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mybatis-plus分頁插件失效探究解決

更新時(shí)間：2023年07月06日 09:35:00 作者：子瞻

這篇文章主要為大家介紹了mybatis-plus分頁插件失效探究解決，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進(jìn)步，早日升職加薪

網(wǎng)上推薦

網(wǎng)上基本上都是推薦配置如下：

@Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor mybatisPlusInterceptor = new MybatisPlusInterceptor();
        mybatisPlusInterceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return mybatisPlusInterceptor;
    }

但是，僅僅這么做，就能達(dá)到我們的預(yù)期嗎？

分頁插件無效原因

其結(jié)局就是分頁插件沒有效果，原因是為什么呢？??

圖1

圖2

通過對(duì)比上面兩張圖可以發(fā)現(xiàn)，圖一DefaultSqlSession.selectList()底層調(diào)用Plugin.invoke();圖二DefaultSqlSession.selectList()底層調(diào)用CachingExecutor.query()。

其中，圖一是分頁插件生效的調(diào)用鏈，圖二是分頁插件失效的調(diào)用鏈。

也就是說，分頁插件失效的原因是，mybatis-plusPlugin類沒有為分頁插件攔截器生成Executor代理。

解決方案

具體應(yīng)該怎么做呢？像下面這樣，在構(gòu)建SqlSessionFactory時(shí)，需要在MybatisSqlSessionFactoryBean顯示設(shè)置Plugin。

@Bean(name = "defaultSqlSessionFactory")
    public SqlSessionFactory defaultSqlSessionFactory(){
        MybatisSqlSessionFactoryBean bean = new MybatisSqlSessionFactoryBean();
        bean.setDataSource(dataSource);
        //設(shè)置攔截器
        bean.setPlugins(mybatisPlusInterceptor);
        SqlSessionFactory sqlSessionFactory = bean.getObject();
        //設(shè)置自動(dòng)提交
        sqlSessionFactory.openSession(true);
        return sqlSessionFactory;
}

那么，為分頁插件生成代理類是在什么時(shí)機(jī)生成呢？先公布答案：

//設(shè)置自動(dòng)提交
sqlSessionFactory.openSession(true);

調(diào)用鏈如下

圖3

咱再看細(xì)節(jié)：DefaultSqlSessionFactory.openSessionFromDataSource()詳情：

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //這步很關(guān)鍵，創(chuàng)建執(zhí)行者實(shí)例
      final Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

Configuration.newExecutor()詳情：

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //對(duì)上面的executor進(jìn)行代理（目的是把插件和執(zhí)行器封裝為代理類）
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

MybatisPlusInterceptor.pluginAll();

public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

通過上面的重點(diǎn)code展示，我們大致了解了部分重要節(jié)點(diǎn)中分頁插件代理類生成的邏輯。接下來我們繼續(xù)了解具體分頁插件工作的效果。

圖4

public boolean willDoQuery(){
        if (countMs != null) {
            countSql = countMs.getBoundSql(parameter);
        } else {
            countMs = buildAutoCountMappedStatement(ms);
            //生成查詢count SQL
            String countSqlStr = autoCountSql(page, boundSql.getSql());
            PluginUtils.MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);
            //構(gòu)建BoundSql
            countSql = new BoundSql(countMs.getConfiguration(), countSqlStr, mpBoundSql.parameterMappings(), parameter);
            PluginUtils.setAdditionalParameter(countSql, mpBoundSql.additionalParameters());
        }
        //查詢 count 數(shù)值
        List<Object> result = executor.query(countMs, parameter, rowBounds, resultHandler, cacheKey, countSql);
}

接下來，PaginationInnerInterceptor.beforeQuery(）生成分頁sql；

最終MybatisPlusInterceptor.intercept()里面的executor.query(）執(zhí)行分頁sql。

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