Mybatis 之 ParameterHandler

╰+哭是因爲堅強的太久メ 2022-06-08 02:14 199阅读 0赞

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。