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

Mybatis's first-level cache is enabled by default and is SqlSession level. This means that multiple queries in the same SqlSession will take advantage of this level of caching. The first-level cache mainly stores query results. When executing a query operation, MyBatis will store the mapping relationship between the mapping statement and the query result, as well as the query result data in the cache. The second-level cache of mybatis is different from the first-level cache. The second-level cache is shared throughout the application, unlike the first-level cache in each SqlSession and so on.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

MyBatis is an excellent persistence layer framework that supports customized SQL, stored procedures and advanced mapping. In MyBatis, caching is an optimization method used to improve the performance of data access. MyBatis provides a two-level cache mechanism, namely first-level cache and second-level cache.

First level cache:

The first level cache is enabled by default and is at the SqlSession level. This means that multiple queries in the same SqlSession will take advantage of this level of caching. The first-level cache mainly stores query results. When executing a query operation, MyBatis will store the mapping relationship between the mapping statement and the query result, as well as the query result data in the cache. If the same SQL statement is executed again, MyBatis will first check whether the mapping relationship and result data of the SQL statement exist in the first-level cache. If it exists, it will directly return the results in the cache, avoiding repeated querying of the database.

The life cycle of the first-level cache is the same as SqlSession. When the SqlSession is closed or cleared, the first-level cache will also be cleared. It should be noted that the first-level cache is shared within a transaction, that is, multiple operations within the same transaction can share data in the first-level cache. However, the first-level caches of different transactions are independent, and the cached data between transactions will not affect each other.

Second-level cache:

Unlike the first-level cache, the second-level cache is shared throughout the application. It is not like the first-level cache in each SqlSession. exist independently. The second-level cache can be shared by multiple SqlSession, so it is also called the global cache. This means that if different SqlSession executes the same SQL statement, they can fetch the data from the second-level cache instead of re-querying the database. This further improves the performance of data access and reduces database load.

The second-level cache stores the result set of the SQL query, not the mapping relationship between the mapping statement and the query result. This means that the second level cache is based on query results, not SQL statements. Therefore, even if two different SQL statements return the same result set, they cannot share data in the second-level cache.

To use the second-level cache, you need to enable it in the global configuration file of MyBatis and configure the corresponding tags in the mapping file that needs to be cached. Another thing to note is that since the second-level cache is shared across the entire application, concurrent access and data consistency issues need to be handled carefully.

Summary:

MyBatis's first-level cache and second-level cache are designed to improve the performance of data access. The first-level cache provides a fast query method at the SqlSession level, while the second-level cache provides a global query method at the entire application level. Choosing to use the first-level cache or the second-level cache or both according to the needs of the actual application can effectively improve the performance of the application.

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