Analyze the caching mechanism of MyBatis: compare the characteristics and usage of first-level cache and second-level cache

MyBatis caching mechanism analysis: the difference and application of first-level cache and second-level cache

In the MyBatis framework, caching is a very important feature that can be effective Improve the performance of database operations. Among them, first-level cache and second-level cache are two commonly used caching mechanisms in MyBatis. This article will analyze the differences and applications of first-level cache and second-level cache in detail, and provide specific code examples to illustrate.

1. First-level cache

The first-level cache is also called the local cache. It is enabled by default and cannot be turned off. The first-level cache is a SqlSession-level cache, that is, query operations performed in the same SqlSession will share the same cache. After the query statement is executed, the query results will be stored in the cache of SqlSession. If the same query statement is executed again, MyBatis will obtain the results directly from the cache without accessing the database again.

The characteristics of the first-level cache are as follows:

1. The first-level cache is enabled by default and cannot be turned off.
2. The life cycle of the first-level cache is the same as the life cycle of SqlSession.
3. The first-level cache is thread-private, and the caches between different SqlSession are independent.

The following is a sample code using the first-level cache:

SqlSession sqlSession = sqlSessionFactory.openSession();
try {
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    User user1 = userMapper.getUserById(1);
    User user2 = userMapper.getUserById(1);
    System.out.println(user1 == user2); // 输出true，表示对象是相同的
} finally {
    sqlSession.close();
}

In the above example, we first obtain a user object with ID 1 and store it in into the first-level cache, and then execute the same query statement again. You can see that the output is true, which means that the objects obtained twice are the same. This is the role of the first-level cache.

2. Second-level cache

The second-level cache is a Mapper-level cache. It is shared across SqlSession and can improve the data sharing efficiency between multiple SqlSession. However, it should be noted that the second-level cache needs to be manually configured and enabled. It is not enabled by default like the first-level cache.

The characteristics of the second-level cache are as follows:

1. Requires manual configuration and activation.
2. The life cycle of the second-level cache is the same as the life cycle of the Mapper.
3. The second-level cache is shared across SqlSession, which can improve the data sharing efficiency between multiple SqlSession.

The following is a sample code for using the second-level cache:

First, configure the following in the MyBatis configuration file:

<settings>
    <setting name="cacheEnabled" value="true"/>
</settings>

Then, in the corresponding Mapper Add the following annotation to the interface:

@CacheNamespace
public interface UserMapper {
    User getUserById(int id);
}

Then, you can add the @Select annotation on the query method that needs to be cached, and set useCache=true:

@Select("select * from user where id = #{id}")
@Options(useCache=true)
User getUserById(int id);

The above configuration makes the getUserById method have two The function of level caching can share data across SqlSession.

To sum up, this article analyzes the differences and applications of first-level cache and second-level cache in MyBatis in detail, and provides specific code examples for explanation. Appropriate use of the caching mechanism can effectively improve the performance of database operations and improve the response speed of the system, which is a part that needs to be focused on during development. I hope this article can help readers better understand and apply the caching mechanism of MyBatis.

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