自己实现一个Mybatis分页插件

1. 原理： 无非就是基于mybaits插件机制,拦截执行过程中某个阶段（Executor, StatementHandler, ParamterStatementHander,ResultSetHandler)， 获取到待执行SQL, 利用反射对SQL重新赋值 （拼接上分页参数）； 

2. SQL触发执行是在 StatementHandler阶段,故这里对StatementHandler拦截 （具体原因,自行看源码  从 Executor #query()方法debug）

2. 上代码

/**
 * @author yangxj
 * @date 2021/6/4-23:21
 */
@Slf4j
@Component
@Intercepts({@Signature(type = StatementHandler.class, method = "prepare",args = {Connection.class, Integer.class})})
public class PageHelper implements Interceptor {

    private static final ThreadLocal<Page> localCache = new ThreadLocal<>();

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        StatementHandler stm = (StatementHandler) invocation.getTarget();

        BoundSql boundSql = stm.getBoundSql();

        Page page = getPage(); // 取到分页参数

        if (page != null) {
            Field SQLField = BoundSql.class.getDeclaredField("sql");   // 利用反射对SQL重新赋值
            SQLField.setAccessible(true);
            SQLField.set(boundSql, page.getPageSQL(boundSql.getSql()));
            clearPage(); // 用完清空分页参数
        }

        log.info("待执行SQL： " + boundSql.getSql().replaceAll("\\n", "").replaceAll("\\t", ""));

        return invocation.proceed();
    }

    @Override
    public Object plugin(Object o) {
        return o instanceof StatementHandler ? Plugin.wrap(o, this) : o;
    }

    @Override
    public void setProperties(Properties properties) {

    }


    public  static void startPage(Long pageNum, Long pageSize) {
        localCache.set(new Page(pageNum, pageSize));
    }

    private Page getPage() {
        return localCache.get();
    }

    private void clearPage() {
        localCache.remove();
    }

    private static class Page {
        private Long pageNum;
        private Long pageSize;

        private Page(Long pageNum, Long pageSize) {
            this.pageNum = pageNum;
            this.pageSize = pageSize;
        }

        private String getPageSQL(String SQL) {
            if (pageNum != null && pageSize != null) {
                return SQL + " limit " + (pageNum - 1) * pageSize + " ," + pageSize;
            }
            return SQL;
        }
    }
}