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： 

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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：

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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。