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。

如下面代码：

1. Session session = factory.openSession();
   6 t5 |8 b: P6 U; C
2. try {
   ' o, G) w+ C0 E
3.         Book book1 = (Book) session.get(Book.class, id);
   
4.         Book book2 = (Book) session.get(Book.class, id);
   . A/ I4 P, r- A* m+ i& c7 O
5. } finally {
   
6.         session.close();
   - p0 S& G: I' T: g  ]" X: ~
7. }

复制代码

' [% q3 r) g/ q+ q7 A' Y% X6 H执行该代码，只会显示一条sql语句。


9 H+ {( s( D, l, |  [$ {如果执行下面代码：

1. Session sess=sf.openSession();
   $ z( [9 S7 {) J. O3 q
2.         Book b=(Book) sess.get(Book.class, "PBN123");
   ( e3 u( Q3 O9 N6 o0 h3 L" O- f! E
3.         assertEquals("spring Recipes", b.getName());
   
4.         sess.close();
   9 f0 l/ _) t0 C% H) I+ a
5.         Session sess1=sf.openSession();
   
6.         Book b1=(Book) sess1.get(Book.class, "PBN123");
   , a6 d. j7 {1 s# k
7.         assertEquals("Spring Recipes", b1.getName());
   # f- J( a! g4 b6 }! |7 y
8.         sess1.close();

复制代码

就会显示2条SQL语句

1. Hibernate: select book0_.ISBN as ISBN1_0_0_, book0_.NAME as NAME2_0_0_, book0_.PUBLISHDATE a
   0 _1 s. @) U: ^9 z1 [
2. s PUBLISHD3_0_0_, book0_.PRICE as PRICE4_0_0_, book0_.PUBLISHERCODE as PUBLISHE5_0_0_, publi
   + a, |# V  j$ \0 s% b
3. sher1_.CODE as CODE1_1_1_, publisher1_.NAME as NAME2_1_1_, publisher1_.ADDRESS as ADDRESS3_1
   * d* ^0 {& X- `- X0 g$ o
4. _1_ from BOOK book0_ left outer join PUBLISHER publisher1_ on book0_.PUBLISHERCODE=publisher
   
5. 1_.CODE where book0_.ISBN=?
   - h( X( [$ }. S$ c
6. Hibernate: select book0_.ISBN as ISBN1_0_0_, book0_.NAME as NAME2_0_0_, book0_.PUBLISHDATE a
   0 [3 y3 E/ }+ L$ \, [9 y8 u
7. s PUBLISHD3_0_0_, book0_.PRICE as PRICE4_0_0_, book0_.PUBLISHERCODE as PUBLISHE5_0_0_, publi
   9 x# l! w5 S. n* ]1 u
8. sher1_.CODE as CODE1_1_1_, publisher1_.NAME as NAME2_1_1_, publisher1_.ADDRESS as ADDRESS3_1
   " z7 Q% E9 I5 D: p$ e; R# e
9. _1_ from BOOK book0_ left outer join PUBLISHER publisher1_ on book0_.PUBLISHERCODE=publisher
   - E) P, M4 \, @% H* G3 T8 @3 m8 q$ V
10. 1_.CODE where book0_.ISBN=?

复制代码

; r6 N, i! J9 K+ c# l