CommunicationsException: Communi

异常1 .CommunicationsException: Communications link failure

create connection SQLException, url: jdbc:mysql:// rds.aliyuncs.com:3306/jiaoliuqu?useUnicode=true&characterEncoding=utf-8, errorCode 0, state 08S01
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.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.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1117)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:350)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2408)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2445)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2230)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:813)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
at sun.reflect.GeneratedConstructorAccessor62.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:399)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:334)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:149)
at com.alibaba.druid.filter.FilterAdapter.connection_connect(FilterAdapter.java:786)
at com.alibaba.druid.filter.FilterEventAdapter.connection_connect(FilterEventAdapter.java:38)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:143)
at com.alibaba.druid.filter.stat.StatFilter.connection_connect(StatFilter.java:218)
at com.alibaba.druid.filter.FilterChainImpl.connection_connect(FilterChainImpl.java:143)
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1470)
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1533)
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2333)
java.net.UnknownHostException: rds.aliyuncs.com
at java.net.InetAddress.getAllByName0(InetAddress.java:1281)
at java.net.InetAddress.getAllByName(InetAddress.java:1193)
at java.net.InetAddress.getAllByName(InetAddress.java:1127)
at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:249)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300)
... 20 more

问题 描述

从上面调用栈上可以看出 jdbc 在初始化 JDBC4Connection的时候 dns rds.aliyuncs.com 无法识别，抛出了异常。

  java.net.UnknownHostException:  rds.aliyuncs.com
    at java.net.InetAddress.getAllByName0(InetAddress.java:1281)
    at java.net.InetAddress.getAllByName(InetAddress.java:1193)
    at java.net.InetAddress.getAllByName(InetAddress.java:1127)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:249)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300)
    ... 20 more

也就是说 jdbc:mysql:// rds.aliyuncs.com:3306/jiaoliuqu?useUnicode=true&characterEncoding=utf-8 中的dns 域名无法解析 。 抛出异常的原因如下图 （详情请看InetAddress源码 https://github.com/ojdkbuild/lookaside_java-1.8.0-openjdk/blob/master/jdk/src/share/classes/java/net/InetAddress.java ）

    private static InetAddress[] getAllByName0 (String host, InetAddress reqAddr, boolean check)
        throws UnknownHostException  {

        /* If it gets here it is presumed to be a hostname */
        /* Cache.get can return: null, unknownAddress, or InetAddress[] */

        /* make sure the connection to the host is allowed, before we
         * give out a hostname
         */
        if (check) {
            SecurityManager security = System.getSecurityManager();
            if (security != null) {
                security.checkConnect(host, -1);
            }
        }

        InetAddress[] addresses = getCachedAddresses(host);

        /* If no entry in cache, then do the host lookup */
        if (addresses == null) {
            addresses = getAddressesFromNameService(host, reqAddr);
        }

        if (addresses == unknown_array)
            throw new UnknownHostException(host);

        return addresses.clone();
    }

在java dns是有缓存策略的， 正向缓存和负向缓存 ，正向缓存 代表第一次dns 成功后 会缓存dns ，缓存解析失败会到负向缓存 ，当前dns 为负向缓存的时候，会直接抛出异常。
下图为缓存添加逻辑

    private static void cacheAddresses(String hostname,
                                       InetAddress[] addresses,
                                       boolean success) {
        hostname = hostname.toLowerCase();
        synchronized (addressCache) {
            cacheInitIfNeeded();
            if (success) {
                addressCache.put(hostname, addresses);
            } else {
                negativeCache.put(hostname, addresses);
            }
        }
    }

可能造成的原因

1. pc-jiaoliuqu.rwlb.zhanbei.rds.aliyuncs.com 解析超时 或者失败 ，和dns 解析器有关，这点需要知道公司java 用的什么dns 解析器 ，失败后，在缓存过期前 ，用的负向缓存，都会造成解析失败

潜在的其他问题

1. rds.aliyuncs.com 对应ip 变过了 ，用的是缓存旧的ip

解决方案

问题 可能很明确了 ， rds.aliyuncs.com dns 无法解析 可能需要和运维同学沟通哈 ，是否有域名对应ip 改变过的情况 ，如果没有，要看下域名为什么解析失败 。
除此之外 我们要修改java缓存超时时间 。不要一次域名解析失败，造成10s 内解析都失败，这是不行的
修改后 ，还是出现大量域名解析问题 ，还是要运维同学看下k8s dns server 稳定性了 。

1 .下面设置 java dns 缓存过期时间为0 ，取消缓存机制会带来性能耗损，但是没办法
jvm启动参数里面配置-Dsun.net.inetaddr.ttl=0
修改配置文件$JDK_HOME/lib/security/java.security相应的参数networkaddress.cache.ttl=0
代码里直接设置：java.security.Security.setProperty(”networkaddress.cache.ttl” , “0”);

2. 有同学建议 改小时间 这个也是可以的 ， 建议还是越短越好，比如说1 s

测试demo

dns解析 fail , 会抛出两种不同信息的UnknownHostException ,第一个解析失败加入 unknown_array 报出异常（失败cache），第二个走cache unknown_array报出异常

import java.net.InetAddress;
import java.net.UnknownHostException;

public class DnsTest {

  public static void main(String[] args) {
    // 5s cache time
    java.security.Security.setProperty("networkaddress.cache.ttl", "1");

    // dns fail , 会抛出两种不同信息的UnknownHostException ,第一个加入失败加入 unknown_array 报出异常（失败cache），
    //  以后5s 内都走cache unknown_array
    //  1. Inet6AddressImpl  单个 java.net.UnknownHostException: www.xxfdsffsd.com: nodename nor servname provided, or not known
    //  2. Inet6AddressImpl  后面会有多个 java.net.UnknownHostException: www.xxfdsffsd.com
    for (int i = 0; i < 10; i++) {
      try {
        // mac 默认ipv6 支持
        InetAddress.getByName("www.xxfdsffsd.com");
      } catch (UnknownHostException e) {
        e.printStackTrace();
      }
    }
    
  }
  
}