我的Java程序无法连接到MySQL服务器

Question

Yesterday I spent several hours trying to solve this problem. When I try to connect to MySQL server from my Java program I get the CommunicationsException: Communications link failure . I have encountered several questions on Stackoverflow with similar problems but none of the solutions that were provided in the answers worked for me. For instance I have read an answer which advises to replace "localhost" with "127.0.0.1" or with my private IP address in the Driver.getConnection method, but that didn't solve the problem.I have read an answer which suggested that it should be checked whether the server is running. So, I typed sudo service mysql status and I saw that the server was active and operational. I have read an answer telling me to add the line Class.forName("com.mysql.jdbc.Driver"); so I did that but that didn't help either.

Here is my Java code

package com.mainpackage;
import java.sql.*;

public class Main {

    public static void main(String[] args) throws SQLException, ClassNotFoundException{
   
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/db1?user=userName&passwword=userPassword");
    }
}

Here is the entire message from the terminal that I have received:

java -classpath .:/home/javaUtil/MySQL/mysql-connector.jar com/mainpackage/Main

Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is com.mysql.cj.jdbc.Driver. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Exception in thread "main" 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:828)
    at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:448)
    at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:241)
    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:198)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:681)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:252)
    at com.mainpackage.Main.main(Main.java:9)
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 java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:480)
    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:89)
    at com.mysql.cj.NativeSession.connect(NativeSession.java:120)
    at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:948)
    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:818)
    ... 6 more
Caused by: java.net.ConnectException: Connection refused
    at java.base/sun.nio.ch.Net.connect0(Native Method)
    at java.base/sun.nio.ch.Net.connect(Net.java:579)
    at java.base/sun.nio.ch.Net.connect(Net.java:568)
    at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:588)
    at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
    at java.base/java.net.Socket.connect(Socket.java:633)
    at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:153)
    at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:63)
    ... 9 more

Edit: I don't have big enough reputation to respond in the comment, so I will add additional information here. I am able to connect to the server from terminal by writting mysql -u username -p . Everything works fine then.

Answer 1

I have found the solution to my problem and I want to share the solution in case someone else faces the same problem. As Jon Skeet suggested in his comment, the server, for whatever reason, wasn't listening on port 3306. I confirmed this by logging into the mysql server via terminal ( mysql -u root -p ) and then I wrote
SHOW GLOBAL VARIABLES LIKE 'PORT';
It said that port was 0 instead of 3306. Here is how I changed the port.
I went to /etc/mysql/mysql.conf.d/
then I edited the file which is named mysqld.cnf .
On my PC, that file was a read-only file, so I needed to first change the permissions.
I wrote sudo chmod +rw mysqld.cnf in the terminal while I was in the /etc/mysql/mysql.conf.d/ directory.
After that, I edited the file by typing sudo gedit mysqld.cnf in the terminal.
In the file, I added port = 3306 in a new line, below [mysqd], so that it looks like this

[mysqld]
port = 3306

I restarted the mysql server by writting sudo service mysql restart in terminal.

Just as a quick note, if you want to run the server with skip-grant-tables option, this is where you add skip-grant-tables, you add it on the same place that I added port = 3306.