项目上线一段时间后，多个环境经常出现com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure错误，堆栈信息如下： The last packet successfully received from the server was 10,003 milliseconds ago. The last packet sent successfully to the server was 10,003 milliseconds ago. at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174) at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64) at com.mysql.cj.jdbc.StatementImpl.executeQuery(StatementImpl.java:1200) at com.alibaba.druid.pool.vendor.MySqlValidConnectionChecker.isValidConnection(MySqlValidConnectionChecker.java:140) at com.alibaba.druid.pool.DruidAbstractDataSource.validateConnection(DruidAbstractDataSource.java:1440) at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1812) at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2877) Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
The last packet successfully received from the server was 10,003 milliseconds ago. The last packet sent successfully to the server was 10,003 milliseconds ago. at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:423) at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61) at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105) at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:151) at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:167) at com.mysql.cj.protocol.a.NativeProtocol.readMessage(NativeProtocol.java:538) at com.mysql.cj.protocol.a.NativeProtocol.checkErrorMessage(NativeProtocol.java:702) at com.mysql.cj.protocol.a.NativeProtocol.sendCommand(NativeProtocol.java:641) at com.mysql.cj.protocol.a.NativeProtocol.sendQueryPacket(NativeProtocol.java:940) at com.mysql.cj.protocol.a.NativeProtocol.sendQueryString(NativeProtocol.java:886) at com.mysql.cj.NativeSession.execSQL(NativeSession.java:1073) at com.mysql.cj.jdbc.StatementImpl.executeQuery(StatementImpl.java:1168) … 4 common frames omitted Caused by: java.net.SocketTimeoutException: Read timed out at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.socketRead(SocketInputStream.java:116) at java.net.SocketInputStream.read(SocketInputStream.java:171) at java.net.SocketInputStream.read(SocketInputStream.java:141) at sun.security.ssl.SSLSocketInputRecord.read(SSLSocketInputRecord.java:475) at sun.security.ssl.SSLSocketInputRecord.readHeader(SSLSocketInputRecord.java:469) at sun.security.ssl.SSLSocketInputRecord.bytesInCompletePacket(SSLSocketInputRecord.java:69) at sun.security.ssl.SSLSocketImpl.readApplicationRecord(SSLSocketImpl.java:1228) at sun.security.ssl.SSLSocketImpl.access
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300(SSLSocketImpl.java:75) at sun.security.ssl.SSLSocketImpl
300(SSLSocketImpl.java:75)atsun.security.ssl.SSLSocketImplAppInputStream.read(SSLSocketImpl.java:915) at java.io.FilterInputStream.read(FilterInputStream.java:133) at com.mysql.cj.protocol.FullReadInputStream.readFully(FullReadInputStream.java:64) at com.mysql.cj.protocol.a.SimplePacketReader.readHeader(SimplePacketReader.java:63) at com.mysql.cj.protocol.a.SimplePacketReader.readHeader(SimplePacketReader.java:45) at com.mysql.cj.protocol.a.TimeTrackingPacketReader.readHeader(TimeTrackingPacketReader.java:52) at com.mysql.cj.protocol.a.TimeTrackingPacketReader.readHeader(TimeTrackingPacketReader.java:41) at com.mysql.cj.protocol.a.MultiPacketReader.readHeader(MultiPacketReader.java:54) at com.mysql.cj.protocol.a.MultiPacketReader.readHeader(MultiPacketReader.java:44) at com.mysql.cj.protocol.a.NativeProtocol.readMessage(NativeProtocol.java:532) … 10 common frames omitted 经过多次排查和测试，终于排查到了原因。 出现问题的原因是因为我们的项目需要动态配置数据源，程序中根据需要即时动态创建DruidDataSource，没有设置SocketTimeout和ConnectTimeout参数，默认值都是0，有文章说：connectTimeout的默认值为0，表示驱动层面不设置超时时间，但这并不意味着不会超时，此时将由操作系统来决定超时时间。实际经过测试，当SocketTimeout和ConnectTimeout为默认值0时，两个超时时间可能都为10秒。 测试代码：
public static void main(String[] args) throws Exception {
//创建数据源
DruidDataSource druidDataSource = new DruidDataSource();
druidDataSource.setUrl("jdbc:mysql://127.0.0.1:3306/test?autoReconnect=true&useUnicode=true&characterEncoding=utf8&allowMultiQueries=true&serverTimezone=GMT%2B8");
druidDataSource.setUsername("admin");
druidDataSource.setPassword("admin");
druidDataSource.setDriverClassName("com.mysql.cj.jdbc.Driver");
String querySql = "SELECT sleep(11)"; //执行语句，该行语句的执行时间≥11秒
druidDataSource.setConnectionErrorRetryAttempts(3); //失败后重连次数
druidDataSource.setBreakAfterAcquireFailure(true);
druidDataSource.setMaxWait(30 * 1000); //保持的最长时间
// 以下配置自动检测链接的可用性
druidDataSource.setTestWhileIdle(true); //明确指定检测空闲链接是否可用
druidDataSource.setValidationQuery("select 1"); //执行该SQL来检测空闲链接，如果不配置，实际不会执行检测
//如果链接空闲时长超过60秒则执行检测，周而复始。如果该时长少于MySQL数据变量：wait_timeout 会出现链接不可用的情况。
druidDataSource.setTimeBetweenEvictionRunsMillis(60 * 1000);
druidDataSource.setSocketTimeout(20 * 1000);//超时时长：20秒，该值要大于执行语句的执行时间
druidDataSource.setConnectTimeout(20 * 1000);//超时时长：20秒，该值要大于执行语句的执行时间
List
System.out.println(JSONUtilExt.toJson(result));
}
请注意测试代码中的这两行代码： druidDataSource.setSocketTimeout(20 * 1000); druidDataSource.setConnectTimeout(20 * 1000); 当我将这两行代码中的任意一行代码注释掉或两行代码都注释掉时，都会出现文章开始的错误，且错误日志相同。
分析到此，基本可以确定，程序里出现上面错误就是因为SocketTimeout和ConnectTimeout没有设置，导致操作系统里实际默认的时间是10秒，而因为项目已经上线一段时间，个别表里数据量比较大，所以在执行语句时偶尔会出现执行时间超过10秒的情况，连接超时。因此，根据需要，在创建数据源时，增加设置SocketTimeout和ConnectTimeout的大小就可以避免再次出现上述错误。
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