当 mysql-connector-java-5 遇上 MySQL8，终究还是错付了 → 门当户对真的很重要！

开心一刻

　　今天，老婆给我发消息

　　老婆：老公，儿子从隔壁邻居家回来了

　　老婆：是先打还是先洗？

　　我：先洗吧，万一打错人了呢

　　老婆：先洗脸吧，没错就边打边洗

起因

　　在我们的固有认知中， mysql-connector-java-5.x.x 连接的是 MySQL5 ，而 mysql-connector-java-8.x.x 连接的是 MySQL8 

　　如果用 mysql-connector-java-5.x.x 连接 MySQL8 ，或者用 mysql-connector-java-8.x.x 连接 MySQL5 ，会出问题

　　至于出什么问题，呃...，我没想过，也没试过，我相信你们也是一样的

　　但无意间，我尝试了下，确实有问题

　　一开始，我的一个 demo 是基于 MySQL 5.7.36 、 mysql-connector-java 5.1.26 、 MyBatis-Plus 3.1.0 

　　示例代码：play_it_safe

　　因为 MySQL 5.7.36 被我删了，而本机（ windows 10 ）上正好有 MySQL 8.0.26 

　　那就用 MySQL 8.0.26 来跑 demo 呗，只修改数据库连接的 url 、 username 和 password 

　　然后就出现了一系列的异常

java.math.BigInteger cannot be cast to java.lang.Long

　　启动直接异常

java.sql.SQLException: java.lang.ClassCastException: java.math.BigInteger cannot be cast to java.lang.Long
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1078) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:975) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:920) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.buildCollationMapping(ConnectionImpl.java:1074) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3593) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2534) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2304) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.26.jar:na]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:346) ~[mysql-connector-java-5.1.26.jar:na]
    at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:138) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:358) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:206) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:477) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:560) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) [HikariCP-3.4.5.jar:na]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.buildSqlSessionFactory(MybatisSqlSessionFactoryBean.java:486) [mybatis-plus-extension-3.1.1.jar:3.1.1]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.afterPropertiesSet(MybatisSqlSessionFactoryBean.java:434) [mybatis-plus-extension-3.1.1.jar:3.1.1]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.getObject(MybatisSqlSessionFactoryBean.java:633) [mybatis-plus-extension-3.1.1.jar:3.1.1]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration.sqlSessionFactory(MybatisPlusAutoConfiguration.java:190) [mybatis-plus-boot-starter-3.1.1.jar:3.1.1]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8802fc19.CGLIB$sqlSessionFactory$1(<generated>) [mybatis-plus-boot-starter-3.1.1.jar:3.1.1]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8802fc19$$FastClassBySpringCGLIB$$f9d1deeb.invoke(<generated>) [mybatis-plus-boot-starter-3.1.1.jar:3.1.1]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) [spring-core-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:331) [spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8802fc19.sqlSessionFactory(<generated>) [mybatis-plus-boot-starter-3.1.1.jar:3.1.1]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_91]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:650) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:635) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1336) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1176) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1509) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1404) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:878) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at com.qsl.PlayItSafeApplication.main(PlayItSafeApplication.java:16) ~[classes/:na]
Caused by: java.lang.ClassCastException: java.math.BigInteger cannot be cast to java.lang.Long
    at com.mysql.jdbc.ConnectionImpl.buildCollationMapping(ConnectionImpl.java:1019) ~[mysql-connector-java-5.1.26.jar:na]
    ... 64 common frames omitted

　　这确实出乎了我的意料，我认为的是：启动不会异常，运行中出现异常

　　既然异常来了，那就好好看看呗

　　怎么看？直接从异常堆栈中看，找到 Caused by 

　　直接点击框住的，来到 mysql-connector-java 5.1.26 的源码

　　这个 for 循环遍历的是 sortedCollationMap ， indexEntry 则表示遍历的当前元素

　　我问个问题： java.math.BigInteger cannot be cast to java.lang.Long 异常是谁抛出来的， getKey() 还是 intValue() ？

　　很简单，用排除法嘛

　　 Long 是有 intValue() 方法的，只会 NPE ，不会 java.math.BigInteger cannot be cast to java.lang.Long 

　　那就是 getKey() 抛出的异常呗，我们猜想下什么情况会抛出这个异常

　　 indexEntry 声明的类型虽然是 Map.Entry<Long, String> ，但实际它的 key 不是 Long 类型，而是 BigInteger 类型？

　　再往上追溯一下， sortedCollationMap 声明的类型是 TreeMap<Long, String> ，但实际存放元素的 key 是 BigInteger 类型？

　　此时你们对这个猜想肯定有疑问：这编译不会报错吗

　　直接 put 是会编译报错的，但是如果我变通下呢

　　编译没问题，但是执行会有如下异常

　　同样也是 int collationIndex = entry.getKey().intValue(); 抛出的异常

　　但两者原因是否一致，我们还需要对 play_it_safe 验证

　　如何验证， debug 调试呗

　　断点不能打在 connectionimpl.java:1019 这一行，因为此时是遍历 sortedCollationMap ，而非往里面 put 元素

　　我们需要看，往 sortedCollationMap 里面 put 的 key 具体是什么类型，所以断点打在 1002 行

　　会走 if 分支

　　F7 跟进去

　　 key 的类型就是 BigInteger ，证明我们的猜想是对的！

　　但是大家别高兴的太早，根因还没找到，我们得继续跟进 com.mysql.jdbc.ResultSetImpl#getObject(int) 

　　重点看这个 switch 

　　继续跟进，你会发现来到 BIGINT 分支

　　注意，重点来了

　　这段代码很简单，相信大家都能看到，但我还是分 3 块给大家说明下

　　1、 unsigned 表示无符号，然后再取反则表示有符号，也就是说如果数据库字段类型是有符号的 bigint ，则用 java.lang.Long 接收字段值并返回

　　2、如果数据库字段类型是无符号的 bigint ，则用 java.lang.String 来接收字段值，如果值为 null 则直接返回 null 

　　3、如果数据库字段类型是无符号的 bigint ，并且字段值不是 null ，则用 java.math.BigInteger 来接收字段值并返回

　　此刻相信大家对异常的根因有点明白了，但又没有完全明白，为什么了？

　　因为没有弄清楚来龙去脉，我给大家捋一下：

　　1、程序启动过程（初始化）中，mysql-connector-java 5.1.26 会构建字段级字符集/字符序到java字符编码名称的映射

　　　　也就是会执行 com.mysql.jdbc.ConnectionImpl#buildCollationMapping 方法

　　2、 buildCollationMapping 方法中会执行 SQL SHOW COLLATION 来获取 MySQL 的字符集和字符序

　　　　Id：字符序id

　　　　Charset：字符集名称

　　　　关于 MySQL 的字符集和字符序的更多信息，可查看记一次字符串末尾空白丢失的排查 → MySQL 是会玩的！

　　3、从查询结果集中，逐行取第 3 列（ Id  ）作为 key ，取第 2 列（ Charset ）作为 value ， put 到 sortedCollationMap 中

　　　　对应代码就是： Util.resultSetToMap(sortedCollationMap, results, 3, 2); 

　　4、在取 Id 列值的时候，会根据数据库列类型，用不同的 java 类型来接收列值

　　　　 MySQL 8.0.26 中， COLLATIONS 表的 ID 列的类型是 bigint unsigned

　　　　那么用什么 java 类型来接收？往上面翻一翻，是不是用 java.math.BigInteger 来接收？

　　　　 MySQL 5.7.36 中， COLLATIONS 表的 ID 列的类型却是 bigint(11)

　　　　所以用 java.lang.Long 来接收

　　5、 for 循环遍历 sortedCollationMap ，完成数据库字段级字符集/字符序到java字符编码名称的映射

　　　　 indexEntry.getKey() 在 MySQL 8.0.26 会抛出异常： java.math.BigInteger cannot be cast to java.lang.Long 

　　　　而在 MySQL 5.7.36 则正常，不抛出异常

　　至此，大家对根因是不是完全明白了？

　　说的简单点就是： mysql-connector-java 5.1.26 配不上 MySQL 8 ！

　　说的详细点就是： mysql-connector-java 5.1.26 适配的是 COLLATIONS 的 bigint 类型的 ID ，而非 bigint unsigned 类型的 ID 

　　说的抽象点就是：上岸第一剑 先斩意中人， MySQL 从 5 上岸到了 8， mysql-connector-java 如果不跟着上岸的话，会被第一个斩杀

Unknown system variable 'query_cache_size'

　　因为用的是都说了能不动就别动，非要去调整，出生产事故了吧中的 demo ：play_it_safe

　　所以我顺手将 mysql-connector-java 调整到了 5.1.37 ， MySQL 还是 8.0.26 ，结果启动出现了另一个异常

java.sql.SQLException: Unknown system variable 'query_cache_size'
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:959) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3870) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3806) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2470) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2617) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2546) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2504) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1370) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.loadServerVariables(ConnectionImpl.java:3861) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3289) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2298) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2084) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:795) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:327) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:138) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:358) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:206) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:477) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:560) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) [HikariCP-3.4.5.jar:na]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.buildSqlSessionFactory(MybatisSqlSessionFactoryBean.java:591) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.afterPropertiesSet(MybatisSqlSessionFactoryBean.java:422) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.getObject(MybatisSqlSessionFactoryBean.java:636) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration.sqlSessionFactory(MybatisPlusAutoConfiguration.java:172) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$eeb10e55.CGLIB$sqlSessionFactory$2(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$eeb10e55$$FastClassBySpringCGLIB$$de0ae008.invoke(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) [spring-core-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:331) [spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$eeb10e55.sqlSessionFactory(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_91]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:650) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:635) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1336) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1176) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1509) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1404) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:878) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at com.qsl.PlayItSafeApplication.main(PlayItSafeApplication.java:16) ~[classes/:na]

　　看到这个异常，不知道你们第一反应想到的是什么？

　　我第一反应想到的是 MySQL 8 取消了查询缓存，而 MySQL 5 是有查询缓存的

　　异常提示的已经很明显了，我们来验证下

　　 MySQL 5.7.36 是有系统变量 query_cache_size 

　　而 MySQL 8.0.26 是没有的

　　后面我又将 mysql-connector-java 升级到 5.1.42 ，启动还是异常： Unknown system variable 'query_cache_size' 

Could not create connection to database server

　　因为需要用 MySQL 5.7.36 做对比，所以还是 Docker 快速安装了一个 MySQL 5.7.36 

　　顺带也安装了 MySQL 8.0.26 （前面用的是 Win10 版，这个相当于是 Linux 版本）

　　然后我又鬼使神差的将连接调整到了 Linux 版的 MySQL 8.0.26 

　　当 mysql-connector-java 是 5.1.42 时，启动异常是： Unknown system variable 'query_cache_size' 

　　当 mysql-connector-java 是 5.1.37 时，启动异常是： Could not create connection to database server 

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.Util.getInstance(Util.java:387) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:919) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:898) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:887) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:862) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2331) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2084) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:795) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:327) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:138) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:358) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:206) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:477) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:560) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) [HikariCP-3.4.5.jar:na]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.buildSqlSessionFactory(MybatisSqlSessionFactoryBean.java:591) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.afterPropertiesSet(MybatisSqlSessionFactoryBean.java:422) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.getObject(MybatisSqlSessionFactoryBean.java:636) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration.sqlSessionFactory(MybatisPlusAutoConfiguration.java:172) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$b0a04890.CGLIB$sqlSessionFactory$2(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$b0a04890$$FastClassBySpringCGLIB$$b0fb4ddc.invoke(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) [spring-core-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:331) [spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$b0a04890.sqlSessionFactory(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_91]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:650) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:635) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1336) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1176) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1509) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1404) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:878) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at com.qsl.PlayItSafeApplication.main(PlayItSafeApplication.java:16) ~[classes/:na]
Caused by: java.lang.NullPointerException: null
    at com.mysql.jdbc.ConnectionImpl.getServerCharset(ConnectionImpl.java:3004) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1908) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1837) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1207) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2254) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2285) ~[mysql-connector-java-5.1.37.jar:5.1.37]
    ... 62 common frames omitted

　　关键异常堆栈是：

　　真正的异常其实是 NPE ，大家可以参考前面讲到的方法去分析

　　当 mysql-connector-java 是 5.1.26 时，启动异常也是： Could not create connection to database server 

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.Util.getInstance(Util.java:386) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:975) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:920) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2568) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2304) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.26.jar:na]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_91]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_91]
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:346) ~[mysql-connector-java-5.1.26.jar:na]
    at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:138) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:358) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:206) ~[HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:477) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:560) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) [HikariCP-3.4.5.jar:na]
    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) [HikariCP-3.4.5.jar:na]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.buildSqlSessionFactory(MybatisSqlSessionFactoryBean.java:591) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.afterPropertiesSet(MybatisSqlSessionFactoryBean.java:422) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.extension.spring.MybatisSqlSessionFactoryBean.getObject(MybatisSqlSessionFactoryBean.java:636) [mybatis-plus-extension-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration.sqlSessionFactory(MybatisPlusAutoConfiguration.java:172) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8bb2ad81.CGLIB$sqlSessionFactory$1(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8bb2ad81$$FastClassBySpringCGLIB$$fce7a55a.invoke(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) [spring-core-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:331) [spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration$$EnhancerBySpringCGLIB$$8bb2ad81.sqlSessionFactory(<generated>) [mybatis-plus-boot-starter-3.1.0.jar:3.1.0]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_91]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_91]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_91]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_91]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:650) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:635) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1336) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1176) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1509) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1404) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:878) ~[spring-beans-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551) ~[spring-context-5.2.9.RELEASE.jar:5.2.9.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.10.RELEASE.jar:2.2.10.RELEASE]
    at com.qsl.PlayItSafeApplication.main(PlayItSafeApplication.java:16) ~[classes/:na]
Caused by: java.lang.NullPointerException: null
    at com.mysql.jdbc.ConnectionImpl.getServerCharacterEncoding(ConnectionImpl.java:3279) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1940) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1866) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1252) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2486) ~[mysql-connector-java-5.1.26.jar:na]
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2519) ~[mysql-connector-java-5.1.26.jar:na]
    ... 62 common frames omitted

　　关键异常堆栈是：

总结

　　 MySQL 驱动版本和 MySQL 版本一定要门当户对，否则就会错付

　　要升就一起升，只升一边，就会出现升的一方看不上没升的那方：上岸第一剑 先斩意中人