java.sql.SQLException: Unknown system variable 'tx_isolation'

问题原因：mysql版本：8.0.21，mysql-connector-java版本：mysql-connector-java-5.1.7-bin.jar，版本不对齐导致报错。

解决办法：去 mysql-connector-java 下载与 mysql 对应的版本。

问题现象：

java.sql.SQLException: Unknown system variable 'tx_isolation'
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3515)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3447)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1951)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2101)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2548)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2477)
    at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1422)
    at com.mysql.jdbc.ConnectionImpl.getTransactionIsolation(ConnectionImpl.java:3131)
    at com.mchange.v2.c3p0.impl.NewPooledConnection.<init>(NewPooledConnection.java:107)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:198)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
    at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
    at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)