我的代码用jRuby编写，我使用Warbler部署在Tomcat中。我正在使用Sequel查询MySQL数据库。我正在使用两层数据库连接池。一种是本机Sequel池，另一种是Tomcat级别的JNDI池。连接字符串为：
DB = Sequel.connect("jdbc:jndi:java:comp/env/test", :logger => $db_log, :max_connections => 10)
此连接字符串在app.rb中定义，仅在进行全新部署或重新启动Tomcat时才会加载。这将创建一个Sequel连接池，并且所有线程都共享该池。我的$CATALINA_OME/conf/context.xml中的JNDI配置为：
<Resource
name="test"
auth="Container"
type="javax.sql.DataSource"
maxActive="10"
maxIdle="5"
maxWait="9000"
username="test_db"
password="test_db"
driverClassName="com.mysql.jdbc.Driver"
testOnBorrow="true"
testWhileIdle="true"
validationQuery="SELECT 1" testOnReturn="true"
timeBetweenEvictionRunsMillis="300000" removeAbandonedTimeout="60"
removeAbandoned="true"
logAbandoned="true"
url="jdbc:mysql://IP:3306/test"
/>
我正在使用DB.disconnect将连接返回到JNDI池，以确保没有线程使用以前使用的连接。我这样做是为了确保wait_timeout错误。 auto_reconnect=true似乎无法正确解决wait_timeout问题。一切正常，直到几天前我开始出现如下错误：
W, [2012-07-26T05:10:30.999000 #29456] WARN -- : 134325951259087 == NativeException: 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.
D, [2012-07-26T05:10:31.001000 #29456] DEBUG -- : 134325951259087 == ["sun.reflect.GeneratedConstructorAccessor23:-1:in `newInstance'", "sun/reflect/DelegatingConstructorAccessorImpl.java:27:in `newInstance'", "java/lang/reflect/Constructor.java:513:in `newInstance'", "com/mysql/jdbc/Util.java:409:in `handleNewInstance'", "com/mysql/jdbc/SQLError.java:1118:in `createCommunicationsException'", "com/mysql/jdbc/MysqlIO.java:343:in `<init>'", "com/mysql/jdbc/ConnectionImpl.java:2308:in `connectOneTryOnly'", "com/mysql/jdbc/ConnectionImpl.java:2122:in `createNewIO'", "com/mysql/jdbc/ConnectionImpl.java:774:in `<init>'", "com/mysql/jdbc/JDBC4Connection.java:49:in `<init>'", "sun.reflect.GeneratedConstructorAccessor25:-1:in `newInstance'", "sun/reflect/DelegatingConstructorAccessorImpl.java:27:in `newInstance'", "java/lang/reflect/Constructor.java:513:in `newInstance'", "com/mysql/jdbc/Util.java:409:in `handleNewInstance'"
和
W, [2012-07-26T10:19:14.029000 #1572] WARN -- : 134327815053548 == NativeException: java.sql.SQLException: Connection com.mysql.jdbc.JDBC4Connection@5a0655d is closed.
D, [2012-07-26T10:19:14.030000 #1572] DEBUG -- : 134327815053548 == ["org/apache/tomcat/dbcp/dbcp/DelegatingConnection.java:398:in `checkOpen'", "org/apache/tomcat/dbcp/dbcp/DelegatingConnection.java:255:in `createStatement'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/adapters/jdbc.rb:523:in `statement'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/adapters/jdbc.rb:233:in `execute'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/connection_pool/threaded.rb:88:in `hold'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/database/connecting.rb:234:in `synchronize'", "file:/usr/local/tomcat-instance/testv/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/adapters/jdbc.rb:232:in `execute'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/lib/santa-gems.jar!/gems/sequel-3.34.0/lib/sequel/dataset/actions.rb:744:in `execute'", "file:/usr/local/tomcat-instance/test/webapps/service/WEB-INF/l
这些错误的发生频率太高，无法忽略。
发生的唯一变化是数据库主机，但是我确保超时变量保持不变。值是：
interactive_timeout=300
connect_timeout=300
wait_timeout=10
还有其他可以创造差异的地方吗？
“通信链接失败”通常意味着您的程序无法连接到数据库[[at all]]。典型的问题是主机名和端口号不正确（很容易再次检查），或者程序和数据库之间存在一个或多个防火墙（包括软件和硬件）。最好（如果可以的话）使用命令行mysql实用程序来验证您可以从运行应用程序的计算机连接到数据库服务器。如果可行，则需要检查<Resource>配置。如果它不起作用，那么您将需要检查防火墙的存在。
请注意，例如，大多数最新版本的Microsoft Windows都默认启用了防火墙，这可能不允许您的程序通过无法识别的端口进行外部连接（3306）。因此，请先检查您的本地（即在运行webapp的计算机上）防火墙配置，然后与您的网络管理员联系，以查看是否还有其他防火墙无法解决问题。