hibernate抓取策略fetch

约定不等于承诺〃 2022-06-12 11:29 191阅读 0赞

抓取策略（fetching strategy）是指：当应用程序需要在（Hibernate实体对象图的）关联关系间经行导航的时候，hibernate如何获取关联对象的策略。抓取策略可以在O/R映射的元数据中声明，也可以在特定的HQL 或`条件查询（Criteria Query）`中重载声明。

Hibernate3 定义了如下几种抓取策略：

*连接抓取（Join fetching）* \- Hibernate通过 在`SELECT`语句使用`OUTER JOIN`（外连接）来 获得对象的关联实例或者关联集合，这种情况lazy无效。

*查询抓取（Select fetching）* \- 另外发送一条`SELECT` 语句抓取当前对象的关联实体或集合。除非你显式的指定`lazy="false"`禁止 延迟抓取（lazy fetching），否则只有当你真正访问关联关系的时候，才会执行第二条select语句。

*子查询抓取（Subselect fetching）* \- 另外发送一条`SELECT` 语句抓取在前面查询到（或者抓取到）的所有实体对象的关联集合。除非你显式的指定`lazy="false"` 禁止延迟抓取（lazy fetching），否则只有当你真正访问关联关系的时候，才会执行第二条select语句。

*批量抓取（Batch fetching）* \- 对查询抓取的优化方案， 通过指定一个主键或外键列表，Hibernate使用单条`SELECT`语句获取一批对象实例或集合。

下面来分别介绍这几种抓取

# *查询抓取（Select fetching）* #

项目结构如图

![Center][]

所有的代码和上一节《[*hibernate加载策略之lazy*][hibernate_lazy]》中的代码一样，可以参考，

现在多的一端设置fetch

Book.hbm.xml代码：

<?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping package="com.myeclipse.pojo">
    	<class name="Book" table="t_book">
    		<id name="id">
    			<generator class="identity" />
    		</id>
    		<many-to-one name="category" class="Category" column="category_id"
    			cascade="save-update" fetch="select" lazy="false"/>
    		<property name="author" />
    		<property name="name" column="book_name" />
    		<property name="price" />
    		<property name="pubDate" />
    		
    		<filter name="bookFilter" condition="id=:id"></filter>
    
    	</class>
    	
    	<filter-def name="bookFilter">
    		<filter-param name="id" type="integer" />
    	</filter-def>
    
    </hibernate-mapping>

Category.hbm.xml代码：

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
       "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    	
    	
    <hibernate-mapping package="com.myeclipse.pojo">
    	<class name="Category" >
    		<id name="id" >
    			<generator class="identity" />
    		</id>
    		<property name="name" />
    		<set name="books" inverse="true" >
    			<key>
    				<column name="category_id" />
    			</key>
    			<one-to-many class="Book" />
    		</set>
    	</class>
    
    </hibernate-mapping>

HibernateTest中TestCreateDB代码：

@Test
    	public void testCreateDB() {
    		Configuration cfg = new Configuration().configure();
    		SchemaExport se = new SchemaExport(cfg);
    		// 第一个参数：是否生成ddl脚本
    		// 第二个参数：是否执行到数据库中
    		se.create(true, true);
    	}

使用Junit4执行，重新生成数据库表。

控制台打印的sql语句如下：

alter table t_book 
            drop 
            foreign key FK_cm584cq6cv5yht4jrqal0ocaq
    
        drop table if exists Category
    
        drop table if exists t_book
    
        create table Category (
            id integer not null auto_increment,
            name varchar(255),
            primary key (id)
        )
    
        create table t_book (
            id integer not null auto_increment,
            category_id integer,
            author varchar(255),
            book_name varchar(255),
            price double precision,
            pubDate datetime,
            primary key (id)
        )
    
        alter table t_book 
            add constraint FK_cm584cq6cv5yht4jrqal0ocaq 
            foreign key (category_id) 
            references Category (id)

HibernateTest类中的TestSave()方法代码：

/**
    	 * 保存数据
    	 */
    	@Test
    	public void testSave() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    
    		Category category = new Category();
    		category.setName("文学");
    		Category category1 = new Category();
    		category1.setName("历史");
    		Category category2 = new Category();
    		category2.setName("仙侠");
    		Category category3 = new Category();
    		category3.setName("科幻");
    		Category category4 = new Category();
    		category4.setName("恐怖");
    
    		Book book = new Book();
    		book.setName("读者");
    		book.setPrice(5.6);
    		book.setAuthor("众人");
    		book.setPubDate(new Date());
    		book.setCategory(category);
    
    		Book book1 = new Book();
    		book1.setName("傲慢与偏见");
    		book1.setPrice(80.0);
    		book1.setAuthor("简.奥斯汀");
    		book1.setPubDate(new Date());
    		book1.setCategory(category1);
    
    		Book book2 = new Book();
    		book2.setName("中国历史");
    		book2.setPrice(30.0);
    		book2.setAuthor("人民出版社");
    		book2.setPubDate(new Date());
    		book2.setCategory(category1);
    
    		Book book3 = new Book();
    		book3.setName("翩眇之旅");
    		book3.setPrice(70.0);
    		book3.setAuthor("萧鼎");
    		book3.setPubDate(new Date());
    		book3.setCategory(category2);
    
    		Book book4 = new Book();
    		book4.setName("蓝血人");
    		book4.setPrice(60.0);
    		book4.setAuthor("卫斯理");
    		book4.setPubDate(new Date());
    		book4.setCategory(category3);
    
    		Book book5 = new Book();
    		book5.setName("我的大学");
    		book5.setPrice(60.5);
    		book5.setAuthor("高尔基");
    		book5.setPubDate(new Date());
    		book5.setCategory(category);
    
    		session.save(book);
    		session.save(book1);
    		session.save(book2);
    		session.save(book3);
    		session.save(book4);
    		session.save(book5);
    		session.save(category4);
    
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

执行保存数据，sql语句如下：

Hibernate: 
        insert 
        into
            Category
            (name) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            Category
            (name) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            Category
            (name) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            Category
            (name) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            t_book
            (category_id, author, book_name, price, pubDate) 
        values
            (?, ?, ?, ?, ?)
    Hibernate: 
        insert 
        into
            Category
            (name) 
        values
            (?)

数据库表中的数据如图：

![Center 1][]

HibernateTest类中的查询代码如下：

/**
    	 * 查询图书
    	 */
    	@Test
    	public void testLoadBook() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    
    		Book book = (Book) session.load(Book.class, 1) ;
    		
    		System.out.println("-------------------------");
    		System.out.println("bookName = "+book.getName());
    		System.out.println("===========================");
    		System.out.println("categoryName = "+book.getCategory().getName());
    		
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

打印的sql语句如下：

INFO: HHH000232: Schema update complete
    -------------------------
    Hibernate: 
        select
            book0_.id as id1_1_0_,
            book0_.category_id as category2_1_0_,
            book0_.author as author3_1_0_,
            book0_.book_name as book_nam4_1_0_,
            book0_.price as price5_1_0_,
            book0_.pubDate as pubDate6_1_0_ 
        from
            t_book book0_ 
        where
            book0_.id=?
    Hibernate: 
        select
            category0_.id as id1_0_0_,
            category0_.name as name2_0_0_ 
        from
            Category category0_ 
        where
            category0_.id=?
    bookName = 读者
    ===========================
    categoryName = 文学

从sql语句可以看出，

当执行Book book = (Book) session.load(Book.class, 1) ;这句话时，并没有打印sql语句，

当查询book.getName()，图书的Name时，打印了查询sql语句：

Hibernate: 
        select
            book0_.id as id1_1_0_,
            book0_.category_id as category2_1_0_,
            book0_.author as author3_1_0_,
            book0_.book_name as book_nam4_1_0_,
            book0_.price as price5_1_0_,
            book0_.pubDate as pubDate6_1_0_ 
        from
            t_book book0_ 
        where
            book0_.id=?
    Hibernate: 
        select
            category0_.id as id1_0_0_,
            category0_.name as name2_0_0_ 
        from
            Category category0_ 
        where
            category0_.id=?

这两条sql语句可以看出，先根据book的id 查询了t\_book表，t\_book表中有category\_id，又根据category\_id查询了Category表。

当我们把Book.hbm.xml中的lazy改成proxy时，如图：

![Center 2][]

执行TestLoadBook()方法，打印的sql语句，如下：

-------------------------
    Hibernate: 
        select
            book0_.id as id1_1_0_,
            book0_.category_id as category2_1_0_,
            book0_.author as author3_1_0_,
            book0_.book_name as book_nam4_1_0_,
            book0_.price as price5_1_0_,
            book0_.pubDate as pubDate6_1_0_ 
        from
            t_book book0_ 
        where
            book0_.id=?
    bookName = 读者
    ===========================
    Hibernate: 
        select
            category0_.id as id1_0_0_,
            category0_.name as name2_0_0_ 
        from
            Category category0_ 
        where
            category0_.id=?
    categoryName = 文学

此时当需要获取对应的数据时，才会执行相应的sql语句。

下面在一的一端设置fetch

Category.hbm.xml代码如下：

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
       "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    	
    	
    <hibernate-mapping package="com.myeclipse.pojo">
    	<class name="Category" >
    		<id name="id" >
    			<generator class="identity" />
    		</id>
    		<property name="name" />
    		<set name="books" inverse="true" fetch="select">
    			<key>
    				<column name="category_id" />
    			</key>
    			<one-to-many class="Book" />
    		</set>
    	</class>
    
    </hibernate-mapping>

fetch设置为select，

testGetCategory()方法的代码如下：

@Test
    	public void testGetCategory() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    
    		Category category = (Category) session.get(Category.class, 1);
    		System.out.println("---------------------------------------------");
    		System.out.println("category_name=" + category.getName());
    		System.out.println("============================================");
    		for (Iterator<Book> iter = category.getBooks().iterator(); iter
    				.hasNext();) {
    			System.out.println(iter.next().getName());
    		}
    
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

打印的sql语句如下：

Hibernate: 
        select
            category0_.id as id1_0_0_,
            category0_.name as name2_0_0_ 
        from
            Category category0_ 
        where
            category0_.id=?
    ---------------------------------------------
    category_name=文学
    ============================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    我的大学
    读者

从上面可以看出来，get方法，当时就查询了Category表的sql语句，当需要查询Book的数据时，才会执行t\_book的数据库表信息。

# *连接抓取（Join fetching）* #

先说多段的Book

Book.hbm.xml代码：

<?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping package="com.myeclipse.pojo">
    	<class name="Book" table="t_book">
    		<id name="id">
    			<generator class="identity" />
    		</id>
    		<many-to-one name="category" class="Category" column="category_id"
    			cascade="save-update" fetch="join" lazy="proxy"/>
    		<property name="author" />
    		<property name="name" column="book_name" />
    		<property name="price" />
    		<property name="pubDate" />
    		
    		<filter name="bookFilter" condition="id=:id"></filter>
    
    	</class>
    	
    	<filter-def name="bookFilter">
    		<filter-param name="id" type="integer" />
    	</filter-def>
    
    </hibernate-mapping>

fetch是join，lazy是proxy

HIbernateTest类中的testLoadBook()方法，代码：

/**
    	 * 查询图书
    	 */
    	@Test
    	public void testLoadBook() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    
    		Book book = (Book) session.load(Book.class, 1);
    
    		System.out.println("-------------------------");
    		System.out.println("bookName = " + book.getName());
    		System.out.println("===========================");
    		System.out.println("categoryName = " + book.getCategory().getName());
    
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

控制台打印的sql语句是：

INFO: HHH000232: Schema update complete
    -------------------------
    Hibernate: 
        select
            book0_.id as id1_1_0_,
            book0_.category_id as category2_1_0_,
            book0_.author as author3_1_0_,
            book0_.book_name as book_nam4_1_0_,
            book0_.price as price5_1_0_,
            book0_.pubDate as pubDate6_1_0_,
            category1_.id as id1_0_1_,
            category1_.name as name2_0_1_ 
        from
            t_book book0_ 
        left outer join
            Category category1_ 
                on book0_.category_id=category1_.id 
        where
            book0_.id=?
    bookName = 读者
    ===========================
    categoryName = 文学

由上面可以看到，在需要book的信息时，sql语句使用 left outer join 把Category表中的信息也查询了出来。

下面从一的一端Category来查询

HibernateTest类中的testGetCategory()方法代码：

控制台打印的sql语句如下：

Hibernate: 
        select
            category0_.id as id1_0_0_,
            category0_.name as name2_0_0_,
            books1_.category_id as category2_0_1_,
            books1_.id as id1_1_1_,
            books1_.id as id1_1_2_,
            books1_.category_id as category2_1_2_,
            books1_.author as author3_1_2_,
            books1_.book_name as book_nam4_1_2_,
            books1_.price as price5_1_2_,
            books1_.pubDate as pubDate6_1_2_ 
        from
            Category category0_ 
        left outer join
            t_book books1_ 
                on category0_.id=books1_.category_id 
        where
            category0_.id=?
    ---------------------------------------------
    category_name=文学
    ============================================
    读者
    我的大学

从一的一端也是从使用left outer join 方法把t\_book表的数据一起查询了出来

接下来我们把Category.hbm.xml和Book.hbm.xml中的lazy和fetch都去掉，执行testLoad()方法，代码如下：

@Test
    	public void testLoad() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    		
    		List<Category> list = session.createCriteria(Category.class).list() ;
    		System.out.println("---------------------------------");
    		System.out.println("类型个数："+list.size());
    		for (Category category : list) {
    			System.out.println("================================");
    			System.out.println(category.getName()+"----数据本书："+category.getBooks().size());
    		}
    		
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

执行后，打印的sql语句如下：

Hibernate: 
        select
            this_.id as id1_0_0_,
            this_.name as name2_0_0_ 
        from
            Category this_
    ---------------------------------
    类型个数：5
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    文学----数据本书：2
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    历史----数据本书：2
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    仙侠----数据本书：1
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    科幻----数据本书：1
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_0_,
            books0_.id as id1_1_0_,
            books0_.id as id1_1_1_,
            books0_.category_id as category2_1_1_,
            books0_.author as author3_1_1_,
            books0_.book_name as book_nam4_1_1_,
            books0_.price as price5_1_1_,
            books0_.pubDate as pubDate6_1_1_ 
        from
            t_book books0_ 
        where
            books0_.category_id=?
    恐怖----数据本书：0

# 子查询（Subselect fetching） #

接下来我们在Category.hbm.xml代码中，加入fetch=“subselect” ，如图：

![Center 3][]

继续testLoad()方法，打印的sql语句如下：

由sql语句可以看出，当我们需要查询t\_book数据时，sql语句的where语句是：

where
            books0_.category_id in (
                select
                    this_.id 
                from
                    Category this_
            )

一次性将所有的分类Category数据全部查询出来。

假设现在我想查询Category的id是1，3，5的数据，那么方法testLoadWhere代码是：

@Test
    	public void testLoadWhere() {
    
    		Session session = HibernateUtil.getSession();
    		Transaction tx = session.beginTransaction();
    		
    		List<Category> list = session.createCriteria(Category.class)
    								.add(Restrictions.in("id", new Integer[]{1,3,5}))
    								.list() ;
    		System.out.println("---------------------------------");
    		System.out.println("类型个数："+list.size());
    		for (Category category : list) {
    			System.out.println("================================");
    			System.out.println(category.getName()+"----数据本书："+category.getBooks().size());
    		}
    		
    		tx.commit();
    		HibernateUtil.closeSession();
    
    	}

控制台打印的sql语句如下：

Hibernate: 
        select
            this_.id as id1_0_0_,
            this_.name as name2_0_0_ 
        from
            Category this_ 
        where
            this_.id in (
                ?, ?, ?
            )
    ---------------------------------
    类型个数：3
    ================================
    Hibernate: 
        select
            books0_.category_id as category2_0_1_,
            books0_.id as id1_1_1_,
            books0_.id as id1_1_0_,
            books0_.category_id as category2_1_0_,
            books0_.author as author3_1_0_,
            books0_.book_name as book_nam4_1_0_,
            books0_.price as price5_1_0_,
            books0_.pubDate as pubDate6_1_0_ 
        from
            t_book books0_ 
        where
            books0_.category_id in (
                select
                    this_.id 
                from
                    Category this_ 
                where
                    this_.id in (
                        ?, ?, ?
                    )
            )
    文学----数据本书：2
    ================================
    仙侠----数据本书：1
    ================================
    恐怖----数据本书：0

#    批量抓取（*Batch fetching*） #

Book.hbm.xml代码修改为：

<?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping package="com.myeclipse.pojo">
    	<class name="Book" table="t_book" batch-size="3">
    		<id name="id">
    			<generator class="identity" />
    		</id>
    		<many-to-one name="category" class="Category" column="category_id"
    			cascade="save-update" />
    		<property name="author" />
    		<property name="name" column="book_name" />
    		<property name="price" />
    		<property name="pubDate" />
    		
    		<filter name="bookFilter" condition="id=:id"></filter>
    
    	</class>
    	
    	<filter-def name="bookFilter">
    		<filter-param name="id" type="integer" />
    	</filter-def>
    
    </hibernate-mapping>

上面的代码中增加了一个batch-size属性，如图：

![Center 4][]

[Center]: /images/20220612/8c847f6005804510aaf76a7e2998fcde.png
[hibernate_lazy]: http://blog.csdn.net/u011159417/article/details/75726196
[Center 1]: /images/20220612/f1cd55572935455c913c793ba4d434d7.png
[Center 2]: /images/20220612/f8958a0ea6c748b596b167324a197176.png
[Center 3]: /images/20220612/d6a4b712533c4e1a89b7e855b4f47c4b.png
[Center 4]: /images/20220612/8944d10e73a145d39b89a0f299710b01.png