使用C3P0连接MySQL 8.0.19数据库出现异常：Unknown system variable 'tx_isolation'

在做架构迁移的时候，使用C3P0连接MySQL 8.0.19数据库出现如下异常：

java.sql.SQLException: Unknown system variable 'tx_isolation' at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1094) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4208) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4140) at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2597) at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2758) at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2820) at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2769) at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1569) at com.mysql.jdbc.ConnectionImpl.getTransactionIsolation(ConnectionImpl.java:3372) at com.mchange.v2.c3p0.impl.NewPooledConnection.(NewPooledConnection.java:70) at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:98) at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1.acquireResource(C3P0PooledConnectionPool.java:83) at com.mchange.v2.resourcepool.BasicResourcePool.assimilateResource(BasicResourcePool.java:884) at com.mchange.v2.resourcepool.BasicResourcePool.acquireUntil(BasicResourcePool.java:601) at com.mchange.v2.resourcepool.BasicResourcePool.access$400(BasicResourcePool.java:31) at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1079) at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:354)

原因：在低版本的MySQL中，tx_isolation是作为transaction_isolation（查询事务隔离级别）的别名被应用的，高版本已经弃用。

解决方法：升级mysql-connector-java.jar的版本。