java连接mysql数据库

stackoverflow:

无法获取数据库连接

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.  at sun.reflect.NativeConstructorAccesso

可能原因:

• 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.

解决办法:

• 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.

stackoverflow: com-mysql-jdbc-exceptions-jdbc4-communicationsexception-communications-link-fai

通过mysql show processlist 命令检查mysql锁的方法

http://blog.sina.com.cn/s/blog_54eeb5d90102wf9l.html