Docker容器应用连接同主机MySQL容器失败，报CommunicationsException

Hey there, I’ve walked through your scenario and can spot a few key issues that are preventing your app container from connecting to MySQL. Let’s break down the fixes and troubleshooting steps:

Core Problem Breakdown #

The main issue here is how you’re addressing the MySQL container from your app container:

• Inside a Docker container, 127.0.0.1 refers to the container’s own loopback interface, not the host machine or the MySQL container. Even with --link mysql, using 127.0.0.1 in your JDBC URL makes the app try to connect to its own 3306 port (which isn’t running MySQL).
• Additionally, your MySQL configuration is set to only listen on its internal loopback (bind-address=127.0.0.1), blocking connections from other containers.

Step-by-Step Fixes & Troubleshooting #

1. Fix MySQL’s Network Listening Configuration #

You’ll need to update MySQL to accept connections from any network interface (so other containers can reach it):

• Enter your MySQL container: docker exec -it mysql bash
• Edit /etc/my.cnf and change bind-address=127.0.0.1 to bind-address=0.0.0.0
• Save the file and restart the MySQL container: docker restart mysql

2. Update Your App’s JDBC URL to Use the Linked Container Name #

When you use --link mysql, Docker adds a hostname entry for mysql in your app container’s /etc/hosts file, pointing directly to the MySQL container’s IP. Update your app’s startup command to use this hostname instead of 127.0.0.1:

docker run -d ... -e DB_URL=jdbc:mysql://mysql:3306/mydatabase --name myApp --restart=always --link mysql myApp_t:latest

3. Verify MySQL User Permissions #

Even if you set up permissions for local access, you need to ensure your MySQL user is allowed to connect from other containers (which are on a different network address):

• Enter the MySQL container and log into MySQL: mysql -u root -p
• Run these commands (replace youruser and yourpassword with your actual credentials):

  GRANT ALL PRIVILEGES ON mydatabase.* TO 'youruser'@'%' IDENTIFIED BY 'yourpassword';
  FLUSH PRIVILEGES;
  

The % wildcard allows the user to connect from any address, which works for container-to-container communication.

4. Test Container-to-Container Network Connectivity #

To confirm the network link is working:

• Enter your app container: docker exec -it myApp bash
• Ping the MySQL container using the linked hostname: ping mysql
  • If you get responses, the network is working. If not, check that both containers are on the same Docker network (the default bridge network should work with --link, but you can verify with docker network inspect bridge to see if both containers are listed).

5. Check Firewall/Port Forwarding (If Needed) #

Since your local Workbench can connect via 127.0.0.1:3306, you likely have port forwarding set up for the MySQL container, but this isn’t needed for container-to-container communication. Just ensure you’re relying on Docker’s internal network for the app-to-MySQL connection, not the host’s port.

After making these changes, restart your app container and check the logs again—this should resolve the communications link failure.

内容的提问来源于stack exchange，提问作者user3595231