Mybatis源码解析十二之ParameterHandler

介绍

ParameterHandler是在SatementHandler执行prepared比如预编译SQL后，设置参数时使用的类

接口

ParameterHandler也只是一个接口

public interface ParameterHandler {

  Object getParameterObject();

  void setParameters(PreparedStatement ps)
      throws SQLException;

}

说明:
- 只有两个方法:获取参数和设置参数。

• 它只有一个实现类DefaultParameterHandler.java

ParameterHandler的创建。

通过源码调用分析，可以看到ParameterHandler是在StatementHandler对象创建时，一起创建的.
具体代码如下：

 1. BaseStatementHandler.java

 protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    ...
    ## ParemeterHandler创建入口
    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    ## ResultHandler创建入口
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }

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

  3. XMLLanguageDriver
  @Override
  public ParameterHandler createParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ## 这里创建 DefaultParameterHandler
    return new DefaultParameterHandler(mappedStatement, parameterObject, boundSql);
  }

创建时 传入三个对象：执行SQL对应的配置信息MappedStatement、参数对象Object，SQL的BoundSql。

注：一个BoundSql对象，代表了一次sql语句的实际执行，而SqlSource对象的责任，就是根据传入的参数对象，动态计算出这个BoundSql，也就是说Mapper文件中的节点的计算，是由SqlSource对象完成的。SqlSource最常用的实现类是DynamicSqlSource

setParement

设置参数

@Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    ## 1.获取所有参数，ParameterMapping是java类型和jdbc类型的对应关系   
    List parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          ## 获取参数名
          String propertyName = parameterMapping.getProperty();
          ## 根据参数名获取参数值
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          //获取参数值对应的jdbc类型  
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          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);
          }
        }
      }
    }
  }

parameterObject 参数，可以是MAP，一个实体对象，或者基本数据类型。

上面大致流程是，根据参数列表，使用BaseTypeHandler来设置参数。BaseTypeHandler其底层调用的是java的PreparedStatement.setXXX 方法来设置参数。

调用时机说明：
setParameter方法是在Executor进行调用的。
以SimpleExecutor为例

 @Override
  public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);

      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
  ## 获取连接、预编译prepare方法，设置参数都在该方法中执行
    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;
  }

参考

• Mybatis 之 ParameterHandler

• MappedStatement,SqlSource,BoundSql