Caching mechanism of Hibernate / session level caching mechanism

Hibernate Two levels of cache are maintained in . The first level cache is created by Session Instance maintenance , The cache holds the data associated with the Session The data of all entity objects associated with the instance , Also known as internal caching . And the second level cache exists in SessionFactory In the example , By the SessionFactory All obtained by the instance Session The instance Shared .

L2 cache needs to be configured and enabled to be effective , L2 cache is stored between memory and disk , When it needs to be used, enter memory , There is no need to use the existing disk , This will not occupy memory .

Hibernate The role of caching mechanism ：

1. If session Be inquired of （ For example, to perform session.load()/session.get()）,session First, you will go to the first level cache to find whether there are queried objects , If found, the object will be returned directly ; If it cannot be found, look in the L2 cache , If found, it will be returned directly to the object ; If it still cannot be found, query the database

2. If the state of the object in the cache changes , Will be marked （ That is, maintain the state of the object ）, When executed session.flush() Changes will be synchronized to the database

When an entity object is in the cache, the object is in a persistent state ; When calling session.evict(Object obj) when , The object will be obj from session Clear from cache , Object obj It is in a free state , Waiting to be destroyed .

Test code ：

  /** * session First level cache test ( Session level caching ) * * @param * @return * @throws * @author liaowenxiong * @date 2022/7/2 20:38 */
  @Test
  public void testSessionLevelCache() {
    
    Session session = HibernateUtils.getSession();
    Transaction tx = session.beginTransaction();
    //  Get... For the first time id=1 Of Employee object , stay Session The object is not in the L1 cache , So I will query the database , Because being Session
    //  The processed object will be in a persistent state , So this object （e1 The object that is pointed to ） It's persistent , Will be cached in session in 
    Employee e1 = session.get(Employee.class, 3);
    //  Get... For the second time id=1 Of Employee object , Will go to session Search in cache , If it exists, the object will be returned directly 
    Employee e2 = session.get(Employee.class, 3);
    //  therefore e1 and e2 Is the same object 
    System.out.println(e1 == e2); //  The result is true
    e1.setNickname("liaowenxiong123");
    session.flush();//  The object will be e1 The updates of are synchronized to the database 
    session.evict(e1); //  Put the object e1 Clear from cache , At this point the object e1 In a free state 
    e1.setNickname("liaowenxiong234");// e1 And session Out of the relationship , Therefore, even if the transaction is committed later flush() Nor will the object e1 The updates of are synchronized to the database 
    tx.commit();
    session.close();
  }