com.mysql.jdbc.exceptions.jdbc4.CommunicationsException MySQL + 阿帕奇 Tomcat 7
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException MySQL + Apache Tomcat 7
我的应用程序 (Spring 4 MVC / Hibernate 4 / MySQL / Maven) 使用基于注释的配置将 Spring 与 Hibernate 集成。几个小时不使用数据库后，我总是看到这个错误：
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully received from the server was 84,510,841 milliseconds ago. The last packet sent successfully to the server was 84,510,841 milliseconds ago. is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
java.lang.reflect.Constructor.newInstance(Constructor.java:526)
com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1121)
com.mysql.jdbc.MysqlIO.send(MysqlIO.java:3938)
com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2551)
com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2731)
com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2809)
com.mysql.jdbc.ConnectionImpl.setAutoCommit(ConnectionImpl.java:5333)
sun.reflect.GeneratedMethodAccessor58.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:606)
org.apache.tomcat.jdbc.pool.ProxyConnection.invoke(ProxyConnection.java:126)
org.apache.tomcat.jdbc.pool.JdbcInterceptor.invoke(JdbcInterceptor.java:109)
org.apache.tomcat.jdbc.pool.DisposableConnectionFacade.invoke(DisposableConnectionFacade.java:80)
com.sun.proxy.$Proxy33.setAutoCommit(Unknown Source)
org.hibernate.resource.jdbc.internal.AbstractLogicalConnectionImplementor.begin(AbstractLogicalConnectionImplementor.java:67)
org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl.begin(LogicalConnectionManagedImpl.java:238)
org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.begin(JdbcResourceLocalTransactionCoordinatorImpl.java:213)
org.hibernate.engine.transaction.internal.TransactionImpl.begin(TransactionImpl.java:52)
org.hibernate.internal.SessionImpl.beginTransaction(SessionImpl.java:1512)
org.hibernate.jpa.internal.TransactionImpl.begin(TransactionImpl.java:45)
org.springframework.orm.jpa.vendor.HibernateJpaDialect.beginTransaction(HibernateJpaDialect.java:170)
org.springframework.orm.jpa.JpaTransactionManager.doBegin(JpaTransactionManager.java:380)
org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(AbstractPlatformTransactionManager.java:373)
org.springframework.transaction.interceptor.TransactionAspectSupport.createTransactionIfNecessary(TransactionAspectSupport.java:427)
org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:276)
org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:208)
com.sun.proxy.$Proxy118.findAll(Unknown Source)
com.ideefe.iot.controller.AppController.listDevices(AppController.java:49)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:606)
org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:222)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:814)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:737)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.web.multipart.support.MultipartFilter.doFilterInternal(MultipartFilter.java:118)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
这是 MySQL my.cnf 文件：
devuser@localhost:/etc/apache2$ more /etc/mysql/my.cnf
#
# The MySQL database server configuration file.
#
# You can copy this to one of:
# - "/etc/mysql/my.cnf" to set global options,
# - "~/.my.cnf" to set user-specific options.
#
# One can use all long options that the program supports.
# Run program with --help to get a list of available options and with
# --print-defaults to see which it would actually understand and use.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html
# This will be passed to all mysql clients
# It has been reported that passwords should be enclosed with ticks/quotes
# escpecially if they contain "#" chars...
# Remember to edit /etc/mysql/debian.cnf when changing the socket location.
[client]
port = 3306
socket = /var/run/mysqld/mysqld.sock
# Here is entries for some specific programs
# The following values assume you have at least 32M ram
# This was formally known as [safe_mysqld]. Both versions are currently parsed.
[mysqld_safe]
socket = /var/run/mysqld/mysqld.sock
nice = 0
[mysqld]
#
# * Basic Settings
#
user = mysql
pid-file = /var/run/mysqld/mysqld.pid
socket = /var/run/mysqld/mysqld.sock
port = 3306
basedir = /usr
datadir = /var/lib/mysql
tmpdir = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking
#
# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
bind-address = 127.0.0.1
#
# * Fine Tuning
#
key_buffer = 32M
max_allowed_packet = 1M
thread_stack = 128K
thread_cache_size = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover = BACKUP
max_connections = 75
table_cache = 32
#thread_concurrency = 10
#
# * Query Cache Configuration
#
query_cache_limit = 1M
query_cache_size = 16M
#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
# As of 5.1 you can enable the log at runtime!
#general_log_file = /var/log/mysql/mysql.log
#general_log = 1
#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log
#
# Here you can see queries with especially long duration
#log_slow_queries = /var/log/mysql/mysql-slow.log
#long_query_time = 2
#log-queries-not-using-indexes
#
# The following can be used as easy to replay backup logs or for replication.
# note: if you are setting up a replication slave, see README.Debian about
# other settings you may need to change.
#server-id = 1
#log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
#binlog_do_db = include_database_name
#binlog_ignore_db = include_database_name
#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem
[mysqldump]
quick
quote-names
max_allowed_packet = 16M
[mysql]
#no-auto-rehash # faster start of mysql but no tab completition
[isamchk]
key_buffer = 16M
#
# * IMPORTANT: Additional settings that can override those from this file!
# The files must end with '.cnf', otherwise they'll be ignored.
#
!includedir /etc/mysql/conf.d/
我使用的连接池定义如下：
<?xml version="1.0" encoding="UTF-8"?>
<Context>
<!-- Especifica un DataSource JDBC. -->
<Resource name="jdbc/devicesDS"
auth="Container"
type="javax.sql.DataSource"
maxActive="10"
maxIdle="4"
username="root"
password="SrbBfsatn640^"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/devices?autoReconnect=true"
validationQuery="select 1 from dual" />
</Context>
看起来这是 MySQL inactivity timeout - there's quite a few similar Whosebug questions around it (e.g. here, here and here)。建议通过添加 testOnBorrow=true 来更改您的连接池设置，即：
<Resource name="jdbc/devicesDS"
auth="Container"
type="javax.sql.DataSource"
maxActive="10"
maxIdle="4"
username="root"
password="SrbBfsatn640^"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/devices?autoReconnect=true"
validationQuery="select 1 from dual"
testOnBorrow=true />
我个人倾向于 testOnBorrow 而不是 testWhileIdle 因为前者完全独立于 wait_timeout value (but not sure how the performance compares). Would also be cautious about using the alternative of autoreconnect=true in the JDBC URL: It isn't recommended by the MySQL Connector/J documentation " 因为它有相关的副作用当应用程序未正确处理 SQLExceptions 时，会话状态和数据一致性，并且仅设计用于当您无法配置应用程序以正确处理由死连接和陈旧连接导致的 SQLExceptions 时使用“.
在您的应用程序中，到数据库 mysql 的连接字符串应该是：
jdbc:mysql://{host}:{port}/{schema}?autoReconnect=true
添加 autoReconnect=true 将解决此问题
我 运行 在尝试了每个配置、自动重新连接等之后，用其他应用程序实现解决了这个问题。
我发现解决这个问题的唯一方法是制作一个计划事件，每小时调用一个测试查询来 ping 数据库，强制它刷新连接。
query("SELECT version()");
我的应用程序 (Spring 4 MVC / Hibernate 4 / MySQL / Maven) 使用基于注释的配置将 Spring 与 Hibernate 集成。几个小时不使用数据库后，我总是看到这个错误：
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully received from the server was 84,510,841 milliseconds ago. The last packet sent successfully to the server was 84,510,841 milliseconds ago. is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
java.lang.reflect.Constructor.newInstance(Constructor.java:526)
com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1121)
com.mysql.jdbc.MysqlIO.send(MysqlIO.java:3938)
com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2551)
com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2731)
com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2809)
com.mysql.jdbc.ConnectionImpl.setAutoCommit(ConnectionImpl.java:5333)
sun.reflect.GeneratedMethodAccessor58.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:606)
org.apache.tomcat.jdbc.pool.ProxyConnection.invoke(ProxyConnection.java:126)
org.apache.tomcat.jdbc.pool.JdbcInterceptor.invoke(JdbcInterceptor.java:109)
org.apache.tomcat.jdbc.pool.DisposableConnectionFacade.invoke(DisposableConnectionFacade.java:80)
com.sun.proxy.$Proxy33.setAutoCommit(Unknown Source)
org.hibernate.resource.jdbc.internal.AbstractLogicalConnectionImplementor.begin(AbstractLogicalConnectionImplementor.java:67)
org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl.begin(LogicalConnectionManagedImpl.java:238)
org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.begin(JdbcResourceLocalTransactionCoordinatorImpl.java:213)
org.hibernate.engine.transaction.internal.TransactionImpl.begin(TransactionImpl.java:52)
org.hibernate.internal.SessionImpl.beginTransaction(SessionImpl.java:1512)
org.hibernate.jpa.internal.TransactionImpl.begin(TransactionImpl.java:45)
org.springframework.orm.jpa.vendor.HibernateJpaDialect.beginTransaction(HibernateJpaDialect.java:170)
org.springframework.orm.jpa.JpaTransactionManager.doBegin(JpaTransactionManager.java:380)
org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(AbstractPlatformTransactionManager.java:373)
org.springframework.transaction.interceptor.TransactionAspectSupport.createTransactionIfNecessary(TransactionAspectSupport.java:427)
org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:276)
org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:208)
com.sun.proxy.$Proxy118.findAll(Unknown Source)
com.ideefe.iot.controller.AppController.listDevices(AppController.java:49)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:606)
org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:222)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:814)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:737)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.web.multipart.support.MultipartFilter.doFilterInternal(MultipartFilter.java:118)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
这是 MySQL my.cnf 文件：
devuser@localhost:/etc/apache2$ more /etc/mysql/my.cnf
#
# The MySQL database server configuration file.
#
# You can copy this to one of:
# - "/etc/mysql/my.cnf" to set global options,
# - "~/.my.cnf" to set user-specific options.
#
# One can use all long options that the program supports.
# Run program with --help to get a list of available options and with
# --print-defaults to see which it would actually understand and use.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html
# This will be passed to all mysql clients
# It has been reported that passwords should be enclosed with ticks/quotes
# escpecially if they contain "#" chars...
# Remember to edit /etc/mysql/debian.cnf when changing the socket location.
[client]
port = 3306
socket = /var/run/mysqld/mysqld.sock
# Here is entries for some specific programs
# The following values assume you have at least 32M ram
# This was formally known as [safe_mysqld]. Both versions are currently parsed.
[mysqld_safe]
socket = /var/run/mysqld/mysqld.sock
nice = 0
[mysqld]
#
# * Basic Settings
#
user = mysql
pid-file = /var/run/mysqld/mysqld.pid
socket = /var/run/mysqld/mysqld.sock
port = 3306
basedir = /usr
datadir = /var/lib/mysql
tmpdir = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking
#
# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
bind-address = 127.0.0.1
#
# * Fine Tuning
#
key_buffer = 32M
max_allowed_packet = 1M
thread_stack = 128K
thread_cache_size = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover = BACKUP
max_connections = 75
table_cache = 32
#thread_concurrency = 10
#
# * Query Cache Configuration
#
query_cache_limit = 1M
query_cache_size = 16M
#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
# As of 5.1 you can enable the log at runtime!
#general_log_file = /var/log/mysql/mysql.log
#general_log = 1
#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log
#
# Here you can see queries with especially long duration
#log_slow_queries = /var/log/mysql/mysql-slow.log
#long_query_time = 2
#log-queries-not-using-indexes
#
# The following can be used as easy to replay backup logs or for replication.
# note: if you are setting up a replication slave, see README.Debian about
# other settings you may need to change.
#server-id = 1
#log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
#binlog_do_db = include_database_name
#binlog_ignore_db = include_database_name
#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem
[mysqldump]
quick
quote-names
max_allowed_packet = 16M
[mysql]
#no-auto-rehash # faster start of mysql but no tab completition
[isamchk]
key_buffer = 16M
#
# * IMPORTANT: Additional settings that can override those from this file!
# The files must end with '.cnf', otherwise they'll be ignored.
#
!includedir /etc/mysql/conf.d/
我使用的连接池定义如下：
<?xml version="1.0" encoding="UTF-8"?>
<Context>
<!-- Especifica un DataSource JDBC. -->
<Resource name="jdbc/devicesDS"
auth="Container"
type="javax.sql.DataSource"
maxActive="10"
maxIdle="4"
username="root"
password="SrbBfsatn640^"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/devices?autoReconnect=true"
validationQuery="select 1 from dual" />
</Context>
看起来这是 MySQL inactivity timeout - there's quite a few similar Whosebug questions around it (e.g. here, here and here)。建议通过添加 testOnBorrow=true 来更改您的连接池设置，即：
<Resource name="jdbc/devicesDS"
auth="Container"
type="javax.sql.DataSource"
maxActive="10"
maxIdle="4"
username="root"
password="SrbBfsatn640^"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/devices?autoReconnect=true"
validationQuery="select 1 from dual"
testOnBorrow=true />
我个人倾向于 testOnBorrow 而不是 testWhileIdle 因为前者完全独立于 wait_timeout value (but not sure how the performance compares). Would also be cautious about using the alternative of autoreconnect=true in the JDBC URL: It isn't recommended by the MySQL Connector/J documentation " 因为它有相关的副作用当应用程序未正确处理 SQLExceptions 时，会话状态和数据一致性，并且仅设计用于当您无法配置应用程序以正确处理由死连接和陈旧连接导致的 SQLExceptions 时使用“.
在您的应用程序中，到数据库 mysql 的连接字符串应该是：
jdbc:mysql://{host}:{port}/{schema}?autoReconnect=true
添加 autoReconnect=true 将解决此问题
我 运行 在尝试了每个配置、自动重新连接等之后，用其他应用程序实现解决了这个问题。 我发现解决这个问题的唯一方法是制作一个计划事件，每小时调用一个测试查询来 ping 数据库，强制它刷新连接。
query("SELECT version()");