mybatis之StatementHandler

StatementHandler数据库交互

StatementHandler接口是mybatis的核心接口之一，StatementHandler接口中功能很多，进行预编译并且调用ParameterHandler的setParameters()方法设置参数，创建Statement对象，通过ParameterHandler为SQL语句绑定实参，执行select、insert、update、delete等多种类型的SQL语句，批量执行SQL语句，通过ResultSetHandler将结果集映射成结果对象。

StatementHandler流程

• 创建： 由Configuration方法中newStatementHandler()方法生成的，生成的是RoutingStatementHandler
• 选择： RoutingStatementHandler的通过适配器模式找到对应（根据上下文）的StatementHandler执行的，并且有SimpleStatementHandler、PrepareStatementHandler、CallableStatementHandler
• Executor会调用StatementHandler的prepare方法进行预编译sql语句，调用parameterize方法调用ParameterHandler设置参数，完成预编译
• 最后调用ResultSetHandler封装结果集

public interface StatementHandler {
    // 从连接中获取一个Statement
  Statement prepare(Connection connection, Integer transactionTimeout)
      throws SQLException;
    // 绑定statement执行时所需的实参
  void parameterize(Statement statement)
      throws SQLException;
    // 批量执行SQL语句
  void batch(Statement statement)
      throws SQLException;
    // 执行update/insert/delete语句
  int update(Statement statement)
      throws SQLException;
    // 执行select语句
  <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException;

  <E> Cursor<E> queryCursor(Statement statement)
      throws SQLException;

  BoundSql getBoundSql();
    // 获取其中封装的ParameterHandler
  ParameterHandler getParameterHandler();
}

StatementHandler有两个实现类RoutingStatementHandler和BaseStatementHandler

BaseStatementHandler

BaseStatementHandler是实现了一个StatementHandler接口的抽象类，只提供了参数绑定相关的方法，

public abstract class BaseStatementHandler implements StatementHandler {
    protected final Configuration configuration;
    protected final ObjectFactory objectFactory;
    protected final TypeHandlerRegistry typeHandlerRegistry;
      // 将结果集映射为对象
    protected final ResultSetHandler resultSetHandler;
      // 主要是为SQL语句绑定实参，使用传入的实参替换SQL语句的中？占位符
    protected final ParameterHandler parameterHandler;
      // 记录执行SQL语句的Executor对象
    protected final Executor executor;
      // 记录SQL语句对应的MappedStatement
    protected final MappedStatement mappedStatement;
      // 记录用户设置的offset和limit，用于用在结果集中定位映射的起始位置和结束位置
    protected final RowBounds rowBounds;
      // 记录SQK语句对应的BoundSql
    protected BoundSql boundSql;

    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 = this.configuration.getTypeHandlerRegistry();
        this.objectFactory = this.configuration.getObjectFactory();
        if (boundSql == null) {
              // 调用keyGenerator的processBefore方法获取主键
            this.generateKeys(parameterObject);
            boundSql = mappedStatement.getBoundSql(parameterObject);
        }

        this.boundSql = boundSql;
        this.parameterHandler = this.configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
        this.resultSetHandler = this.configuration.newResultSetHandler(executor, mappedStatement, rowBounds, this.parameterHandler, resultHandler, boundSql);
    }

    public BoundSql getBoundSql() {
        return this.boundSql;
    }

    public ParameterHandler getParameterHandler() {
        return this.parameterHandler;
    }
        // 实现StatementHandler的prepare方法
    public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
        ErrorContext.instance().sql(this.boundSql.getSql());
        Statement statement = null;

        try {
              // 初始化Statement对象，由子类实现(模板方法模式)
            statement = this.instantiateStatement(connection);
            this.setStatementTimeout(statement, transactionTimeout);
            this.setFetchSize(statement);
            return statement;
        } catch (SQLException var5) {
            this.closeStatement(statement);
            throw var5;
        } catch (Exception var6) {
            this.closeStatement(statement);
            throw new ExecutorException("Error preparing statement.  Cause: " + var6, var6);
        }
    }

    protected abstract Statement instantiateStatement(Connection var1) throws SQLException;

    protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {
        Integer queryTimeout = null;
        if (this.mappedStatement.getTimeout() != null) {
            queryTimeout = this.mappedStatement.getTimeout();
        } else if (this.configuration.getDefaultStatementTimeout() != null) {
            queryTimeout = this.configuration.getDefaultStatementTimeout();
        }

        if (queryTimeout != null) {
            stmt.setQueryTimeout(queryTimeout);
        }

        StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);
    }

    protected void setFetchSize(Statement stmt) throws SQLException {
        Integer fetchSize = this.mappedStatement.getFetchSize();
        if (fetchSize != null) {
            stmt.setFetchSize(fetchSize);
        } else {
            Integer defaultFetchSize = this.configuration.getDefaultFetchSize();
            if (defaultFetchSize != null) {
                stmt.setFetchSize(defaultFetchSize);
            }

        }
    }

    protected void closeStatement(Statement statement) {
        try {
            if (statement != null) {
                statement.close();
            }
        } catch (SQLException var3) {
        }

    }

    protected void generateKeys(Object parameter) {
        KeyGenerator keyGenerator = this.mappedStatement.getKeyGenerator();
        ErrorContext.instance().store();
        keyGenerator.processBefore(this.executor, this.mappedStatement, (Statement)null, parameter);
        ErrorContext.instance().recall();
    }
}

在BoundSql中记录的SQL语句中可能包含”?”占位符，而每个”?”占位符都对应了BoundSql.parameterMappings集合中的一个元素，ParameterMapping对象中记录了对应参数的名称以及相关属性
在ParameterHandler中的setParameters方法负责为SQL语句绑定实参，该接口只有一个实现类DefaultParameterHandler，遍历BoundSql.parameterMappings集合，根据ParameterMapping对象中记录的参数名称查找相应的实参，通过TypeHandler转换为JdbcType

SimpleStatementHandler

SimpleStatementHandler继承了BaseStatementHandler抽象类，底层直接使用java.sql.Statement对象来操作数据库，SQL中不能存在占位符，parameterize方法为空实现

public class SimpleStatementHandler extends BaseStatementHandler {
    public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
    }
        
      //增删改都会走update方法
    public int update(Statement statement) throws SQLException {
        String sql = this.boundSql.getSql();
        Object parameterObject = this.boundSql.getParameterObject();
        KeyGenerator keyGenerator = this.mappedStatement.getKeyGenerator();
        int rows;
        if (keyGenerator instanceof Jdbc3KeyGenerator) {
            statement.execute(sql, 1);
              // 受影响的行数
            rows = statement.getUpdateCount();
              // 数据库生成的主键添加到parameterObject中
            keyGenerator.processAfter(this.executor, this.mappedStatement, statement, parameterObject);
        } else if (keyGenerator instanceof SelectKeyGenerator) {
            statement.execute(sql);
            rows = statement.getUpdateCount();
            keyGenerator.processAfter(this.executor, this.mappedStatement, statement, parameterObject);
        } else {
            statement.execute(sql);
            rows = statement.getUpdateCount();
        }

        return rows;
    }

    public void batch(Statement statement) throws SQLException {
        String sql = this.boundSql.getSql();
        statement.addBatch(sql);
    }

    public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        String sql = this.boundSql.getSql();
        statement.execute(sql);
        return this.resultSetHandler.handleResultSets(statement);
    }

    public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
        String sql = this.boundSql.getSql();
        statement.execute(sql);
        return this.resultSetHandler.handleCursorResultSets(statement);
    }
        
      // 初始化Statement
    protected Statement instantiateStatement(Connection connection) throws SQLException {
          // 直接使用Connection对象创建Statement对象
        return this.mappedStatement.getResultSetType() == ResultSetType.DEFAULT ? connection.createStatement() : connection.createStatement(this.mappedStatement.getResultSetType().getValue(), 1007);
    }

    public void parameterize(Statement statement) {
    }
}

PreparedStatementHandler

PreparedStatementHandler继承了BaseStatementHandler抽象类，底层使用java.sql.PreparedStatement对象来操作数据库，PreparedStatement是预编译的，可以在sql语句中存在”?”占位符，parameterize方法通过调用ParameterHandler.setParameters()方法完成SQL语句的参数绑定

public class PreparedStatementHandler extends BaseStatementHandler {
    public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
    }

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

    public void batch(Statement statement) throws SQLException {
        PreparedStatement ps = (PreparedStatement)statement;
        ps.addBatch();
    }

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

    public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
        PreparedStatement ps = (PreparedStatement)statement;
        ps.execute();
        return this.resultSetHandler.handleCursorResultSets(ps);
    }
        
      //初始化Statement，进行预编译
    protected Statement instantiateStatement(Connection connection) throws SQLException {
        String sql = this.boundSql.getSql();
          // 根据mappedStatement.getKeyGenerator()字段的值，创建PrepareStatement
        if (this.mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
            String[] keyColumnNames = this.mappedStatement.getKeyColumns();
            return keyColumnNames == null ? connection.prepareStatement(sql, 1) : connection.prepareStatement(sql, keyColumnNames);
        } else {
            return this.mappedStatement.getResultSetType() == ResultSetType.DEFAULT ? connection.prepareStatement(sql) : connection.prepareStatement(sql, this.mappedStatement.getResultSetType().getValue(), 1007);
        }
    }

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

CallableStatementHandler

CallableStatementHandler继承了BaseStatementHandler抽象类，底层使用java.sql.CallableStatement调用指定的存储过程，其parameterize方法通过调用ParameterHandler.setParameters()方法完成SQL语句的参数绑定，并指定输出参数的索引位置和JDBC类型。

public class CallableStatementHandler extends BaseStatementHandler {
    public CallableStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
    }

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

    public void batch(Statement statement) throws SQLException {
        CallableStatement cs = (CallableStatement)statement;
        cs.addBatch();
    }

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

    public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
        CallableStatement cs = (CallableStatement)statement;
        cs.execute();
        Cursor<E> resultList = this.resultSetHandler.handleCursorResultSets(cs);
        this.resultSetHandler.handleOutputParameters(cs);
        return resultList;
    }

    protected Statement instantiateStatement(Connection connection) throws SQLException {
        String sql = this.boundSql.getSql();
        return this.mappedStatement.getResultSetType() == ResultSetType.DEFAULT ? connection.prepareCall(sql) : connection.prepareCall(sql, this.mappedStatement.getResultSetType().getValue(), 1007);
    }

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

    private void registerOutputParameters(CallableStatement cs) throws SQLException {
        List<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings();
        int i = 0;

        for(int n = parameterMappings.size(); i < n; ++i) {
            ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
            if (parameterMapping.getMode() == ParameterMode.OUT || parameterMapping.getMode() == ParameterMode.INOUT) {
                if (null == parameterMapping.getJdbcType()) {
                    throw new ExecutorException("The JDBC Type must be specified for output parameter.  Parameter: " + parameterMapping.getProperty());
                }

                if (parameterMapping.getNumericScale() != null && (parameterMapping.getJdbcType() == JdbcType.NUMERIC || parameterMapping.getJdbcType() == JdbcType.DECIMAL)) {
                    cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE, parameterMapping.getNumericScale());
                } else if (parameterMapping.getJdbcTypeName() == null) {
                    cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE);
                } else {
                    cs.registerOutParameter(i + 1, parameterMapping.getJdbcType().TYPE_CODE, parameterMapping.getJdbcTypeName());
                }
            }
        }

    }
}

RoutingStatementHandler

RoutingStatementHandler是StatementHandler接口的另一个实现类，会根据MappedStatement中指定的statementType来创建对应的StatementHandler接口实现。

public class RoutingStatementHandler implements StatementHandler {
    private final StatementHandler delegate;

    public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
          // 根据statementType来设置相应的接口实现
        switch(ms.getStatementType()) {
        case STATEMENT:
            this.delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
        case PREPARED:
            this.delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
        case CALLABLE:
            this.delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
            break;
        default:
            throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
        }

    }

    public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
        return this.delegate.prepare(connection, transactionTimeout);
    }

    public void parameterize(Statement statement) throws SQLException {
        this.delegate.parameterize(statement);
    }

    public void batch(Statement statement) throws SQLException {
        this.delegate.batch(statement);
    }

    public int update(Statement statement) throws SQLException {
        return this.delegate.update(statement);
    }

    public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        return this.delegate.query(statement, resultHandler);
    }

    public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
        return this.delegate.queryCursor(statement);
    }

    public BoundSql getBoundSql() {
        return this.delegate.getBoundSql();
    }

    public ParameterHandler getParameterHandler() {
        return this.delegate.getParameterHandler();
    }
}