SqlSession对象之StatementHandler

我不是女神ヾ 2021-09-28 00:14 409阅读 0赞

### 文章目录 ###

*   *  一、StatementHandler介绍
     *   *  1、RoutingStatementHandler类
         *  2、BaseStatementHandler抽象类
     *  二、StatementHandler接口方法介绍
     *   *  1、prepare方法
         *  2、parameterize方法
         *  3、query方法
         *  4、update方法
     *  三、StatementHandler对象创建

上一篇讲了SqlSession对象中的Executor，接下来将对SqlSession的另一个对象StatementHandler进行讲解。

## 一、StatementHandler介绍 ##

StatementHandler是Mybatis中最重要的一个对象，它负责操作Statement与数据库进行交流，在此过程中还会调用ParameterHandler进行参数配置，使用ResultHandler将查询结果与实体类对象进行绑定。因而ParameterHandler与ResultHandler的创建是与StatementHandler相关联的。

StatementHandler是一个顶级接口，它的类图如下所示：

![1147448-20181207214014097-1290733592.png][]

StatementHandler接口下有两个直接实现类BaseStatementHandler和RoutingStatementHandler

### 1、RoutingStatementHandler类 ###

RoutingStatementHandler：是一个具体实现类，在这个类中并没有对Statement对象进行具体使用，只是根据得到Executor类型决定创建何种类型StatementHandler对象。在MyBatis工作时，使用的StatementHandler接口对象实际上就是RoutingStatementHandler对象。

可以简单理解为：

StatementHandler statmentHandler = new RountingStatementHandler();

### 2、BaseStatementHandler抽象类 ###

BaseStatementHandler：是StatementHandler接口的另一个实现类，本身是一个抽象类，用于简化StatementHandler接口实现的难度，属于适配器设计模式体现。它有三个实现类：SimpleStatementHandler，PreparedStatementHandler和CallableStatementHandler。

在RountingStatementHandler创建时,就根据接收的传递的SQL语句来创建这个三个类型的对象。

**SimpleStatementHandler**：管理Statement对象向数据库中推送不需要预编译的SQL语句。

**PreparedStatementHandler**：管理PreparedStatementHandler对象向数据库推送预编译的SQL语句。

**CallableStatementHandler**：管理CallableStatement对象调用数据库中构造函数，即有存储过程的SQL语句。

## 二、StatementHandler接口方法介绍 ##

StatementHandler是一个接口，其代码如下：

public interface StatementHandler { 
    
      Statement prepare(Connection connection, Integer transactionTimeout)throws SQLException;
    
      void parameterize(Statement statement)throws SQLException;
    
      void batch(Statement statement)throws SQLException;
    
      int update(Statement statement)throws SQLException;
    
      <E> List<E> query(Statement statement, ResultHandler resultHandler)throws SQLException;
    
      <E> Cursor<E> queryCursor(Statement statement)throws SQLException;
    
      BoundSql getBoundSql();
    
      ParameterHandler getParameterHandler();
    
    }

我们主要关注StatementHandler中的四个重要方法：prepare()、parameterize()、update()、query()

### 1、prepare方法 ###

prepare方法主要是在BaseStatementHandler类中实现的，其代码如下：

public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { 
        ErrorContext.instance().sql(boundSql.getSql());
        Statement statement = null;
        try { 
          statement = instantiateStatement(connection);
          setStatementTimeout(statement, transactionTimeout);
          setFetchSize(statement);
          return statement;
        } catch (SQLException e) { 
          closeStatement(statement);
          throw e;
        } catch (Exception e) { 
          closeStatement(statement);
          throw new ExecutorException("Error preparing statement. Cause: " + e, e);
        }
      }

prepare方法用于创建一个(Statement or PreparedStatement or CallableStatement)对象，并设置Statement对象的最大工作时间和一次性读取的最大数据量，然后将生成的Statement对象返回。

prepare方法只在BaseStatementHandler被实现，在其三个子类中没有被重写。它主要用于三个子类调用获得对应的Statement接口对象，依靠instantiateStatement(connection)方法来返回具体Statement接口对象。instantiateStatement方法是BaseStatementHandler中定义的抽象方法,由三个子类来具体实现，这里采用了模板方法模式。

SimpleStatementHandler的instantiateStatement方法

protected Statement instantiateStatement(Connection connection) throws SQLException { 
        if (mappedStatement.getResultSetType() != null) { 
          return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
        } else { 
          return connection.createStatement();
        }
      }

PreparedStatementHandler的instantiateStatement方法

protected Statement instantiateStatement(Connection connection) throws SQLException { 
        String sql = boundSql.getSql();
        if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) { 
          String[] keyColumnNames = mappedStatement.getKeyColumns();
          if (keyColumnNames == null) { 
            return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
          } else { 
            return connection.prepareStatement(sql, keyColumnNames);
          }
        } else if (mappedStatement.getResultSetType() != null) { 
          return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
        } else { 
          return connection.prepareStatement(sql);
        }
      }

CallableStatementHandler的instantiateStatement方法

protected Statement instantiateStatement(Connection connection) throws SQLException { 
        String sql = boundSql.getSql();
        if (mappedStatement.getResultSetType() != null) { 
          return connection.prepareCall(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
        } else { 
          return connection.prepareCall(sql);
        }
      }

### 2、parameterize方法 ###

parameterize方法用于传递参数，因此只在PreparedStatementHandler和CallableStatementHandler中被重写。

PreparedStatementHandler的parameterize方法

public void parameterize(Statement statement) throws SQLException { 
        parameterHandler.setParameters((PreparedStatement) statement);
    }

CallableStatementHandler的parameterize方法

public void parameterize(Statement statement) throws SQLException { 
        registerOutputParameters((CallableStatement) statement);
        parameterHandler.setParameters((CallableStatement) statement);
    }

他们都调用了parameterHandler进行参数赋值。

### 3、query方法 ###

query方法主要用于输送查询查询语句，并将查询结果转换对应的实体类对象。

SimpleStatementHandler的query方法：

public <E> Cursor<E> queryCursor(Statement statement) throws SQLException { 
        String sql = boundSql.getSql();
        statement.execute(sql);
        return resultSetHandler.<E>handleCursorResultSets(statement);
    }

PreparedStatementHandler的query方法：

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { 
        PreparedStatement ps = (PreparedStatement) statement;
        ps.execute();
        return resultSetHandler.<E> handleResultSets(ps);
      }

CallableStatementHandler的query方法：

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { 
        CallableStatement cs = (CallableStatement) statement;
        cs.execute();
        List<E> resultList = resultSetHandler.<E>handleResultSets(cs);
        resultSetHandler.handleOutputParameters(cs);
        return resultList;
      }

由代码可知：在得到查询结果后，都是使用ResultSetHandler对结果进行转换的。

### 4、update方法 ###

update方法用于输送insert、update、delete语句并返回处理数据行。

SimpleStatementHandler的update方法：

public int update(Statement statement) throws SQLException { 
        String sql = boundSql.getSql();
        Object parameterObject = boundSql.getParameterObject();
        KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
        int rows;
        if (keyGenerator instanceof Jdbc3KeyGenerator) { 
          statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
          rows = statement.getUpdateCount();
          keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
        } else if (keyGenerator instanceof SelectKeyGenerator) { 
          statement.execute(sql);
          rows = statement.getUpdateCount();
          keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
        } else { 
          statement.execute(sql);
          rows = statement.getUpdateCount();
        }
        return rows;
      }

PreparedStatementHandler的update方法：

public int update(Statement statement) throws SQLException { 
        PreparedStatement ps = (PreparedStatement) statement;
        ps.execute();
        int rows = ps.getUpdateCount();
        Object parameterObject = boundSql.getParameterObject();
        KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
        keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
        return rows;
      }

CallableStatementHandler的update方法：

public int update(Statement statement) throws SQLException { 
        CallableStatement cs = (CallableStatement) statement;
        cs.execute();
        int rows = cs.getUpdateCount();
        Object parameterObject = boundSql.getParameterObject();
        KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
        keyGenerator.processAfter(executor, mappedStatement, cs, parameterObject);
        resultSetHandler.handleOutputParameters(cs);
        return rows;
      }

## 三、StatementHandler对象创建 ##

StatementHandler对象是在SqlSession对象接收到操作命令后由Configuraion中newStatementHandler方法负责调用的。代码如下：

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 
        StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
    }

由代码知：statementHandler的创建调用了RoutingStatementHandler的构造方法。

RoutingStatementHandler构造方法,将会根据StatementType决定创建SimpleStatementHandler，PreparedStatementHandler，CallableStatementHandler实例对象。

public enum StatementType { 
      STATEMENT, PREPARED, CALLABLE
    }

构造方法如下所示：

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 
    
        switch (ms.getStatementType()) { 
          case STATEMENT:
            delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          case PREPARED:
            delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          case CALLABLE:
            delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
          default:
            throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
        }
      }

而创建三种StatementHandler的构造方法都采用的是BaseStatementHandler的构造方法，其代码如下：

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 
        this.configuration = mappedStatement.getConfiguration();
        this.executor = executor;
        this.mappedStatement = mappedStatement;
        this.rowBounds = rowBounds;
    
        this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
        this.objectFactory = configuration.getObjectFactory();
    
        if (boundSql == null) {  // issue #435, get the key before calculating the statement
          generateKeys(parameterObject);
          boundSql = mappedStatement.getBoundSql(parameterObject);
        }
    
        this.boundSql = boundSql;
    
        this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
        this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
      }

这里需要注意的是最后两行代码，创建了parameterHandler和resultSetHandler，这正好和前面所说的StatementHandler、parameterHandler和resultSetHandler是在一起创建的说法保持一致。

创建好handler，执行器的方法会调用prepareStatement方法产生Statment.，代码如下：

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { 
        Statement stmt;
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        if (hasStatementFor(sql)) { 
          stmt = getStatement(sql);
          applyTransactionTimeout(stmt);
        } else { 
          Connection connection = getConnection(statementLog);
          stmt = handler.prepare(connection, transaction.getTimeout());
          putStatement(sql, stmt);
        }
        handler.parameterize(stmt);
        return stmt;
      }

此过程中会调用BaseStatementHandler的prepare方法，而在方法中又会调用instantiateStatement方法，通过Connection产生相应的Statment对象。

[1147448-20181207214014097-1290733592.png]: /images/20210923/b500f545c87644be813d780806003af2.png