Analyze the internal mechanism and execution process of MyBatis

MyBatis is an excellent persistence layer framework that simplifies the database access process and provides flexible mapping configuration and parameter processing capabilities. This article will introduce the working principle and process of MyBatis in detail, and provide specific code examples to help readers better understand this framework.

1. Working Principle
The working principle of MyBatis mainly includes four key components: configuration file, SQL mapping file, SQL session and executor.

1. Configuration file:
   MyBatis configuration file (mybatis-config.xml) is the core configuration file of the entire framework. It defines the global properties of MyBatis, such as database connection information, type processor wait. In addition, the configuration file also contains some important configuration items, such as plug-ins, mappers, etc.
2. SQL mapping file:
   SQL mapping file (Mapper.xml) is another important part of MyBatis. It defines the logic of database access, including SQL statements, parameters and result mapping, etc. Through the tag in the configuration file, you can associate the SQL mapping file with the Java interface or class.
3. SQL session:
   SQL session is one of the core objects of MyBatis, which acts as the "middle layer" to connect to the database. Through the SQL session, you can execute SQL statements and get corresponding results. In MyBatis, the SQL session is created through the SqlSessionFactory factory class and the instance is obtained by calling the openSession method.
4. Executor:
   The executor is another core object of MyBatis. It is responsible for executing SQL statements and processing parameters and results. MyBatis provides two executor types, namely simple executor (SimpleExecutor) and reuse executor (ReuseExecutor). The simple executor creates a new Statement object for each SQL statement, while the reuse executor reuses Statement objects for the same SQL statement.

2. Workflow
The workflow of MyBatis mainly includes steps such as configuration loading, SQL mapping, SQL execution and result processing. Each step is described in detail below, with corresponding code examples.

1. Configuration loading:
   First, MyBatis will load the configuration file (mybatis-config.xml) and create an instance of the SqlSessionFactory factory class based on the information in the configuration file.

The sample code is as follows:

InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

2. SQL mapping:
   Next, MyBatis will load the SQL mapping file (Mapper.xml) and parse the SQL statements and Parameter mapping. Through the tag in the configuration file, you can associate the SQL mapping file with the Java interface or class.

The sample code is as follows:

SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

User user = userMapper.getUserById(1);

3. SQL execution:
   In the SQL execution phase, MyBatis will generate the corresponding executable Statement based on the SQL statement and parameter mapping. object and pass parameters to the Statement object. Then, execute the SQL statement through the executor (Executor) and obtain the execution results.

The sample code is as follows:

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

public interface UserMapperXml {
    String getUserById = "SELECT * FROM user WHERE id = #{id}";
}

4. Result processing:
   After the SQL execution is completed, MyBatis will map the database query results into Java objects and return them to the caller By. The result mapping relationship can be defined through the tag in the configuration file.

The sample code is as follows:

<resultMap id="userResultMap" type="com.example.User">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <result property="age" column="age" />
</resultMap>

Summary:
Through the introduction of the working principle and process of MyBatis, we can clearly understand how it works. The configuration file plays a key role, defining global properties and various configuration items. The SQL mapping file provides the definition of SQL statements and parameter mapping, and realizes the flexibility of database access through the association between them. SQL sessions and executors are responsible for specific SQL execution and result processing.

I hope the code examples provided in this article can help readers better understand and use the MyBatis framework. At the same time, it is also recommended that readers gain an in-depth grasp of more features and usage of MyBatis through further study and practice.

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