概念

职责

ParameterHandler是用来设置参数规则的。StatementHandler中介绍到，其SimpleExecutor中调用prepare()方法之后，接下来StatementHandler就是使用parameterize来设置参数。以SampleExecutor为例，具体代码如下:

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    //解析并设置参数
    handler.parameterize(stmt);
    return stmt;
  }
  
 @Override
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

类图

进入源码，该接口很简单，且只有一个默认实现类DefaultParameterHandler

public interface ParameterHandler {
  Object getParameterObject();
  void setParameters(PreparedStatement ps)
      throws SQLException;

}

getParameterObject()用于读取参数；
setParameter():用于对PreparedStatementHandler的参数赋值；

源码

ParameterHandler对象创建

对于ParameterHandler对象的创建过程，首先抛出结论：该对象是在创建StatementHandler对象的同时被创建完成。StatementHandler文章中我们谈论到其依赖ParameterHandler和ResultSetHandler，下面我们进入正题。

上面谈到了该对象在StatementHandler对象创建时被创建，所以我们从StatementHandler对象位置开始跟踪：

SimpleExecutor

@Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      // 获取环境配置
      Configuration configuration = ms.getConfiguration();
      //Configuration中获取StatementHandler,跟进去
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

Configuration

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

跟进RoutingStatementHandler，我们以SimpleStatementHandler为例，其余*StatementHandler对象创建一样，调用其父类BaseStatementHandler构造方法,所以我们跟踪到其父类的构造方法中。

SimpleStatementHandler

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

BaseStatementHandler

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;
// 创建参数处理器
    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    // 创建结果映射器
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }

从代码注释上，我们可以清晰看到parameterHandler 和resultSetHandler 对象的创建交给Configuration类操作；从上面几步源码的跟踪：Configuration类依次完成了StatementHandler、parameterHandler 、resultSetHandler 对象的创建过程。那么继续回归到Configuration中。

Configuration

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

ParameterHandler解析参数

上面完成了StatementHandler，ParameterHandler和ResultSetHandler对象的创建，基本工作已准备完成，下面继续回归到SimpleExecutor#prepareStatement()

 @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      // 获取环境配置
      Configuration configuration = ms.getConfiguration();
      //Configuration中获取StatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //准备Statement对象
      stmt = prepareStatement(handler, ms.getStatementLog());
      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, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }

PrepareStatementHandler

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

代码里看到是parameterHandler对象调用，上面我们了解到ParameterHandler接口仅有一个实现类即DefaultParameterHandler，因此Debug进去。

DefaultParameterHandler

 @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    // parameterMappings 就是对 #{} 或者 ${} 里面参数的封装
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      // 如果是参数化的SQL，便需要循环取出并设置参数的值
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        // 如果参数类型不是 OUT ，这个类型与 CallableStatementHandler 有关
        // 因为存储过程不存在输出参数，所以参数不是输出参数的时候，就需要设置。
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          // 得到#{}  中的属性名
          String propertyName = parameterMapping.getProperty();
          // 如果 propertyName 是 Map 中的key
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            // 通过key 来得到 additionalParameter 中的value值
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            // 如果不是 additionalParameters 中的key，而且传入参数是 null， 则value 就是null
            value = null;
          }
          // 如果 typeHandlerRegistry 中已经注册了这个参数的 Class对象，即它是Primitive 或者是String 的话
          else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            // 否则就是 Map
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          // 在通过SqlSource 的parse 方法得到parameterMappings 的具体实现中，我们会得到parameterMappings的typeHandler
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          // 获取typeHandler 的jdbc type
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

总结

总结一下ParameterHandler整体大致流程走向，以下面时序图为例

Mybatis源码笔记之浅析ParameterHandler

概念

职责

类图

源码

ParameterHandler对象创建

ParameterHandler解析参数

总结