问题描述：使用JDBC连接Mysql数据库时，出现“Connection reset”错误，抛出如下异常：
Connection couldn't be established to jdbc:mysql://***/***?user=***&password=********
com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:
** BEGIN NESTED EXCEPTION **
com.mysql.jdbc.CommunicationsException
MESSAGE: Communications link failure due to underlying exception: ** BEGIN NESTED EXCEPTION **
java.net.SocketException
MESSAGE: Connection reset
STACKTRACE:java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(Unknown Source)
at com.mysql.jdbc.util.ReadAheadInputStream.fill(ReadAheadInputStream.java:113)
at com.mysql.jdbc.util.ReadAheadInputStream.readFromUnderlyingStreamIfNecessary(ReadAheadInputStream.java:160)
at com.mysql.jdbc.util.ReadAheadInputStream.read(ReadAheadInputStream.java:188)
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1910)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2304)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2803)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1573)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3170)
at com.mysql.jdbc.Connection.configureClientCharacterSet(Connection.java:2436)
at com.mysql.jdbc.Connection.initializePropsFromServer(Connection.java:3913)
at com.mysql.jdbc.Connection.createNewIO(Connection.java:2683)
at com.mysql.jdbc.Connection.<init>(Connection.java:1531)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
……** END NESTED EXCEPTION **
Last packet sent to the server was 203 ms ago.
STACKTRACE:
com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:
** BEGIN NESTED EXCEPTION ** java.net.SocketException
MESSAGE: Connection reset
STACKTRACE:
java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(Unknown Source)
at com.mysql.jdbc.util.ReadAheadInputStream.fill(ReadAheadInputStream.java:113)
at com.mysql.jdbc.util.ReadAheadInputStream.readFromUnderlyingStreamIfNecessary(ReadAheadInputStream.java:160)
at com.mysql.jdbc.util.ReadAheadInputStream.read(ReadAheadInputStream.java:188)
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1910)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2304)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2803)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1573)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3170)
at com.mysql.jdbc.Connection.configureClientCharacterSet(Connection.java:2436)
at com.mysql.jdbc.Connection.initializePropsFromServer(Connection.java:3913)
at com.mysql.jdbc.Connection.createNewIO(Connection.java:2683)
at com.mysql.jdbc.Connection.<init>(Connection.java:1531)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
……
** END NESTED EXCEPTION **
问题分析：这个错误是Java程序没有连接上Mysql，导致无法进行任何与数据库相关的操作。这个错误的本质是一个网络的问题。
问题解决：首先，需要分别排查以下情况：
(1)MySql服务器是否支持TCP/IP连接；
(2)服务器端以及运行Java程序的客户端是否启用了防火墙；
(3)JDBC的驱动是否正确合适；
(4)是否启用了IPv6？如果现在使用的IPv6，那么换成IPv4再试试看；
如果上述情况都排除之后，还是出现如上的异常错误，那么很有可能是你电脑所处的网络存在问题，你可以尝试一下换个地方上网运行这个包含JDBC的Java程序，或者换个时间再次运行。