已解决： JDBC连接Mysql失败报错： 'The last packet sent successfully to the server was 0 milliseconds ago. '

第一次尝试JDBC连接Mysql， 然后一直报错。

报错代码：

com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failureThe 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:172)    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:862)    at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:444)    at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:230)    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:226)    at connection.connectionTest.connectionTest1(connectionTest.java:44)    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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:220)    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:53)Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failureThe 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:59)    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:103)    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:149)    at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:165)    at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:92)    at com.mysql.cj.NativeSession.connect(NativeSession.java:152)    at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:982)    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:852)    ... 26 moreCaused by: java.net.ConnectException: Connection refused (Connection refused)    at java.net.PlainSocketImpl.socketConnect(Native Method)    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:476)    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:218)    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:200)    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:394)    at java.net.Socket.connect(Socket.java:606)    at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:173)    at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:66)    ... 29 more

运行结果及报错内容

尝试过修改时区、字符集、wait_timeout之类的，但是没有效果。然后驱动使用的是：“mysql-connector-java-8.0.17.jar”， mysql也是8.0版本以上的， 匹配应该没有问题。

url=jdbc:mysql://localhost:3306/test?serverTimezone=UTC&userUnicode=true&charsetEncoding=utf-8

url=jdbc:mysql://localhost:3306/testdriverClass=com.mysql.cj.jdbc.Driver

希望可以帮忙看看， 正在学习JDBC，但是第一步连接失败。

后来遇到的问题：

1. ping不通3306

2. sudo /usr/local/mysql/support-files/mysql.server startStarting MySQLSUCCESS! 

   开始mysql之后，连接仍然失败。
   打开Navicat又发现： Can't connect to local server through socket '/tmp/mysql.sock' (2)
   等等。 就是一系列的问题， 于是决定今天卸载mysql。

通过命令行无法删除mysql后，我直接在系统偏好设置的Mysql下选择uninstall。 然后在官网https://downloads.mysql.com/archives/community/
下载了符合电脑版本的dmg文件，直接安装。 选择的是Strong password， 之前选择的是legacy password(就是旧的方法). 之前第一次下载mysql后，尝试在Navicat上建立mysql连接经常失败， 后来勾选了使用套文件又把主机的位置那里写成“127.0.0.1”， 连接名写成"localhost"才成功。

现在卸载后重新安装后，竟然JDBC连接mysql成功了！谢谢大家！