什么是Hibernate中的一级缓存

拧巴人

一级缓存是在事务级别或工作单元级别。hibernate默认启用一级缓存，一级缓存与session关联，如果相同的query在相同的session中执行多次，那么与该query关联的data就会缓存，一级缓存的scope是session。
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( R) N' g! @" a! R. h4 m如下面代码：

1. Session session = factory.openSession();
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2. try {
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3.         Book book1 = (Book) session.get(Book.class, id);
   
4.         Book book2 = (Book) session.get(Book.class, id);
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5. } finally {
   
6.         session.close();
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7. }

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$ K/ a& W: c3 m' N执行该代码，只会显示一条sql语句。
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如果执行下面代码：

1. Session sess=sf.openSession();
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2.         Book b=(Book) sess.get(Book.class, "PBN123");
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3.         assertEquals("spring Recipes", b.getName());
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4.         sess.close();
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5.         Session sess1=sf.openSession();
   
6.         Book b1=(Book) sess1.get(Book.class, "PBN123");
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7.         assertEquals("Spring Recipes", b1.getName());
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8.         sess1.close();

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就会显示2条SQL语句
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1. Hibernate: select book0_.ISBN as ISBN1_0_0_, book0_.NAME as NAME2_0_0_, book0_.PUBLISHDATE a
   
2. s PUBLISHD3_0_0_, book0_.PRICE as PRICE4_0_0_, book0_.PUBLISHERCODE as PUBLISHE5_0_0_, publi
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3. sher1_.CODE as CODE1_1_1_, publisher1_.NAME as NAME2_1_1_, publisher1_.ADDRESS as ADDRESS3_1
   
4. _1_ from BOOK book0_ left outer join PUBLISHER publisher1_ on book0_.PUBLISHERCODE=publisher
   
5. 1_.CODE where book0_.ISBN=?
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6. Hibernate: select book0_.ISBN as ISBN1_0_0_, book0_.NAME as NAME2_0_0_, book0_.PUBLISHDATE a
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7. s PUBLISHD3_0_0_, book0_.PRICE as PRICE4_0_0_, book0_.PUBLISHERCODE as PUBLISHE5_0_0_, publi
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8. sher1_.CODE as CODE1_1_1_, publisher1_.NAME as NAME2_1_1_, publisher1_.ADDRESS as ADDRESS3_1
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9. _1_ from BOOK book0_ left outer join PUBLISHER publisher1_ on book0_.PUBLISHERCODE=publisher
   
10. 1_.CODE where book0_.ISBN=?

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