4. 实现⼀个分页插件

数据库分页插件

@Intercepts({
    @Signature(
        type = Executor.class, // 目标类
        method = "query", // 目标方法
        args ={MappedStatement.class, 
            Object.class, RowBounds.class, ResultHandler.class}
    )
})
public class MySqlPagingPlugin implements Interceptor {
    private static final Integer MAPPED_STATEMENT_INDEX = 0;
    private static final Integer PARAMETER_INDEX = 1;
    private static final Integer ROW_BOUNDS_INDEX = 2;
    
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Object[] args = invocation.getArgs();
        RowBounds rb = (RowBounds) args[ROW_BOUNDS_INDEX];
        // 无需分页
        if (rb == RowBounds.DEFAULT) {
            return invocation.proceed();
        }
        // 将原 RowBounds 参数设为 RowBounds.DEFAULT，关闭 MyBatis 内置的分页机制
        args[ROW_BOUNDS_INDEX] = RowBounds.DEFAULT;
        MappedStatement ms = (MappedStatement) args[MAPPED_STATEMENT_INDEX];
        BoundSql boundSql = ms.getBoundSql(args[PARAMETER_INDEX]);
        // 获取 SQL 语句，拼接 limit 语句
        String sql = boundSql.getSql();
        String limit = String.format(
            "LIMIT %d,%d", rb.getOffset(), rb.getLimit());
        sql = sql + " " + limit;
        // 创建一个 StaticSqlSource，并将拼接好的 sql 传入
        SqlSource sqlSource = new StaticSqlSource(
            ms.getConfiguration(), sql, boundSql.getParameterMappings());
        // 通过反射获取并设置 MappedStatement 的 sqlSource 字段
        Field field = MappedStatement.class.getDeclaredField("sqlSource");
        field.setAccessible(true);
        field.set(ms, sqlSource);
        // 执行被拦截方法
        return invocation.proceed();
    }
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }
    @Override
    public void setProperties(Properties properties) {
    }
}

上面的分页插件通过 RowBounds 参数获取分页信息，并生成相应的 limit 语句。之后拼
接 sql，并使用该 sql 作为参数创建StaticSqlSource。最后通过反射替换MappedStatement 对象中的 sqlSource 字段。

测试

public interface StudentDao {
    List<Student> findByPaging(@Param("id") Integer id, RowBounds rb);
}

<mapper namespace="xyz.coolblog.chapter7.dao.StudentDao">
    <select id="findByPaging"
        resultType="xyz.coolblog.chapter7.model.Student">
        SELECT
            `id`, `name`, `age`
        FROM
            student
        WHERE
            id > #{id}
    </select>
</mapper>

public class PluginTest {
    private SqlSessionFactory sqlSessionFactory;
    
    @Before
    public void prepare() throws IOException {
        String resource = "chapter7/mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
        inputStream.close();
    }
    
    @Test
    public void testPlugin() {
        SqlSession session = sqlSessionFactory.openSession();
        try {
            StudentDao studentDao = session.getMapper(StudentDao.class);
            studentDao.findByPaging(1, new RowBounds(20, 10));
        } finally {
            session.close();
        }
    }
}

SQL 语句中包含了 LIMIT 字样，这说明插件生效了