What are mybatis first-level cache and second-level cache?

Mybatis first-level cache and second-level cache are two different levels of caching mechanisms provided by mybatis. The first-level cache is the cache mechanism enabled by default in mybatis. It is a local cache based on threads. The second-level cache is Cache based on namespace level can be shared by multiple SqlSession objects.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

MyBatis is an open source persistence layer framework that provides some caching mechanisms to improve the performance of database queries. Among them, MyBatis first-level cache and second-level cache are two different levels of caching mechanisms.

The first-level cache is the cache mechanism enabled by default in MyBatis. It is a thread-based local cache. In other words, each SqlSession object has its own first-level cache. When executing a query operation, MyBatis will first check whether the same query exists in the first-level cache. If it exists, the results will be obtained directly from the cache without querying the database. This can reduce the number of database accesses and improve query performance.

The life cycle of the first-level cache is consistent with the life cycle of SqlSession. When the SqlSession is closed or the cache is cleared, the first-level cache will also be cleared. The first-level cache is enabled by default. If you want to close or clear the first-level cache, you can call the clearCache() method of SqlSession.

Although the first-level cache can improve query performance, there are also some problems. First of all, since the first-level cache is a thread-based local cache, data inconsistency may occur in a multi-threaded environment. Secondly, if update, delete or insert operations are performed in the same SqlSession, the first-level cache will be cleared, and the database will be queried again during the next query. Therefore, the first-level cache is suitable for single-threaded scenarios with more reads and less writes.

In order to solve the problem of first-level cache, MyBatis provides second-level cache. The second-level cache is based on the namespace level cache, which can be shared by multiple SqlSession objects. When multiple SqlSession objects execute the same query, if the query results exist in the second-level cache, the results are obtained directly from the cache without querying the database.

The life cycle of the second-level cache is consistent with the life cycle of the Mapper. When the Mapper's SqlSessionFactory is closed or the cache is cleared, the second-level cache will also be cleared. If you want to use the second level cache, you need to configure it accordingly in the Mapper configuration file.

Compared with the first-level cache, the second-level cache has wider applicability. It can solve the problem of data inconsistency in multi-threaded environments, and is suitable for scenarios where multiple SqlSession objects share the same query results. However, there are also some problems with second-level cache. First of all, since the second-level cache is based on the namespace level, the second-level cache is independent of each other under different namespaces. Secondly, if an update, delete, or insert operation is performed under the same namespace, the second-level cache will be cleared.

In general, first-level cache and second-level cache are two different levels of caching mechanisms provided by MyBatis. The first-level cache is a local cache based on threads, which is suitable for single-threaded scenarios with more reads and fewer writes; while the second-level cache is a namespace-level cache that can be shared by multiple SqlSession objects and is suitable for scenarios with multiple threads and frequent reads and writes. Scenes. In actual development, according to specific business needs and performance requirements, an appropriate caching mechanism can be selected to improve query performance.

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