mybatis-plus分页插件失效探究

网上基本上都是推荐配置如下：

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

但是，仅仅这么做，就能达到我们的预期吗？其结局就是分页插件没有效果，原因是为什么呢？😔

图1

图2

通过对比上面两张图可以发现，图一DefaultSqlSession.selectList()底层调用Plugin.invoke();图二DefaultSqlSession.selectList()底层调用CachingExecutor.query()。其中，图一是分页插件生效的调用链，图二是分页插件失效的调用链。

也就是说，分页插件失效的原因是，mybatis-plusPlugin类没有为分页插件拦截器生成Executor代理。具体应该怎么做呢？像下面这样，在构建SqlSessionFactory时，需要在MybatisSqlSessionFactoryBean显示设置Plugin。

@Bean(name = "defaultSqlSessionFactory")
    public SqlSessionFactory defaultSqlSessionFactory(){
        MybatisSqlSessionFactoryBean bean = new MybatisSqlSessionFactoryBean();
        bean.setDataSource(dataSource);
        //设置拦截器
        bean.setPlugins(mybatisPlusInterceptor);
        SqlSessionFactory sqlSessionFactory = bean.getObject();
        //设置自动提交
        sqlSessionFactory.openSession(true);
        return sqlSessionFactory;
}

那么，为分页插件生成代理类是在什么时机生成呢？先公布答案：

//设置自动提交
sqlSessionFactory.openSession(true);

调用链如下：

图3

咱再看细节：
DefaultSqlSessionFactory.openSessionFromDataSource()详情：

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);
      //这步很关键，创建执行者实例
      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()详情：

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);
    }
    //对上面的executor进行代理（目的是把插件和执行器封装为代理类）
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

MybatisPlusInterceptor.pluginAll();

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

通过上面的重点code展示，我们大致了解了部分重要节点中分页插件代理类生成的逻辑。接下来我们继续了解具体分页插件工作的效果。

图4

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

}

接下来，PaginationInnerInterceptor.beforeQuery(）生成分页sql；最终MybatisPlusInterceptor.intercept()里面的executor.query(）执行分页sql。