当我们查询条件中的值不确定是否有空值时，我们期待查询时，Mybatis+能自动帮我们判空。

比如这样：

this.page(
      new Query<OperatorsEntity>().getPage(queryFields),
      new QueryWrapper<OperatorsEntity>()
      .eq("province_id",queryFields.getProvinceId())
      .eq("city_id",queryFields.getCityId())
      .eq("area_id",queryFields.getAreaId())
      .like("name",queryFields.getNameLike())
);

我们期待的结果是：“province_id”、“city_id”、“area_id”、"name"等几个字段中，如果有空的(比如查询表单中没有填这个查询条件)，就不查询。
然而事实上，如果这样去写，结果会是：Mybatis+把所有条件都构造出来，为空的就按null查询，like则是查询: %null%

解决方案:

• 提供一个有条件排空功能的ExcludeEmptyQueryWrapper(后附代码)，有类似需求的用该queryWrapper来构建条件，一劳永逸解决问题，使用：

this.page(
      new Query<OperatorsEntity>().getPage(queryFields),
      new ExcludeEmptyQueryWrapper<OperatorsEntity>()
      .eq("province_id",queryFields.getProvinceId())
      .eq("city_id",queryFields.getCityId())
      .eq("area_id",queryFields.getAreaId())
      .like("name",queryFields.getNameLike())
);

• ExcludeEmptyQueryWrapper 代码如下：

/**
 * 继承QueryWrapper，重写其构造条件方法，使条件为null的查询条件，不再执行。
 * @since Mybatis+ version: 3.x.x
 * @author firewolf
 */
public class ExcludeEmptyQueryWrapper<T> extends QueryWrapper<T> {
    
    @Override
    public QueryWrapper<T> eq(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.eq(condition, column, val);
    }

    @Override
    public QueryWrapper<T> ne(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.ne(condition, column, val);
    }

    @Override
    public QueryWrapper<T> gt(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.gt(condition, column, val);
    }

    @Override
    public QueryWrapper<T> ge(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.ge(condition, column, val);
    }

    @Override
    public QueryWrapper<T> lt(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.lt(condition, column, val);
    }

    @Override
    public QueryWrapper<T> le(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.le(condition, column, val);
    }

    @Override
    public QueryWrapper<T> like(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.like(condition, column, val);
    }

    @Override
    public QueryWrapper<T> notLike(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.notLike(condition, column, val);
    }

    @Override
    public QueryWrapper<T> likeLeft(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.likeLeft(condition, column, val);
    }

    @Override
    public QueryWrapper<T> likeRight(boolean condition, String column, Object val) {
    
        condition = !Objects.isNull(val);
        return super.likeRight(condition, column, val);
    }

    @Override
    public QueryWrapper<T> between(boolean condition, String column, Object val1, Object val2) {
    
        if(Objects.isNull(val1) && Objects.isNull(val2)){
    
            condition = false;
        }else if(Objects.isNull(val1)){
    
            return super.lt(true,column,val2);
        }else if(Objects.isNull(val2)){
    
            return super.ge(true,column,val1);
        }
        return super.between(condition, column, val1, val2);
    }

    @Override
    public QueryWrapper<T> notBetween(boolean condition, String column, Object val1, Object val2) {
    
        if(Objects.isNull(val1) && Objects.isNull(val2)){
    
            condition = false;
        }else if(Objects.isNull(val1)){
    
            return super.lt(true,column,val2);
        }else if(Objects.isNull(val2)){
    
            return super.ge(true,column,val1);
        }
        return super.notBetween(condition, column, val1, val2);
    }
}