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公司使用的 freemarker自定义代码生成模板，Mapper中在使用 SELECT * ，这无疑对性能造成了极大的影响，造成了很多不必要的IO读写。因此在想着改进一下代码，灵活的在使用中指定哪些字段需要查询，提升性能。

Builder模式

建造者模式（Builder Pattern）也叫做生成器模式，其定义如下：Separate the construction of a complex object from its representation so that the same construction process can create different representations.（将一个复杂对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。）

Builder 模式适用于需要多个构造方法，且参数不同的情况。

案例

Table

原代码

查询条件Dto

@Setter@Getterpublic class DemoQueryDto {    private Long id;    private List
   
     ids;        private List
    
      idNotIn;    private Long idLike;    private Long idNotEquals;    private Long idIsNull;        private String content;    private List
     
       contents;        private List
      
        contentsNotIn;    private String contentLike;    private String contentNotEquals;    private String contentIsNull;    private String siteName;    private List
       
         siteNames;        private List
        
          siteNamesNotIn; private String siteNameLike; private String siteNameNotEquals; private String siteNameIsNull;}
        
       
      
     
    
   

Dao

/**     * 根据查询条件获取对应信息     * @param condition     * @return list     */    List
   
     selectList(@Param("condition")  DemoQueryDto condition);
   

Mapper

   
    		id, content, site_name	
   	..........省略select 
    from A where 1=1 
   	..........省略	
   		
    			
     
      				 and id = #{condition.id,jdbcType=BIGINT}			
     			
     
      				and id in				
      
       					#{item}				
      			
     			
     
      				and id not in				
      
       					#{item}				
      			
     			
     
      				 and id like concat('%',#{condition.idLike,jdbcType=BIGINT},'%')			
     			
     
      				 and id != #{condition.idNotEquals,jdbcType=BIGINT}			
     			
     
      				 and id is null			
     			 ..........类似省略		
    	
   

Service

......参数等省略DemoQueryDto  queryDto = new DemoQueryDto();queryDto.setIds（ids）;List
   
     demos = demoMapper.selectList(queryDto);
   

按照上述方法查询的时候，执行的Sql就是Select全部字段的Sql

Builder方法改造

@Getter@Setterpublic class DemoBuilderDto {    private DemoQueryDto  demoQueryDto ;    //需要判断是否查询的字段    private boolean iscontent;    private boolean issiteName;    public static class Builder{   		 private DemoQueryDto  demoQueryDto ;        //需要判断是否查询的字段 默认false        private boolean iscontent = false;        private boolean issiteName = false;        public Builder(){        }        public Builder DemoQueryDto  (DemoQueryDto  conditionQueryDto){            demoQueryDto = conditionQueryDto;            return this;        }        public Builder iscontent(boolean flag){            iscontent = flag;            return this;        }        public Builder issiteName(boolean flag){            issiteName = flag;            return this;        }        public DemoBuilderDto build(){            return new DemoBuilderDto (this);        }    }    private DemoBuilderDto (Builder builder){        demoQueryDto = builder.demoQueryDto ;        iscontent = builder.iscontent;        issiteName = builder.issiteName;    }}

Mapper

   
    		id		
    
     		,content		
     		
    
     		,site_name		
     	
   	..........省略select 
    from A where 1=1 
   	..........省略	
   		
    			
     			
      
       				 and id = #{condition.demoQueryDto.id,jdbcType=BIGINT}			
      			 ..........类似省略			 
     		
    	
   

Dao

List
   
     selectList(@Param("condition")  DemoBuilderDto condition);
   

Service

DemoQueryDto  queryDto = new DemoQueryDto();queryDto.setIds（ids）;//现在我只需要查询contentDemoBuilderDto buildDto = new DemoBuilderDto.Builder().demoQueryDto(queryDto).iscontent(true).build();List
   
     demos = demoMapper.selectList(buildDto );
   

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