.execute(sql);
           //使用ResultSetHandler封裝結果返回給呼叫者
           return resultSetHandler.<E>handleResultSets(statement);
         }   

3. PreparedStatementHandler

     public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
       //sql編譯已經在之前都做好了，所以這裡直接執行就可以了
       PreparedStatement ps = (PreparedStatement) statement;
       ps.execute();
       return resultSetHandler.<E> 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. 程式碼

   public interface ParameterHandler {
    //返回引數物件   
     Object getParameterObject();
    //設定預編譯sql語句的引數
     void setParameters(PreparedStatement ps)
         throws SQLException;
   
   }

   mybatis提供一個預設的DefaultParameterHandler
   public void setParameters(PreparedStatement ps)
         throws SQLException {
       ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
       List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
       if (parameterMappings != null) {
         //從parameterObject物件中獲取引數，然後使用typeHandler進行引數處理
         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. 結果集處理器

    public interface ResultSetHandler {
    //包裝結果集
     <E> List<E> handleResultSets(Statement stmt) throws SQLException;
      //處理儲存過程輸出引數的
     void handleOutputParameters(CallableStatement cs) throws SQLException;
   
   }