Sql communication link failure ошибка

I have had the same problem in two of my programs. My error was this:

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.

I spent several days to solve this problem. I have tested many approaches that have been mentioned in different web sites, but non of them worked. Finally I changed my code and found out what was the problem. I’ll try to tell you about different approaches and sum them up here.

While I was seeking the internet to find the solution for this error, I figured out that there are many solutions that worked for at least one person, but others say that it doesn’t work for them! why there are many approaches to this error?
It seems this error can occur generally when there is a problem in connecting to the server. Maybe the problem is because of the wrong query string or too many connections to the database.

So I suggest you to try all the solutions one by one and don’t give up!

Here are the solutions that I found on the internet and for each of them, there is at least on person who his problem has been solved with that solution.

Tip: For the solutions that you need to change the MySQL settings, you can refer to the following files:

• Linux: /etc/mysql/my.cnf or /etc/my.cnf (depending on the Linux distribution and MySQL package used)

• Windows: C:**ProgramData**MySQLMySQL Server 5.6my.ini (Notice it’s ProgramData, not Program Files)

Here are the solutions:

• changing bind-address attribute:

  Uncomment bind-address attribute or change it to one of the following IPs:

   bind-address="127.0.0.1"
  

  or

   bind-address="0.0.0.0"
  

• commenting out «skip-networking»

  If there is a skip-networking line in your MySQL config file, make it comment by adding # sign at the beginning of that line.

• change «wait_timeout» and «interactive_timeout»

  Add these lines to the MySQL config file:

  [wait_timeout][1] = *number*
  
  interactive_timeout = *number*
  
  connect_timeout = *number*
  

• Make sure Java isn’t translating ‘localhost’ to [:::1] instead of [127.0.0.1]

  Since MySQL recognizes 127.0.0.1 (IPv4) but not :::1 (IPv6)

  This could be avoided by using one of two approaches:

  1. In the connection string use 127.0.0.1 instead of localhost to avoid localhost being translated to :::1

  2. Run java with the option -Djava.net.preferIPv4Stack=true to force java to use IPv4 instead of IPv6. On Linux, this could also be achieved by running (or placing it inside /etc/profile:

      export _JAVA_OPTIONS="-Djava.net.preferIPv4Stack=true"
     

• check Operating System proxy settings, firewalls and anti-virus programs

  Make sure the Firewall, or Anti-virus software isn’t blocking MySQL service.

  Stop iptables temporarily on linux. If iptables are misconfigured they may allow tcp packets to be sent to mysql port, but block tcp packets from coming back on the same connection.

   # Redhat enterprise and CentOS
   systemctl stop iptables.service
   # Other linux distros
   service iptables stop
  

  Stop anti-virus software on Windows.

• change connection string

  Check your query string. your connection string should be some thing like this:

  dbName = "my_database";
  dbUserName = "root";
  dbPassword = "";
  String connectionString = "jdbc:mysql://localhost/" + dbName + "?user=" + dbUserName + "&password=" + dbPassword + "&useUnicode=true&characterEncoding=UTF-8";
  

Make sure you don’t have spaces in your string. All the connection string should be continues without any space characters.

Try to replace «localhost» with the loopback address 127.0.0.1.
Also try to add port number to your connection string, like:

String connectionString = "jdbc:mysql://localhost:3306/my_database?user=root&password=Pass&useUnicode=true&characterEncoding=UTF-8";

Usually default port for MySQL is 3306.

Don’t forget to change username and password to the username and password of your MySQL server.

• update your JDK driver library file
• test different JDK and JREs (like JDK 6 and 7)
• don’t change max_allowed_packet

«max_allowed_packet» is a variable in MySQL config file that indicates the maximum packet size, not the maximum number of packets. So it will not help to solve this error.

• change tomcat security

change TOMCAT6_SECURITY=yes to TOMCAT6_SECURITY=no

• use validationQuery property

use validationQuery=»select now()» to make sure each query has responses

• AutoReconnect

Add this code to your connection string:

&autoReconnect=true&failOverReadOnly=false&maxReconnects=10

Although non of these solutions worked for me, I suggest you to try them. Because there are some people who solved their problem with following these steps.

But what solved my problem?

My problem was that I had many SELECTs on database. Each time I was creating a connection and then closing it. Although I was closing the connection every time, but the system faced with many connections and gave me that error. What I did was that I defined my connection variable as a public (or private) variable for whole class and initialized it in the constructor. Then every time I just used that connection. It solved my problem and also increased my speed dramatically.

#Conclusion#
There is no simple and unique way to solve this problem. I suggest you to think about your own situation and choose above solutions. If you take this error at the beginning of the program and you are not able to connect to the database at all, you might have problem in your connection string. But If you take this error after several successful interaction to the database, the problem might be with number of connections and you may think about changing «wait_timeout» and other MySQL settings or rewrite your code how that reduce number of connections.