com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

具体报错–MySQL服务没启动同样报此错

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:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:981)
    at com.mysql.jdbc.MysqlIO.readPacket(MysqlIO.java:628)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1014)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2255)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2286)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2085)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:795)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:327)
    at org.mybatis.generator.internal.db.ConnectionFactory.getConnection(ConnectionFactory.java:68)
    at org.mybatis.generator.config.Context.getConnection(Context.java:526)
    at org.mybatis.generator.config.Context.introspectTables(Context.java:436)
    at org.mybatis.generator.api.MyBatisGenerator.generate(MyBatisGenerator.java:222)
    at org.mybatis.generator.api.MyBatisGenerator.generate(MyBatisGenerator.java:133)
    at GeneratorSqlmap.generator(GeneratorSqlmap.java:27)
    at GeneratorSqlmap.main(GeneratorSqlmap.java:33)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
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)
    at com.mysql.jdbc.MysqlIO.readPacket(MysqlIO.java:560)
    ... 25 more

分析

报错问题： The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server. 最后一个发送成功的数据包在0秒前 没有收到任何来自服务器的数据包。

简单来说就是连不上MySQL服务器了

检查MySQL连接是否有错 这时候你就得检查下你的msyql连接url有没有写错了

driverClass="com.mysql.jdbc.Driver" connectionURL="jdbc:mysql://IP:3306/库名" userId="root" password="密码"

ip 端口 库名 仔细检查下

如果到这还没有解决问题 那么继续往下看

检查Hosts文件 查看jdbc中的IP是否在hosts文件中有解析

检查MySQL配置文件 查看是否绑定ip地址 如果有这行 注释bind-address=xxx.xxx.xxx.xxx

检查MySQL运行在哪个端口

mysql> show global variables like 'port';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| port          | 3306  |
+---------------+-------+
1 row in set (0.10 sec)

检查MySQL服务是否真正启动 telnet Ip 3306

检查MySQL是否不接受远程连接

SELECT USER,HOST FROM user;
//正确
+-----------+-----------+
| USER      | HOST      |
+-----------+-----------+
| root      | %         |
| mysql.sys | localhost |
| root      | localhost |
+-----------+-----------+

如果没有授权的话 mysql> grant all privileges on . to root@’%’ identified by ‘我是密码‘;

检查MySQL是否已经用完了所有连接 重启MySQL 出现运行程序 如果不报错就是此问题

检查有没有什么东西在阻止Java程序和MySQL连接 比如关闭防火墙 或者设置防火墙规则 比如有没有开什么代理 比如拔网线 再连接MySQL试试