場(chǎng)景

PageHelper.startPage(page, pageSize);

看到這句話后開始思考為何它可以直接實(shí)現(xiàn)分頁邏輯，它是怎么實(shí)現(xiàn)的？為何在數(shù)據(jù)庫查詢的前面加上了這句話就可以實(shí)現(xiàn)分頁，里面到底發(fā)生了什么？

介紹

com.github.pagehelper.PageHelper是一個(gè)開源的分頁源碼工具

		<dependency>
			<groupId>com.github.pagehelpergroupId>
			<artifactId>pagehelper-spring-boot-starterartifactId>
			<version>1.2.12version>
		dependency>

配置文件中也需要配置

# 分頁插件配置
pagehelper:
  helperDialect: mysql
  supportMethodsArguments: true

源碼分析

分頁參數(shù)的傳遞

public static  Page startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
        Page page = new Page(pageNum, pageSize, count);
        page.setReasonable(reasonable);
        page.setPageSizeZero(pageSizeZero);
        Page oldPage = getLocalPage();
        if (oldPage != null && oldPage.isOrderByOnly()) {
            page.setOrderBy(oldPage.getOrderBy());
        }

        setLocalPage(page);
        return page;
    }

這段代碼中第一行是寫入?yún)?shù)，第二行和第三行都是null，可以不考慮

主要的代碼是這兩行：

getLocalPage();
setLocalPage(page);

這里用到了一個(gè)ThreadLocal；

ThreadLocal，很多地方叫做線程本地變量，為變量在每個(gè)線程中都創(chuàng)建了一個(gè)副本，那么每個(gè)線程可以訪問自己內(nèi)部的副本變量。

展示到這就看不到其他了，這就很尷尬了，然后繼續(xù)看jar包中的代碼，然后發(fā)現(xiàn)了PageInterceptor這個(gè)類，分頁攔截器，前面已經(jīng)設(shè)置了分頁參數(shù)在本地變量中，然后就該是調(diào)用了，很大可能就是在這個(gè)攔截器中進(jìn)行實(shí)現(xiàn)。

PageInterceptor 實(shí)現(xiàn)Mybatis的Interceptor 接口，進(jìn)行攔截

 public Object intercept(Invocation invocation) throws Throwable {
        try {
            Object[] args = invocation.getArgs();
            MappedStatement ms = (MappedStatement)args[0];
            Object parameter = args[1];
            RowBounds rowBounds = (RowBounds)args[2];
            ResultHandler resultHandler = (ResultHandler)args[3];
            Executor executor = (Executor)invocation.getTarget();
            CacheKey cacheKey;
            BoundSql boundSql;
            if (args.length == 4) {
                boundSql = ms.getBoundSql(parameter);
                cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
            } else {
                cacheKey = (CacheKey)args[4];
                boundSql = (BoundSql)args[5];
            }

            this.checkDialectExists();
            List resultList;
            if (!this.dialect.skip(ms, parameter, rowBounds)) {
                if (this.dialect.beforeCount(ms, parameter, rowBounds)) {
                    Long count = this.count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
                    if (!this.dialect.afterCount(count, parameter, rowBounds)) {
                        Object var12 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);
                        return var12;
                    }
                }

                resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
            } else {
                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
            }

            Object var16 = this.dialect.afterPage(resultList, parameter, rowBounds);
            return var16;
        } finally {
            if (this.dialect != null) {
                this.dialect.afterAll();
            }

        }
    }

然后進(jìn)入查看ExecutorUtil.pageQuery，看是怎么實(shí)現(xiàn)的：

查看分頁的sql語句，繼續(xù)：

public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
        String sql = boundSql.getSql();
        Page page = this.getLocalPage();
        String orderBy = page.getOrderBy();
        if (StringUtil.isNotEmpty(orderBy)) {
            pageKey.update(orderBy);
            sql = OrderByParser.converToOrderBySql(sql, orderBy);
        }

        return page.isOrderByOnly() ? sql : this.getPageSql(sql, page, pageKey);
    }

看到最后的三則表達(dá)式后后的getPageSql

進(jìn)入到MySqlDialect類的getPageSql方法進(jìn)行SQL封裝，根據(jù)page對(duì)象信息增加Limit。分頁的信息就是這么拼裝起來的

public String getPageSql(String sql, Page page, CacheKey pageKey) {
        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
        sqlBuilder.append(sql);
        if (page.getStartRow() == 0) {
            sqlBuilder.append(" LIMIT ? ");
        } else {
            sqlBuilder.append(" LIMIT ?, ? ");
        }

        return sqlBuilder.toString();
    }

總結(jié)

原來PageHelper的分頁功能是在我們執(zhí)行SQL語句之前動(dòng)態(tài)的將SQL語句拼接了分頁的語句，從而實(shí)現(xiàn)了從數(shù)據(jù)庫中分頁獲取的過程。