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Solving a “communications link failure” with JDBC and MySQL [duplicate]

• 对于此问题，找了很大概4天才解决。
• 描述
  
  • jdbc时不时地抛出此错误，不是很频繁第抛错。
  • 试了很多种，比如更改dbpool,更改mysql的配置等等，都不起作用
  • 解决：是mysql所在的服务器内存不够导致。mysql服务器上运行着其他很多php脚本，导致留给Mysql的内存不足1g,但mysql没有报很明显的内存不够错误，这个点很郁闷，只有一总方法一种方法地去试，最终解决了。
  • 下面是查找相关解决方案时，发现的一个比较好的回答：
    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 spend some 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.

point: 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\MySQL\MySQL Server 5.6\my.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 = 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:

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

Option #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:

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.

http://stackoverflow.com/questions/6865538/solving-a-communications-link-failure-with-jdbc-and-mysql