MyBatis框架的使用及源码分析(十二) ParameterHandler

在StatementHandler使用prepare()方法后，接下来就是使用ParameterHandler来设置参数，让我们看看它的定义：

package org.apache.ibatis.executor.parameter;  

import java.sql.PreparedStatement;
import java.sql.SQLException;  

/**
 * A parameter handler sets the parameters of the {@code PreparedStatement}
 *
 * @author Clinton Begin
 */
public interface ParameterHandler {  

  Object getParameterObject();  

  void setParameters(PreparedStatement ps)
      throws SQLException;  

}

getParameterObject()是获取参数的，而setParameters()是设置参数的，相当于对一条sql所有的参数都执行ps.setXXX(value);

DefaultParameterHandler是它的实现类：

package org.apache.ibatis.scripting.defaults;  

import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.util.List;  

import org.apache.ibatis.executor.ErrorContext;
import org.apache.ibatis.executor.parameter.ParameterHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.ParameterMapping;
import org.apache.ibatis.mapping.ParameterMode;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.type.JdbcType;
import org.apache.ibatis.type.TypeException;
import org.apache.ibatis.type.TypeHandler;
import org.apache.ibatis.type.TypeHandlerRegistry;  

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class DefaultParameterHandler implements ParameterHandler {  

  private final TypeHandlerRegistry typeHandlerRegistry;  

  private final MappedStatement mappedStatement;
  private final Object parameterObject;  //所有的参数值
  private BoundSql boundSql;
  private Configuration configuration;  

  public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    this.mappedStatement = mappedStatement;
    this.configuration = mappedStatement.getConfiguration();
    this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
    this.parameterObject = parameterObject;
    this.boundSql = boundSql;
  }  

  @Override
  public Object getParameterObject() {
    return parameterObject;
  }  

  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    //获取所有参数，ParameterMapping是java类型和jdbc类型的对应关系
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          //参数值
          Object value;
          //获取参数名称
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            //获取参数值
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass()))
            //如果是单个值则直接赋值
            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 {
            //设置参数值和jdbc类型的对应关系
            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);
          }
        }
      }
    }
  }  

}

getParameterObject是获取参数，这个参数值就是你传递进来的值，可能是个实体、map或单个基本类型数据。

重点看setParameters()，首先它读取了ParameterObject参数对象，然后用typeHandler对参数进行设置，而typeHandler里面需要对jdbcType和javaType进行处理，然后就设置参数了。也很好理解。所以当我们使用TypeHandler的时候完全可以控制如何设置SQL参数。设置参数，其实就是你在sql语句中配置的java对象和jdbc类型对应的关系，例如#{id,jdbcType=INTEGER}，id默认类型是javaType=class java.lang.Integer。