MyBatis源码阅读——MyBatis插件PageHelper的实现过程

迷南。 2022-01-28 16:36 279阅读 0赞

# 前言 #

在之前的文章 [MyBatis源码阅读——MyBatis插件原理][MyBatis_MyBatis]中我们已经知道MyBatis的插件是如何运行的，这篇文章中，我们将去了解MyBatis常用插件com.github.pagehelper.PageHelper的实现过程

# 示例代码 #

[https://github.com/yuyufeng1994/learn-mybatis/blob/master/mybatis-demo/src/main/java/top/yuyufeng/learn/mybatis/demo/Demo5Plugin.java][https_github.com_yuyufeng1994_learn-mybatis_blob_master_mybatis-demo_src_main_java_top_yuyufeng_learn_mybatis_demo_Demo5Plugin.java]

# PageHelper拦截过程 #

设置完成后，Executor的每个method = “query” 的方法执行都会被代理执行  
PageHelper.java->intercept()

/**
    	     * Mybatis拦截器方法
    	     *
    	     * @param invocation 拦截器入参
    	     * @return 返回执行结果
    	     * @throws Throwable 抛出异常
    	     */
    	    public Object intercept(Invocation invocation) throws Throwable {
    	        if (autoDialect) {
    	            initSqlUtil(invocation);
    	        }
    	        return sqlUtil.processPage(invocation);
    	    }

sqlUtil.processPage(invocation)

/**
         * Mybatis拦截器方法
         *
         * @param invocation 拦截器入参
         * @return 返回执行结果
         * @throws Throwable 抛出异常
         */
        private Object _processPage(Invocation invocation) throws Throwable {
            final Object[] args = invocation.getArgs();
            RowBounds rowBounds = (RowBounds) args[2];
            if (SqlUtil.getLocalPage() == null && rowBounds == RowBounds.DEFAULT) {
                if (OrderByHelper.getOrderBy() != null) {
                    OrderByHelper.processIntercept(invocation);
                }
                return invocation.proceed();
            } else {
                //获取原始的ms
                MappedStatement ms = (MappedStatement) args[0];
                //判断并处理为PageSqlSource
                if (!isPageSqlSource(ms)) {
                    processMappedStatement(ms, parser);
                }
                //忽略RowBounds-否则会进行Mybatis自带的内存分页
                args[2] = RowBounds.DEFAULT;
                //分页信息
                Page page = getPage(rowBounds);
                //pageSizeZero的判断
                if ((page.getPageSizeZero() != null && page.getPageSizeZero()) && page.getPageSize() == 0) {
                    COUNT.set(null);
                    //执行正常（不分页）查询
                    Object result = invocation.proceed();
                    //得到处理结果
                    page.addAll((List) result);
                    //相当于查询第一页
                    page.setPageNum(1);
                    //这种情况相当于pageSize=total
                    page.setPageSize(page.size());
                    //仍然要设置total
                    page.setTotal(page.size());
                    //返回结果仍然为Page类型 - 便于后面对接收类型的统一处理
                    return page;
                }
    
                //简单的通过total的值来判断是否进行count查询
                if (page.isCount()) {
                    COUNT.set(Boolean.TRUE);
                    //替换MS
                    args[0] = msCountMap.get(ms.getId());
                    //查询总数
                    Object result = invocation.proceed();
                    //还原ms
                    args[0] = ms;
                    //设置总数
                    page.setTotal((Integer) ((List) result).get(0));
                    if (page.getTotal() == 0) {
                        return page;
                    }
                }
                //pageSize>0的时候执行分页查询，pageSize<=0的时候不执行相当于可能只返回了一个count
                if (page.getPageSize() > 0 &&
                        ((rowBounds == RowBounds.DEFAULT && page.getPageNum() > 0)
                                || rowBounds != RowBounds.DEFAULT)) {
                    //将参数中的MappedStatement替换为新的qs
                    COUNT.set(null);
                    BoundSql boundSql = ms.getBoundSql(args[1]);
                    args[1] = parser.setPageParameter(ms, args[1], boundSql, page);
                    COUNT.set(Boolean.FALSE);
                    //执行分页查询
                    Object result = invocation.proceed();
                    //得到处理结果
                    page.addAll((List) result);
                }
                //返回结果
                return page;
            }
        }

[MyBatis_MyBatis]: https://blog.csdn.net/qq_18860653/article/details/80654087
[https_github.com_yuyufeng1994_learn-mybatis_blob_master_mybatis-demo_src_main_java_top_yuyufeng_learn_mybatis_demo_Demo5Plugin.java]: https://github.com/yuyufeng1994/learn-mybatis/blob/master/mybatis-demo/src/main/java/top/yuyufeng/learn/mybatis/demo/Demo5Plugin.java