Study the design concept of MyBatis paging plug-in

Exploring the design ideas of MyBatis paging plug-in

MyBatis is an excellent persistence layer framework that is widely used in Java development. In actual project development, we often encounter the need to display database query results in pages. MyBatis does not provide an official paging plug-in, so developers usually use third-party plug-ins to achieve this function. This article will delve into how to design a simple and efficient MyBatis paging plug-in and provide specific code examples.

1. Design ideas

To design a MyBatis paging plug-in, you first need to clearly implement the core principle of paging: limit the query results through LIMIT and OFFSET in the SQL query statement quantity and offset. Therefore, our plug-in needs to intercept SQL query statements and dynamically add LIMIT and OFFSET parameters to implement paging functionality.

The specific design ideas are as follows:

1. Create a Page class to encapsulate relevant parameters of paging queries, such as page number, page size, total number of records, etc.
2. Customize an Interceptor, implement the MyBatis Interceptor interface, rewrite the intercept() method, intercept the SQL query statement in this method, and dynamically add LIMIT and OFFSET parameters according to the Page object.
3. Configure the Interceptor in the MyBatis configuration file to make it effective.

2. Specific implementation

First, we define a Page class to encapsulate the relevant parameters of the paging query:

public class Page {
    private int pageNo; // 当前页码
    private int pageSize; // 每页大小
    private int total; // 总记录数

    // 省略getter和setter方法
}

Then , we create a CustomPaginationInterceptor class, implement the MyBatis Interceptor interface, and override the intercept() method:

@Intercepts({
    @Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})
})
public class CustomPaginationInterceptor implements Interceptor {
    
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        if (invocation.getTarget() instanceof StatementHandler) {
            StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
            MetaObject metaObject = SystemMetaObject.forObject(statementHandler);
            
            while (metaObject.hasGetter("h")) {
                Object obj = metaObject.getValue("h");
                metaObject = SystemMetaObject.forObject(obj);
            }
            
            MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("delegate.mappedStatement");
            if ("SELECT".equals(mappedStatement.getSqlCommandType().name())) {
                BoundSql boundSql = (BoundSql) metaObject.getValue("delegate.boundSql");
                String originalSql = boundSql.getSql();
                Page page = PageHelper.getPage();

                if (page != null) {
                    String limitSql = String.format("%s LIMIT %s OFFSET %s",
                            originalSql, page.getPageSize(), (page.getPageNo() - 1) * page.getPageSize());
                    metaObject.setValue("delegate.boundSql.sql", limitSql);
                }
            }
        }
        
        return invocation.proceed();
    }

    @Override
    public Object plugin(Object target) {
        if (target instanceof StatementHandler) {
            return Plugin.wrap(target, this);
        }
        return target;
    }

    @Override
    public void setProperties(Properties properties) {
        // 空实现
    }
}

Finally, we configure the interceptor in the MyBatis configuration file:

<plugins>
    <plugin interceptor="com.example.CustomPaginationInterceptor">
</plugins>

3. Usage example

In the Mapper interface that requires paging query, define a selectByPage method:

public interface UserMapper {
    List<User> selectByPage(Page page);
}

Write the SQL statement in the corresponding XML file:

<select id="selectByPage" resultMap="userResultMap">
    SELECT * FROM user
</select>

In the business logic, call the selectByPage method and pass in the Page object:

Page page = new Page(1, 10);
List<User> userList = userMapper.selectByPage(page);

Through the above design ideas and code examples, we successfully implemented a simple and efficient MyBatis paging plug-in. Developers can customize and optimize according to actual project needs to achieve more flexible and powerful paging functions. I hope this article will help you understand the design ideas of the MyBatis paging plug-in.

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