Detailed explanation of the principle of MyBatis paging plug-in

MyBatis is an excellent persistence layer framework. It supports operating databases based on XML and annotations. It is simple and easy to use, and also provides a rich plug-in mechanism. Among them, the paging plug-in is one of the more frequently used plug-ins. This article will delve into the principles of the MyBatis paging plug-in and illustrate it with specific code examples.

1. Principle of paging plug-in

MyBatis itself does not provide native paging function, but you can use plug-ins to implement paging queries. The principle of the paging plug-in is to intercept the query statement of MyBatis, and then add paging-related statements, such as LIMIT, OFFSET, etc., to the query statement to achieve paging.

Specifically, paging plug-ins usually need to implement the Interceptor interface and override the intercept method. In the intercept method, the paging query logic is implemented by intercepting the query method of the Executor object and modifying the MappedStatement object.

The use of paging plug-in generally needs to be configured in the MyBatis configuration file, specify the interceptor class to be used, and set the corresponding parameter configuration, such as the number of records displayed on each page, the current page number, etc.

2. Code Example

The following is a simple example that shows how to use the paging plug-in to implement paging query operations based on the MySQL database.

1. Write the paging plug-in class:

public class PaginationInterceptor implements Interceptor {

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        if (invocation.getTarget() instanceof Executor) {
            Object[] args = invocation.getArgs();
            MappedStatement ms = (MappedStatement) args[0];
            Object parameter = args[1];
            RowBounds rowBounds = (RowBounds) args[2];

            if (rowBounds != null && rowBounds != RowBounds.DEFAULT) {
                BoundSql boundSql = ms.getBoundSql(parameter);
                String sql = boundSql.getSql();
                int offset = rowBounds.getOffset();
                int limit = rowBounds.getLimit();

                String pageSql = sql + " LIMIT " + offset + ", " + limit;
                MetaObject metaObject = SystemMetaObject.forObject(boundSql);
                metaObject.setValue("sql", pageSql);
            }
        }

        return invocation.proceed();
    }
    
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    @Override
    public void setProperties(Properties properties) {
        // 设置额外的属性
    }
}

2. Configure the MyBatis configuration file:

In the MyBatis configuration file, configure Use this paging plug-in:

<plugins>
    <plugin interceptor="com.example.PaginationInterceptor">
        <!-- 设置分页插件的额外属性 -->
        <property name="XXX" value="XXX"/>
    </plugin>
</plugins>

3. Write Mapper interface and corresponding SQL statement:

public interface UserMapper {
    List<User> selectUsersWithPagination(RowBounds rowBounds);
}

<select id="selectUsersWithPagination" resultType="com.example.User">
    SELECT * FROM user
</select>

4. Call paging query:

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession();
UserMapper userMapper = session.getMapper(UserMapper.class);

RowBounds rowBounds = new RowBounds(0, 10);
List<User> users = userMapper.selectUsersWithPagination(rowBounds);

Through the above code example, you can implement paging query operations on the user table in the database. The paging plug-in intercepts the query method of the Executor object and adds LIMIT and OFFSET to the query statement, thereby realizing the paging query function.

Summary:

The paging plug-in of MyBatis provides us with a convenient and fast paging query function. By intercepting the query method of the Executor object, the operation of adding paging parameters to the SQL query statement is realized. When we need to implement paging queries in our project, we can simply configure the paging plug-in and write the code according to the steps in the example to realize the use of the paging function.

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