Could not open JPA EntityManager for transaction; nested exception is javax.persistence.PersistenceException

Question

when i run my code, i get this exception:

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

The last packet sent successfully         


        

Answer 1

I was not unable to figure out the exact cause, but I realized that it was because of some thread pools since I was able to connect to the db using the terminal. for checking the connectivity you can use the below command.

psql -h hostname -U username -d database

so restarting the server worked for me.

Answer 2

DB is down. You must start the DB server and check connectivity with either telnet or mysql command line client first.

Answer 3

There is CommunicationsException: Communications link failure.

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:

• IP address or hostname in JDBC URL is wrong.
• Hostname in JDBC URL is not recognized by local DNS server.
• Port number is missing or wrong in JDBC URL.
• DB server is down.
• DB server doesn't accept TCP/IP connections.
• DB server has run out of connections.
• 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:

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

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