Hibernate – fetching strategies examples
来源:互联网 发布:电驴tcp端口 编辑:程序博客网 时间:2024/05/19 00:40
Hibernate has few fetching strategies to optimize the Hibernate generated select statement, so that it can be as efficient as possible. The fetching strategy is declared in the mapping relationship to define how Hibernate fetch its related collections and entities.
Fetching Strategies
There are four fetching strategies
- fetch-“join” = Disable the lazy loading, always load all the collections and entities.
- fetch-“select” (default) = Lazy load all the collections and entities.
- batch-size=”N” = Fetching up to ‘N’ collections or entities, Not record.
- fetch-“subselect” = Group its collection into a sub select statement.
Fetching strategies examples
Here’s a “one-to-many relationship” example for the fetching strategies demonstration. A stock is belong to many stock daily records.
Example to declare fetch strategies in XML file
...<hibernate-mapping> <class name="com.mkyong.common.Stock" table="stock"> <set name="stockDailyRecords" cascade="all" inverse="true" table="stock_daily_record" batch-size="10" fetch="select"> <key> <column name="STOCK_ID" not-null="true" /> </key> <one-to-many class="com.mkyong.common.StockDailyRecord" /> </set> </class></hibernate-mapping>
Example to declare fetch strategies in annotation
...@Entity@Table(name = "stock", catalog = "mkyong")public class Stock implements Serializable{... @OneToMany(fetch = FetchType.LAZY, mappedBy = "stock") @Cascade(CascadeType.ALL) @Fetch(FetchMode.SELECT) @BatchSize(size = 10) public Set<StockDailyRecord> getStockDailyRecords() { return this.stockDailyRecords; }...}
Let explore how fetch strategies affect the Hibernate generated SQL statement.
1. fetch=”select”
or @Fetch(FetchMode.SELECT)
This is the default fetching strategy. it enabled the lazy loading of all it’s related collections. Let see the example…
//call select from stockStock stock = (Stock)session.get(Stock.class, 114); Set sets = stock.getStockDailyRecords();//call select from stock_daily_recordfor ( Iterator iter = sets.iterator();iter.hasNext(); ) { StockDailyRecord sdr = (StockDailyRecord) iter.next(); System.out.println(sdr.getDailyRecordId()); System.out.println(sdr.getDate());}
Output
Hibernate: select ...from mkyong.stock where stock0_.STOCK_ID=?Hibernate: select ...from mkyong.stock_daily_record where stockdaily0_.STOCK_ID=?
Hibernate generated two select statements
- Select statement to retrieve the
Stock
records –session.get(Stock.class, 114)
- Select its related collections –
sets.iterator()
2. fetch=”join”
or @Fetch(FetchMode.JOIN)
The “join
” fetching strategy will disabled the lazy loading of all it’s related collections. Let see the example…
//call select from stock and stock_daily_recordStock stock = (Stock)session.get(Stock.class, 114); Set sets = stock.getStockDailyRecords();//no extra selectfor ( Iterator iter = sets.iterator();iter.hasNext(); ) { StockDailyRecord sdr = (StockDailyRecord) iter.next(); System.out.println(sdr.getDailyRecordId()); System.out.println(sdr.getDate());}
Output
Hibernate: select ... from mkyong.stock stock0_ left outer join mkyong.stock_daily_record stockdaily1_ on stock0_.STOCK_ID=stockdaily1_.STOCK_ID where stock0_.STOCK_ID=?
Hibernate generated only one select statement, it retrieve all its related collections when the Stock
is initialized. –session.get(Stock.class, 114)
- Select statement to retrieve the
Stock
records and outer join its related collections.
3. batch-size=”10″
or @BatchSize(size = 10)
This ‘batch size’ fetching strategy is always misunderstanding by many Hibernate developers. Let see the misunderstand concept here…
Stock stock = (Stock)session.get(Stock.class, 114); Set sets = stock.getStockDailyRecords();for ( Iterator iter = sets.iterator();iter.hasNext(); ) { StockDailyRecord sdr = (StockDailyRecord) iter.next(); System.out.println(sdr.getDailyRecordId()); System.out.println(sdr.getDate());}
What is your expected result, is this per-fetch 10 records from collection? See the output
Output
Hibernate: select ...from mkyong.stock where stock0_.STOCK_ID=?Hibernate: select ...from mkyong.stock_daily_record where stockdaily0_.STOCK_ID=?
The batch-size did nothing here, it is not how batch-size work. See this statement.
The batch-size fetching strategy is not define how many records inside in the collections are loaded. Instead, it defines how many collections should be loaded.
— Repeat N times until you remember this statement —
Another example
Let see another example, you want to print out all the stock records and its related stock daily records (collections) one by one.
List<Stock> list = session.createQuery("from Stock").list();for(Stock stock : list){ Set sets = stock.getStockDailyRecords(); for ( Iterator iter = sets.iterator();iter.hasNext(); ) { StockDailyRecord sdr = (StockDailyRecord) iter.next(); System.out.println(sdr.getDailyRecordId()); System.out.println(sdr.getDate()); }}
No batch-size fetching strategy
Output
Hibernate: select ... from mkyong.stock stock0_Hibernate: select ... from mkyong.stock_daily_record stockdaily0_ where stockdaily0_.STOCK_ID=?Hibernate: select ... from mkyong.stock_daily_record stockdaily0_ where stockdaily0_.STOCK_ID=?
Keep repeat the select statements….depend how many stock records in your table.
If you have 20 stock records in the database, the Hibernate’s default fetching strategies will generate 20+1 select statements and hit the database.
- Select statement to retrieve all the Stock records.
- Select its related collection
- Select its related collection
- Select its related collection
…. - Select its related collection
The generated queries are not efficient and caused a serious performance issue.
Enabled the batch-size=’10’
fetching strategy
Let see another example with batch-size=’10
’ is enabled.
Output
Hibernate: select ... from mkyong.stock stock0_Hibernate: select ... from mkyong.stock_daily_record stockdaily0_ where stockdaily0_.STOCK_ID in ( ?, ?, ?, ?, ?, ?, ?, ?, ?, ? ) ```Now, Hibernate will per-fetch the collections, with a select *in* statement. If you have 20 stock records, it will generate 3 select statements.1. Select statement to retrieve all the Stock records.2. Select In statement to per-fetch its related collections (10 collections a time)3. Select In statement to per-fetch its related collections (next 10 collections a time)With `batch-size` enabled, it simplify the select statements from 21 select statements to 3 select statements.##4. `fetch=”subselect`” or `@Fetch(FetchMode.SUBSELECT)`This fetching strategy is enable all its related collection in a sub select statement. Let see the same query again..
List list = session.createQuery(“from Stock”).list();
for(Stock stock : list){
Set sets = stock.getStockDailyRecords();for ( Iterator iter = sets.iterator();iter.hasNext(); ) { StockDailyRecord sdr = (StockDailyRecord) iter.next(); System.out.println(sdr.getDailyRecordId()); System.out.println(sdr.getDate());}
}
###Output
Hibernate:
select …
from mkyong.stock stock0_
Hibernate:
select …
from
mkyong.stock_daily_record stockdaily0_
where
stockdaily0_.STOCK_ID in (
select
stock0_.STOCK_ID
from
mkyong.stock stock0_
) ``
subselect`” enabled, it will create two select statements.
With “
- Select statement to retrieve all the Stock records.
- Select all its related collections in a sub select query.
Conclusion
The fetching strategies are highly flexible and a very important tweak to optimize the Hibernate query, but if you used it in a wrong place, it will be a total disaster.
- Hibernate – fetching strategies examples
- Hibernate – fetching strategies examples
- hibernate 抓取策略(Fetching strategies)
- Hibernate 优化技术之抓取策略(Fetching strategies)
- 抓取策略(Fetching strategies)
- 抓取策略(Fetching strategies)
- Hibernate Examples
- Hibernate性能:fetching策略
- Hibernate: Understanding Lazy Fetching
- Hibernate抓取--fetching
- Hibernate: Understanding of Hibernate Fetch Strategies
- strategies
- Hibernate Query examples (HQL)
- Hibernate parameter binding examples
- Hibernate Criteria examples
- Hibernate named query examples
- Learn Hibernate by Examples
- Hibernate中的数据的获取策略(fetching)
- 混合IT架构的最佳实践
- uval1297 Palindrome 后缀数组求最长回文字串,lcp,rmq
- CH Contraction Hierarchies
- Linux-CenOS7 Samba的安装与配置
- Ubuntu 14.04 Storm(单机版)安装
- Hibernate – fetching strategies examples
- android在线下载并安装APK
- Android去掉标题栏的方法
- JavaScript中函数命名机制
- z-index无效,无论设置多大都被其他的元素覆盖
- mysql知识点
- android ui分析 - 今日头条UI主界面
- 全自动遮罩层插件layer2.0版本发布(可传参),兼容IE7以上
- JS判断移动设备的终端类型(浏览器UserAgent)