关于mybatis-plus:mybatisplus如何禁用一级缓存

前言

用过mybatis-plus的敌人可能会晓得,mybatis-plus提供了多租户插件的性能,这个性能能够让开发人员不必手动写租户语句,由该插件主动帮你加上租户语句。明天的素材起源就是取自业务开发人员应用多租户插件时,遇到的一个神奇的问题

问题重现

业务开发人员要实现依据手机号码更新租户的明码性能,其代码形如下

 for(Tenant t : tenantList){
            ApplicationChainContext.getCurrentContext().put(ApplicationChainContext.TENANT_ID,t.getId()+"");
            Optional<SaasUser> user = this.findByUserPhone(req.getUserPhone());
            user.ifPresent(u -> {
                count.getAndSet(count.get() + 1);
                LambdaUpdateWrapper<SaasUser> wrapper = new LambdaUpdateWrapper<>();
                String md5Pwd = Md5Utils.hash(req.getNewUserPwd());
                wrapper.eq(SaasUser::getId,user.get().getId());
                wrapper.set(SaasUser::getUserPwd,md5Pwd);
                this.update(wrapper);
            });
        }

从代码上看起来没啥问题,因为应用了多租户插件,当咱们执行this.findByUserPhone(req.getUserPhone());就会主动带上租户的信息。但在执行的时候发现一个问题,如下图

从图中咱们能够发现,当查问不到用户信息时,后续的查问操作,都没有sql语句呈现。这阐明2点,sql语句要么被零碎吃了,要么零碎没有执行sql

问题剖析

后面说了sql语句没有打印进去,阐明sql要么没执行,要么就sql语句被零碎吃了。到底是哪种,与其猜想,倒不如去官网找找问题的答案,很遗憾在mybatis-plus官网或者issue上并没找到答案,于是只好跟踪源码进行剖析。最初发现mybatis-plus果然如他官网介绍的只做加强不做扭转,他最终调用查问的逻辑,走的是原生mybatis的查问逻辑。其查问的外围代码如下

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }

            List list;
            try {
                ++this.queryStack;
                list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
                if (list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                --this.queryStack;
            }

            if (this.queryStack == 0) {
                Iterator var8 = this.deferredLoads.iterator();

                while(var8.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)var8.next();
                    deferredLoad.load();
                }

                this.deferredLoads.clear();
                if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }

            return list;
        }
    }

从代码咱们能够得出一个重要的信息,如下

 list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
                if (list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }

当resultHandler为空 ,list的取值是this.localCache.getObject(key),即会走本地缓存,而不会进行数据库查问

问题破解

从源码能够得悉,原生的mybatis默认会走本地缓存,即所谓的一级缓存,而mybatis-plus作为mybatis的增强版,其逻辑和mybatis原生逻辑是一样的。那如何禁用mybatis-plus的一级缓存呢,从源码剖析,咱们能够得悉,当list为空时,则不会走缓存,而会查问数据。而list的缓存取值,来源于
this.localCache.getObject(key)。因而禁用缓存的逆向思维就是要么清空localCache,要么就是变更key,使this.localCache.getObject(key)取到的值为null。因而解决方案至多有以下两种

计划一:清空localCache

那要怎么清空?
通过源码咱们能够看到清空localCache的中央有两处,一处是

if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }

另外一处是

if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }

因而咱们能够通过变更configuration.getLocalCacheScope()为STATEMENT进行清空。能够通过在yml做如下配置

mybatis-plus:
    configuration:
        local-cache-scope: statement

计划二:变更localcache的key,使this.localCache.getObject(key)取到的值为null

咱们先看下key的形成

public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            CacheKey cacheKey = new CacheKey();
            cacheKey.update(ms.getId());
            cacheKey.update(rowBounds.getOffset());
            cacheKey.update(rowBounds.getLimit());
            cacheKey.update(boundSql.getSql());
            List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            Iterator var8 = parameterMappings.iterator();

            while(var8.hasNext()) {
                ParameterMapping parameterMapping = (ParameterMapping)var8.next();
                if (parameterMapping.getMode() != ParameterMode.OUT) {
                    String propertyName = parameterMapping.getProperty();
                    Object value;
                    if (boundSql.hasAdditionalParameter(propertyName)) {
                        value = boundSql.getAdditionalParameter(propertyName);
                    } else if (parameterObject == null) {
                        value = null;
                    } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                        value = parameterObject;
                    } else {
                        MetaObject metaObject = this.configuration.newMetaObject(parameterObject);
                        value = metaObject.getValue(propertyName);
                    }

                    cacheKey.update(value);
                }
            }

            if (this.configuration.getEnvironment() != null) {
                cacheKey.update(this.configuration.getEnvironment().getId());
            }

            return cacheKey;
        }
    }

从源码能够看出key是由statementId+原生sql+value(查问进去的对象)+ sqlsession.hashcode组成。

因而变更key,咱们能够从原生sql动手。看到这边可能有敌人会说,租户id失常不都不一样吗,既然mybatis-plus通过多租户插件,其产生的sql语句不都不一样吗,进而产生的key不就都不一样了吗。如果跟踪源码就会发现其原生的sql是没有加上租户信息的。因而咱们能够取巧在查问的sql语句中增加一个随机数,形如下

 public Optional<SaasUser> findByUserPhone(String userPhone) {
        LambdaQueryWrapper<SaasUser> wrapper = new LambdaQueryWrapper<>();
        wrapper.eq(SaasUser::getUserPhone,userPhone);
        wrapper.apply("{0} = {0}",UUID.randomUUID().toString());
        SaasUser saasUser = getBaseMapper().selectOne(wrapper);
        return Optional.ofNullable(saasUser);
    }

即在原先的查问代码语句多加

wrapper.apply("{0} = {0}",UUID.randomUUID().toString());

此时sql语句如下

 Preparing: SELECT id, user_code, user_name, main_accout_flag, user_pwd, user_phone, admin_flag, user_status, last_login_time, login_ip, pwd_update_time, tenant_id, create_date, created_by, created_by_id, last_updated_by, last_updated_by_id, last_update_date, object_version_number, delete_flag FROM saas_user WHERE delete_flag = 0 AND (user_phone = ? AND ? = ?) AND tenant_id = 424210194470490118 
==> Parameters: 111111(String), edcda7fe-ee43-481a-90f7-8b41cb51a3d1(String), edcda7fe-ee43-481a-90f7-8b41cb51a3d1(String)

这样每次产生的sql就会不一样,导致取到不一样key,进而使this.localCache.getObject(key)为空,这样就能够让mybatis每次都进行数据库查问,从而达到禁用一级缓存的目标

总结

计划一的配置是基于全局配置,计划二是基于部分配置。就集体而言,是比拟举荐计划二,即通过增加随机值的形式。因为mybatis配置一级缓存的意义,自身就是出于提供性能思考。不过计划要站在业务的视角进行思考,为了确保性能能正确运行,有时候就义一些性能也无伤大雅

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