在本文中，我们将为您详细介绍com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接的相关知识，此外，我们还会提供一些关于Cause:c
在本文中，我们将为您详细介绍com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接的相关知识，此外，我们还会提供一些关于Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully rece...、Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:、Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure、com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure的有用信息。
本文目录一览：- com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接
- Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully rece...
- Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:
- Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
- com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接
我正在使用Mac OS X
Lion版本10.8。我通过XAMPP安装了MySQL。并且我已经将mysql和jdbc驱动程序（5.1.22）添加到了我的类路径中。以下简单代码无法正常工作：
package learning.database;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class Simple {
Connection getConnection() throws SQLException{
String url = "jdbc:mysql://localhost/";
return DriverManager.getConnection(url,"username","password");
}
void createDatabase(String database,Connection conn) throws SQLException{
Statement stmt = null;
try {
stmt = conn.createStatement();
stmt.executeUpdate("drop database if exists "+database);
stmt.executeUpdate("create database "+database);
}finally {
if(null != stmt)
stmt.close();
}
}
public static void main(String args[]) throws Exception{
String school = "school";
Classroom cr = new Classroom();
Connection conn = cr.getConnection();
cr.createDatabase(school,conn);
}
}
运行代码时，出现以下错误：
Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1117)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:350)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2408)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2445)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2230)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:813)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:399)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:334)
at java.sql.DriverManager.getConnection(DriverManager.java:582)
at java.sql.DriverManager.getConnection(DriverManager.java:185)
at learning.database.Classroom.getConnection(Classroom.java:43)
at learning.database.Simple.main(Simple.java:33)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:432)
at java.net.Socket.connect(Socket.java:529)
at java.net.Socket.connect(Socket.java:478)
at java.net.Socket.<init>(Socket.java:375)
at java.net.Socket.<init>(Socket.java:218)
at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:259)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300)
... 16 more
有谁知道如何解决这个问题？
Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully rece...
今天访问已经架上服务器的网站，报错：
Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException:
The last packet successfully received from the server was 78,050,512 milliseconds ago.
The last packet sent successfully to the server was 78,050,512 milliseconds ago.
is longer than the server configured value of ''wait_timeout''.
You should consider either expiring and/or testing connection validity before use in your application,
increasing the server configured values for client timeouts,
or using the Connector/J connection property ''autoReconnect=true'' to avoid this problem.
翻译过来：上次访问时，发送的包已经多少多少秒了，比设置的‘wait_timeout’要长，建议修改 autoReconnect=true。
解决方案：
修改或添加三个参数：将testConnectionOnCheckout 设为 false
将testConnectionOnCheckin 设为 true
将idleConnectionTestPeriod 设为 30，这个数字要根据项目情况设定，比8小时小就好具体原理参考：https://blog.csdn.net/frankcheng5143/article/details/50589264
Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:
### Error updating database. Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
### The error may involve com.qingmu.core.dao.good.BrandDao.insertSelective-Inline
### The error occurred while setting parameters
### SQL: insert into tb_brand
### Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
; bad SQL grammar []; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1, dubbo version: 2.8.4, current host: 127.0.0.1
org.springframework.jdbc.BadSqlGrammarException:
### Error updating database. Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
### The error may involve com.qingmu.core.dao.good.BrandDao.insertSelective-Inline
### The error occurred while setting parameters
### SQL: insert into tb_brand
### Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
; bad SQL grammar []; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
at org.springframework.jdbc.support.SQLErrorCodeSQLExceptionTranslator.doTranslate(SQLErrorCodeSQLExceptionTranslator.java:231)
at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:73)
at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:73)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:371)
at com.sun.proxy.$Proxy11.insert(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate.insert(SqlSessionTemplate.java:240)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:51)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:52)
at com.sun.proxy.$Proxy13.insertSelective(Unknown Source)
at com.qingmu.core.service.BrandServiceImpl.add(BrandServiceImpl.java:64)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:302)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:208)
at com.sun.proxy.$Proxy18.add(Unknown Source)
at com.alibaba.dubbo.common.bytecode.Wrapper0.invokeMethod(Wrapper0.java)
at com.alibaba.dubbo.rpc.proxy.javassist.JavassistProxyFactory$1.doInvoke(JavassistProxyFactory.java:46)
at com.alibaba.dubbo.rpc.proxy.AbstractProxyInvoker.invoke(AbstractProxyInvoker.java:72)
at com.alibaba.dubbo.rpc.protocol.InvokerWrapper.invoke(InvokerWrapper.java:53)
at com.alibaba.dubbo.rpc.filter.ExceptionFilter.invoke(ExceptionFilter.java:64)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.monitor.support.MonitorFilter.invoke(MonitorFilter.java:75)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.filter.TimeoutFilter.invoke(TimeoutFilter.java:42)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.protocol.dubbo.filter.TraceFilter.invoke(TraceFilter.java:78)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.filter.ContextFilter.invoke(ContextFilter.java:70)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.filter.GenericFilter.invoke(GenericFilter.java:132)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.filter.ClassLoaderFilter.invoke(ClassLoaderFilter.java:38)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.filter.EchoFilter.invoke(EchoFilter.java:38)
at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
at com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol$1.reply(DubboProtocol.java:113)
at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.handleRequest(HeaderExchangeHandler.java:84)
at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(HeaderExchangeHandler.java:170)
at com.alibaba.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:52)
at com.alibaba.dubbo.remoting.transport.dispatcher.ChannelEventRunnable.run(ChannelEventRunnable.java:82)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '''' at line 1
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:408)
at com.mysql.jdbc.Util.getInstance(Util.java:383)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1062)
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:2826)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2082)
at com.mysql.jdbc.PreparedStatement.execute(PreparedStatement.java:1302)
at com.alibaba.druid.pool.DruidPooledPreparedStatement.execute(DruidPooledPreparedStatement.java:493)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.ibatis.logging.jdbc.PreparedStatementLogger.invoke(PreparedStatementLogger.java:62)
at com.sun.proxy.$Proxy26.execute(Unknown Source)
at org.apache.ibatis.executor.statement.PreparedStatementHandler.update(PreparedStatementHandler.java:44)
at org.apache.ibatis.executor.statement.RoutingStatementHandler.update(RoutingStatementHandler.java:69)
at org.apache.ibatis.executor.SimpleExecutor.doUpdate(SimpleExecutor.java:48)
at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:105)
at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:71)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:62)
at com.sun.proxy.$Proxy24.update(Unknown Source)
at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:152)
at org.apache.ibatis.session.defaults.DefaultSqlSession.insert(DefaultSqlSession.java:141)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:358)
... 47 more由于我的sql语句是使用逆向工程生成的,然后sql语句拼写错误,就可以省略,
找到的原因是在页面想后端发送数据的时候,是以json格式进行传输的,然后没有解析,想直接存入到数据库中,
报出来的sql语句异常
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
很长的报错，截取
-
ERROR c.a.d.p.DruidDataSource - discard connection
-
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
-
-
The last packet successfully received from the server was 44,866 milliseconds ago. The last packet sent successfully to the server was 0 milliseconds ago.
-
at sun.reflect.GeneratedConstructorAccessor103.newInstance(Unknown Source) ~[na:na]
-
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java: 45) ~[na:1.7.0_79]
-
at java.lang.reflect.Constructor.newInstance(Constructor.java: 526) ~[na:1.7.0_79]
-
at com.mysql.jdbc.Util.handleNewInstance(Util.java: 404) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java: 981) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java: 3465) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java: 3365) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java: 3805) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java: 2478) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java: 2625) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java: 2551) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java: 1861) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java: 1962) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.alibaba.druid.pool.DruidPooledPreparedStatement.executeQuery(DruidPooledPreparedStatement.java: 227) ~[druid-1.0.5.jar:1.0.5]
-
at com.jfinal.plugin.activerecord.DbPro.query(DbPro.java: 73) [jfinal-2.2.jar:na]
-
at com.jfinal.plugin.activerecord.DbPro.query(DbPro.java: 100) [jfinal-2.2.jar:na]
-
at com.jfinal.plugin.activerecord.Db.query(Db.java: 47) [jfinal-2.2.jar:na]
-
...
-
-
Caused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost.
-
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java: 2957) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java: 3375) ~[mysql-connector-java-5.1.38.jar:5.1.38]
-
... 55 common frames omitted
参考链接:
http://blog.csdn.net/pandajava/article/details/41946251
http://blog.csdn.net/shiqidide/article/details/7642531
https://stackoverflow.com/questions/2983248/com-mysql-jdbc-exceptions-jdbc4-communicationsexception-communications-link-fai
起因和现象：
项目运行后，莫名其妙就在log中看到这个报错，但是数据也能获取到，不影响日常使用，但是看到了总归不爽。
看了下网上的原因：
MySQL服务器默认的“wait_timeout”是28800秒即8小时，意味着如果一个连接的空闲时间超过8个小时，MySQL将自动断开该连接，而连接池却认为该连接还是有效的(因为并未校验连接的有效性)，当应用申请使用该连接时，就会导致上面的报错。
出现异常”The last packet sent successfully to the server was 0 milliseconds ago.“的大部分原因
是由于数据库回收了连接，而系统的缓冲池不知道，继续使用被回收的连接所致的。
解决方法：
1.JDBC的URL中加上属性（旧版本可用，不推荐）
按照错误的提示，可以在JDBC URL中使用autoReconnect属性，实际测试时使用了autoReconnect=true&failOverReadOnly=false，不过并未起作用，
使用的是5.1版本，可能真像网上所说的只对4之前的版本有效。
2.修改MYSQL的配置文件my.ini/my.cnf，添加超时等待参数的最长时间（推荐）
没办法，只能修改MySQL的参数了，wait_timeout最大为31536000即1年，在my.cnf中加入：
[mysqld]
wait_timeout=86400 (1天)
interactive_timeout=7200
重启生效，需要同时修改这两个参数。
3.修改配置，让缓冲池验证链接是否有效
#SQL查询,用来验证从连接池取出的连接
dbcp.validationQuery=SELECT 1
#指明连接是否被空闲连接回收器(如果有)进行检验，如果检测失败，则连接将被从池中去除
dbcp.testWhileIdle=true
#在空闲连接回收器线程运行期间休眠的时间值,以毫秒为单位，一般比minEvictableIdleTimeMillis小
dbcp.timeBetweenEvictionRunsMillis=300000
#在每次空闲连接回收器线程(如果有)运行时检查的连接数量，最好和maxActive一致
dbcp.numTestsPerEvictionRun=50
#连接池中连接，在时间段内一直空闲，被逐出连接池的时间(1000*60*60)，以毫秒为单位
dbcp.minEvictableIdleTimeMillis=3600000
如果在wait_timeout秒期间内，数据库连接(java.sql.Connection)一直处于等待状态，mysql5就将该连接关闭。这时，你的Java应用的连接池仍然合法地持有该连接的引用。当用该连接来进行数据库操作时，就碰到上述错误。这解释了为什么我的程序第二天不能登录 的问题。
你可能会想到在tomcat的数据源配置中有没有办法解决？的确，在jdbc连接url的配置中，你可以附上“autoReconnect=true”，但这仅对mysql5以前的版本起作用。增加“validation query”似乎也无济于事。
原因分析
当数据库重启或数据库空闲连接超过设置的最大timemout时间，数据库会强行断开已有的链接，最大timeout时间可以通过命令show global variables like "wait_timeout";查询：
mysql> show global variables like "wait_timeout";
+---------------+-------+
| VARIABLE_NAME | VALUE |
+---------------+-------+
| wait_timeout | 28800 |
+---------------+-------+
1 row in set (0.00 sec)解决办法
为了解决这个异常，我们在配置数据库连接池的时候需要做一些检查连接有效性的配置，这里以Druid为例，相关配置如下（更多配置）：
| 字段名 | 默认值 | 说明 |
|---|---|---|
| validationQuery | 用来检测连接是否有效的sql，要求是一个查询语句，常用select ''x''。如果validationQuery为null，testOnBorrow、testOnReturn、testWhileIdle都不会起作用。 | |
| validationQueryTimeout | 单位：秒，检测连接是否有效的超时时间。底层调用jdbc Statement对象的void setQueryTimeout(int seconds)方法 | |
| testOnBorrow | true | 申请连接时执行validationQuery检测连接是否有效，做了这个配置会降低性能。 |
| testOnReturn | false | 归还连接时执行validationQuery检测连接是否有效，做了这个配置会降低性能。 |
| testWhileIdle | false | 建议配置为true，不影响性能，并且保证安全性。申请连接的时候检测，如果空闲时间大于timeBetweenEvictionRunsMillis，执行validationQuery检测连接是否有效。 |
| timeBetweenEvictionRunsMillis | 1分钟（1.0.14） | 有两个含义：1) Destroy线程会检测连接的间隔时间，如果连接空闲时间大于等于minEvictableIdleTimeMillis则关闭物理连接。2) testWhileIdle的判断依据，详细看testWhileIdle属性的说明 |
为了避免空闲时间过长超过最大空闲时间而被断开，我们设置三个配置：
validationQuery: SELECT 1
testWhileIdle: true timeBetweenEvictionRunsMillis: 28000其中timeBetweenEvictionRunsMillis需要小于mysql的wait_timeout。
但是这种方法无法避免重启的情况，不过一般数据库不会频繁重启，影响不大，如果非得频繁重启，可以通过设置testOnBorrow，即申请连接的时候先试一试连接是否可用，不过带来的影响就是性能降低，需要根据实际需求合理取舍。
转自:
https://blog.csdn.net/tuntun1120/article/details/78866991
com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure
com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:827)
at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:447)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:237)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:199)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
at java.sql.DriverManager.getConnection(DriverManager.java:247)
at com.baomidou.mybatisplus.generator.config.DataSourceConfig.getConn(DataSourceConfig.java:183)
at com.baomidou.mybatisplus.generator.config.builder.ConfigBuilder.handlerDataSource(ConfigBuilder.java:270)
at com.baomidou.mybatisplus.generator.config.builder.ConfigBuilder.<init>(ConfigBuilder.java:136)
at com.baomidou.mybatisplus.generator.AutoGenerator.execute(AutoGenerator.java:92)
at com.chinanums.order.admin.code.CodeGenerator.main(CodeGenerator.java:104)
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:151)
at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:167)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:91)
at com.mysql.cj.NativeSession.connect(NativeSession.java:150)
at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:947)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:817)
... 10 more
Caused by: java.net.ConnectException: Connection timed out: connect
at java.net.DualStackPlainSocketImpl.connect0(Native Method)
at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:155)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:65)
... 13 more
Exception in thread "main" java.lang.NullPointerException
at com.baomidou.mybatisplus.generator.config.builder.ConfigBuilder.getTablesInfo(ConfigBuilder.java:442)
at com.baomidou.mybatisplus.generator.config.builder.ConfigBuilder.handlerStrategy(ConfigBuilder.java:282)
at com.baomidou.mybatisplus.generator.config.builder.ConfigBuilder.<init>(ConfigBuilder.java:143)
at com.baomidou.mybatisplus.generator.AutoGenerator.execute(AutoGenerator.java:92)
at com.chinanums.order.admin.code.CodeGenerator.main(CodeGenerator.java:104)因为是使用的公司内的数据库，没有连接到数据库，所以才报这样的异常。
能够连接上公司内的数据库，就不会出现这样的异常情况了’
今天关于com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接的讲解已经结束，谢谢您的阅读，如果想了解更多关于Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully rece...、Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:、Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure、com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure的相关知识，请在本站搜索。
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