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

Question

So, you have a

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
java.net.ConnectException: Connection refused

I'm quoting from this answer which also contains a step-by-step MySQL+JDBC tutorial:

If you get a SQLException: Connection refused or Connection timed out or a MySQL specific CommunicationsException: Communications link failure, then it means that the DB isn't reachable at all. This can have one or more of the following causes:

1. IP address or hostname in JDBC URL is wrong.
2. Hostname in JDBC URL is not recognized by local DNS server.
3. Port number is missing or wrong in JDBC URL.
4. DB server is down.
5. DB server doesn't accept TCP/IP connections.
6. DB server has run out of connections.
7. Something in between Java and DB is blocking connections, e.g. a firewall or proxy.
   
   

To solve the one or the other, follow the following advices:

1. Verify and test them with ping.
2. Refresh DNS or use IP address in JDBC URL instead.
3. Verify it based on my.cnf of MySQL DB.
4. Start the DB.
5. Verify if mysqld is started without the --skip-networking option.
6. Restart the DB and fix your code accordingly that it closes connections in finally.
7. Disable firewall and/or configure firewall/proxy to allow/forward the port.
   
   

See also:

• How should I connect to JDBC database / datasource in a servlet based application?
• Is it safe to use a static java.sql.Connection instance in a multithreaded system?

In my case, I needed to do a replacement of Localhost to the actual database server IP address

Instead of

 Connection con = DriverManager.getConnection(
 "jdbc:mysql://localhost:3306/DBname", "root", "root");

I needed

 Connection con = DriverManager.getConnection(
 "jdbc:mysql://192.100.0.000:3306/DBname", "root", "root");

I catch this exception when Java out of heap. If I try to put in RAM many data items - first I catch "Communications link failure" and next "OutOfMemoryError".

I logged it and I decrease memory consumption (delete 1/2 data) and all ok.

This com.mysql.jdbc.exceptions.jdbc4.CommunicationsException exception occurs if your database connection is idle for long time.

This idle connection returns true on connection.isClosed(); but if we try to execute statement then it will fire this exception so I will suggest to go with database pooling.

In my case, turn out to be that the version of mysql-connector-java was too old.

In my demo, I somehow use mysql-connector-java like this:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.9</version>
</dependency>

But in the develop environment, I use this:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.31</version>
</dependency>

And my MySQL version was 5.1.48(yes, it is old, just for mimic the product version). So I met the same error.

Since the reason is found, the solution is found, too. Match the version!

I've been having the same problem for hours. I'm using MAMP Server

Instead of using localhost:[Apache Port], use your MySQL port.

Below is the default MySQL Port for MAMP server.

String url = "jdbc:mysql://localhost:8889/db_name";

Connection conn = DriverManager.getConnection(url, dbUsername, dbPassword);

I might be barking up the wrong tree here, but your exception seems to indicate your MySQL server isn't available.

Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.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...

What happens if you try (from the terminal)

mysql -u username -p

You will be prompted for the password associated with the username. After you give the correct password does the mysql client connect?

You may have to start MySQL from the Preferences if not. You can also set it to run at startup.