Hibernate's caching mechanism mainly includes first-level cache and second-level cache.
1. First-level cache (Session cache):
The first-level cache is Hibernate's Session-level cache, which is associated with each Session object. When you perform query, save, or update operations through the Session object, Hibernate automatically utilizes the first-level cache to improve performance. The main features of the first-level cache are as follows:
1.1. Query cache: When using the Session object to perform a query operation, Hibernate will first look for data in the first-level cache. If the data exists in the cache, the results will be returned directly; otherwise, the database will be queried and the results will be stored in the first-level cache.
1.2. Update cache: When using the Session object to perform an update operation, Hibernate will store the updated data in the first-level cache. When the transaction commits, Hibernate updates the data in the cache to the database.
1.3. Save cache: When using the Session object to perform a save operation, Hibernate will store the new data in the first-level cache. When the transaction commits, Hibernate inserts the data in the cache into the database.
1.4. Life cycle: The life cycle of the first-level cache is the same as the life cycle of the Session object. When the Session object is closed, the data in the first-level cache will be cleared.
2. Second level cache (SessionFactory cache):
The second level cache is Hibernate's SessionFactory level cache and is shared with all Session objects. Second level cache requires manual configuration to enable and specify a cache policy for each entity class. The main function of the second-level cache is to share cached data across Sessions, thereby reducing query operations on the database. The main features of the second level cache are as follows:
2.1. Query cache: When using the Session object to perform a query operation, if there is no data in the first-level cache, Hibernate will look for data in the second-level cache. If the data exists in the second-level cache, the result will be returned directly; otherwise, the database will be queried and the results will be stored in the first-level cache and the second-level cache.
2.2. Update cache: When using the Session object to perform an update operation, Hibernate will store the updated data in the first-level cache and update the data in the second-level cache according to the caching strategy of the entity class.
2.3. Save cache: When using the Session object to perform a save operation, Hibernate will store the new data in the first-level cache and insert the data into the second-level cache according to the caching strategy of the entity class.
2.4. Life cycle: The life cycle of the second-level cache is the same as the life cycle of the SessionFactory object. When the SessionFactory object is closed, the data in the second-level cache will be cleared.
2.5. Cache provider: Hibernate supports a variety of second-level cache providers, such as EhCache, Infinispan, Redis, etc. You can choose the right one based on your project needs
Precautions
When using Hibernate's second-level cache, you do need to pay attention to some issues, including data consistency, dirty reads, etc. Here are some suggestions:
1. Data consistency:
Since the second-level cache is across Sessions, multiple Sessions may operate the data in the cache at the same time, which may cause data consistency problems. To ensure data consistency, you can adopt the following strategies:
1.1. Use optimistic locking: By adding a version field (such as version) to the entity class, Hibernate will automatically check the version number when performing an update operation. If the version numbers do not match, an exception will be thrown to avoid data inconsistencies.
```java
@Version
private int version;
```
1.2. Choose an appropriate caching strategy: Choose an appropriate caching strategy (such as Read-Only, Read-Write, Nonstrict-Read-Write, Transactional, etc.) according to the characteristics and needs of the entity class to ensure data consistency.
2. Dirty read problem:
Since the second-level cache caches data, expired data (dirty data) may be read. To resolve this issue you can:
2.1. Set a reasonable cache expiration time: Set a reasonable expiration time for the second-level cache to reduce the possibility of dirty reads. When cached data expires, Hibernate automatically reloads the data from the database.
2.2. Use refresh operation: When you need to ensure that the acquired data is the latest, you can use the refresh method of Session to force refresh the data. This will skip the cache and reload the data directly from the database.
3. Performance issues:
Although the second-level cache can improve application performance, in some cases, using the second-level cache may cause performance degradation. Therefore, you need:
3.1. Reasonably select entity classes to cache: Based on the access frequency and data change frequency of the entity class, reasonably select the entity classes that need to be cached. For entity classes that are accessed frequently and have few data changes, using the second-level cache can achieve a large performance improvement; while for entity classes with frequent data changes, the second-level cache may cause performance degradation.
3.2. Monitor cache performance: Monitor the performance of the second-level cache through monitoring tools (such as JMX, cache provider's monitoring component, etc.) to discover and solve performance problems in a timely manner.
In short, when using Hibernate second-level cache, you need to pay attention to issues such as data consistency, dirty reads, and performance, and adopt corresponding strategies to solve these problems. Hope these tips are helpful!