mybatis 源码分析(六)StatementHandler 主体结构分析

分析到这里的时候,mybatis 初始化、接口、事务、缓存等主要功能都已经讲完了,现在就还剩下 StatementHandler 这个真正干活的家伙没有分析了;所以接下来的博客内容主要和数据库的关系比较密切,而 StatementHandler 的主要流程也基本是和 JDBC 的流程是一一对应的;

一、StatementHandler 执行流程

在 mybatis 系列文章的第一篇,我放了一张 mybatis 整体的执行流程图:

mybatis 源码分析(六)StatementHandler 主体结构分析

从上面的图中也能比较清楚的看到 StatementHandler 的职责:获取 Statement -> 设置参数 -> 查询数据库 -> 将查询结果映射为 JavaBean,从这里也能看到是和我们使用原生 JDBC 的流程是一样的;而整个过程 StatementHandler 又将其拆分成了部分:

  • KeyGenerator:主键设置
  • ParameterHandler:参数设置
  • ResultSetHandler:结果集设置

这里我们首先介绍 StatementHandler 的类结构:

mybatis 源码分析(六)StatementHandler 主体结构分析

  • RoutingStatementHandler:路由处理器,这个相当于一个静态代理,根据 MappedStatement.statementType 创建对应的对处理器;
  • SimpleStatementHandler:不需要预编译的简单处理器;
  • PreparedStatementHandler:预编译的 SQL 处理器;
  • CallableStatementHandler:主要用于存储过程的调度;

这里的 SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler 同时也和 JDBC 的三个 Statement 一一对应;首先我们还是先看一下接口方法:

public interface StatementHandler {
  Statement prepare(Connection connection) throws SQLException; // 获取 Statement
  void parameterize(Statement statement) throws SQLException;   // 参数化
  void batch(Statement statement) throws SQLException;          // 批处理
  int update(Statement statement) throws SQLException;
  <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException;
  BoundSql getBoundSql();                 // 获取绑定sql
  ParameterHandler getParameterHandler(); // 得到参数处理器
}

其中公共的方法都封装到了 BaseStatementHandler 中(这里使用的是模版模式);

// 获取 Statement
@Override
public Statement prepare(Connection connection) throws SQLException {
  ErrorContext.instance().sql(boundSql.getSql());
  Statement statement = null;
  try {
    statement = instantiateStatement(connection); // 实例化 Statement
    setStatementTimeout(statement);               // 设置超时
    setFetchSize(statement);                      // 设置读取条数
    return statement;
  } catch (SQLException e) {
    closeStatement(statement);
    throw e;
  } catch (Exception e) {
    closeStatement(statement);
    throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
  }
}

// 由子类提供不同的实例化
protected abstract Statement instantiateStatement(Connection connection) throws SQLException;

二、StatementHandler 子类

1. RoutingStatementHandler

静态代理模式,根据 MappedStatement.statementType 创建对应的对处理,默认是 PREPARED,可以使用 XML 配置或者注解的方式指定;

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  // 创建路由选择语句处理器
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  // 插件拦截
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  switch (ms.getStatementType()) {  // 根据语句类型,委派到不同的语句处理器(STATEMENT|PREPARED|CALLABLE)
    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());
  }
}

2. SimpleStatementHandler

其功能和 JDBC.Statement 对应,从初始化方法也可以看到使用的是简单 Statement;

@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
  // 调用 Connection.createStatement
  if (mappedStatement.getResultSetType() != null) {
    return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
  } else {
    return connection.createStatement();
  }
}
@Override
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 {
    //如果没有keyGenerator,直接调用Statement.execute和Statement.getUpdateCount
    statement.execute(sql);
    rows = statement.getUpdateCount();
  }
  return rows;
}

这里在更新的时候需要区分 KeyGenerator,因为使用的是简单 Statement,所以需要在查询的时候指定返回主键 statement.execute(sql, Statement.RETURN_GENERATED_KEYS);

3. PreparedStatementHandler

其功能和 PreparedStatement 对应,这里初始化的时候可以看到比 SimpleStatementHandler 要复杂一些,因为 PreparedStatement 更新返回主键有三个方法,详细分析后面会单独放一篇详细讲解;

@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
  //调用Connection.prepareStatement
  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);
  }
}

还有一点不同的就是 PreparedStatement 需要参数化设置,就是设置预编译 SQL 对应占位符的参数;

// DefaultParameterHandler
@Override
public void setParameters(PreparedStatement ps) throws SQLException {
  ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
  List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
  if (parameterMappings != null) {
    // 循环设参数
    for (int i = 0; i < parameterMappings.size(); i++) {
      ParameterMapping parameterMapping = parameterMappings.get(i);
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        // 如果不是OUT,才设进去
        Object value;
        String propertyName = parameterMapping.getProperty();
        if (boundSql.hasAdditionalParameter(propertyName)) {
          // 若有额外的参数, 设为额外的参数
          value = boundSql.getAdditionalParameter(propertyName);
        } else if (parameterObject == null) {
          // 若参数为null,直接设null
          value = null;
        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
          // 若参数有相应的TypeHandler,直接设object
          value = parameterObject;
        } else {
          // 除此以外,MetaObject.getValue反射取得值设进去
          MetaObject metaObject = configuration.newMetaObject(parameterObject);
          value = metaObject.getValue(propertyName);
        }
        TypeHandler typeHandler = parameterMapping.getTypeHandler();
        JdbcType jdbcType = parameterMapping.getJdbcType();
        if (value == null && jdbcType == null) {
          // 不同类型的set方法不同,所以委派给子类的setParameter方法
          jdbcType = configuration.getJdbcTypeForNull();
        }
        typeHandler.setParameter(ps, i + 1, value, jdbcType);
      }
    }
  }
}

另外还有 CallableStatementHandler ,代码也很简单,这里就不详细分析了;

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