这几天我把项目里的spring boot升级到了最新版本：2.4.2，然后启动的时候发现redisson报错了，确切的说是redisson依赖的netty报错了。报错内容如下：

2021-01-31 20:01:30.788  WARN 1764 --- [           main] i.n.r.d.DnsServerAddressStreamProviders  : Unable to load io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider, fallback to system defaults. This may result in incorrect DNS resolutions on MacOS.

java.lang.ClassNotFoundException: io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382) ~[na:1.8.0_251]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[na:1.8.0_251]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:355) ~[na:1.8.0_251]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[na:1.8.0_251]
	at java.lang.Class.forName0(Native Method) ~[na:1.8.0_251]
	at java.lang.Class.forName(Class.java:348) ~[na:1.8.0_251]
	at io.netty.resolver.dns.DnsServerAddressStreamProviders$1.run(DnsServerAddressStreamProviders.java:50) ~[netty-resolver-dns-4.1.58.Final.jar:4.1.58.Final]
	at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_251]
	at io.netty.resolver.dns.DnsServerAddressStreamProviders.<clinit>(DnsServerAddressStreamProviders.java:46) ~[netty-resolver-dns-4.1.58.Final.jar:4.1.58.Final]
	at org.redisson.connection.MasterSlaveConnectionManager.<init>(MasterSlaveConnectionManager.java:203) [redisson-3.15.0.jar:3.15.0]
	at org.redisson.connection.MasterSlaveConnectionManager.<init>(MasterSlaveConnectionManager.java:155) [redisson-3.15.0.jar:3.15.0]
	at org.redisson.connection.SingleConnectionManager.<init>(SingleConnectionManager.java:34) [redisson-3.15.0.jar:3.15.0]
        ....

乍一看ClassNotFoundException很明显是少包了，但是maven导入都是正常的，于是我去找这个class在哪个包下，最终找到了这个包：

<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-resolver-dns-native-macos</artifactId>
</dependency>

然而，问题就这么解决了吗？显然不是。当我导完包一运行，发现报了另一个异常：

2021-01-31 20:11:18.987  WARN 1949 --- [           main] i.n.r.d.DnsServerAddressStreamProviders  : Unable to load io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider, fallback to system defaults. This may result in incorrect DNS resolutions on MacOS.

java.lang.reflect.InvocationTargetException: null
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_251]
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_251]
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_251]
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_251]
	at io.netty.resolver.dns.DnsServerAddressStreamProviders.<clinit>(DnsServerAddressStreamProviders.java:64) ~[netty-resolver-dns-4.1.58.Final.jar:4.1.58.Final]
	at org.redisson.connection.MasterSlaveConnectionManager.<init>(MasterSlaveConnectionManager.java:203) [redisson-3.15.0.jar:3.15.0]
	at org.redisson.connection.MasterSlaveConnectionManager.<init>(MasterSlaveConnectionManager.java:155) [redisson-3.15.0.jar:3.15.0]
        ....
Caused by: java.lang.UnsatisfiedLinkError: io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider.resolvers()[Lio/netty/resolver/dns/macos/DnsResolver;
        ....

然后我去追了一下源码:

MacOSDnsServerAddressStreamProvider.class

/**
 * {@link DnsServerAddressStreamProvider} implementation which makes use of the same mechanism as
 * <a href="https://opensource.apple.com/tarballs/mDNSResponder/">Apple's open source mDNSResponder</a> to retrieve the
 * current nameserver configuration of the system.
 */
public final class MacOSDnsServerAddressStreamProvider implements DnsServerAddressStreamProvider{
    private static void loadNativeLibrary() {
      String name = SystemPropertyUtil.get("os.name").toLowerCase(Locale.UK).trim();
      if (!name.startsWith("mac")) {
          throw new IllegalStateException("Only supported on MacOS");
      }
      String staticLibName = "netty_resolver_dns_native_macos";
      String sharedLibName = staticLibName + '_' + PlatformDependent.normalizedArch();
      ClassLoader cl = PlatformDependent.getClassLoader(MacOSDnsServerAddressStreamProvider.class);
      try {
          NativeLibraryLoader.load(sharedLibName, cl);
      } catch (UnsatisfiedLinkError e1) {
          try {
              NativeLibraryLoader.load(staticLibName, cl);
              logger.debug("Failed to load {}", sharedLibName, e1);
          } catch (UnsatisfiedLinkError e2) {
              ThrowableUtil.addSuppressed(e1, e2);
              throw e1;
          }
      }
    }
    // ...
    private static native DnsResolver[] resolvers();
}

可以看到报错的resolvers()是一个native方法。看类名这个类应该是在MacOS系统中解析Dns的一个类，看注释应该会调用mDNSResponder服务，但为啥会调用不到呢？重启项目，跟一下loadNativeLibrary这个方法，发现里面两处load逻辑全部抛异常了，很显然，应该是native库丢了。于是我去netty的github上找这个库。在resolve-dns-native-macos这个包下面找到了源码：

https://github.com/netty/netty/tree/4.1/resolver-dns-native-macos/src/main/c

于是我又翻了下netty-all这个包，发现这个库在netty-all下是有的。

我又看了下下面那几个库，发现也都是只在netty-all这个包下有，而在独立的包中都没有。所以当netty遇到无法加载原生库时，一定要引入netty-all这个包。最终，问题解决。当然，如果不想引入netty-all这么重的包，也可以把里面的library复制出来放到自己项目的classpath下META-INF/native/目录下，然后引入netty-resolver-dns-native-macos这个包即可。

我们再来分析下为啥老版本中不会报错呢？新版本的netty修改了什么？

我把spring-boot的版本调整为2.4.1，果然，这个版本并没有报这个错，看一下这两个版本下netty版本的区别

spring-boot:2.4.1 -> netty 4.1.55.Final

spring-boot:2.4.2 -> netty 4.1.58.Final

对比了一下两个版本中的io.netty.resolver.dns.DnsServerAddressStreamProviders类，发现了一下区别：

public final class DnsServerAddressStreamProviders {
  static {
    Constructor<? extends DnsServerAddressStreamProvider> constructor = null;
    if (PlatformDependent.isOsx()) {
        try {
            // ....
        } catch (Throwable cause) {
            // 这是4.1.58.Final版本下的日志输出
            LOGGER.warn(
                    "Unable to load {}, fallback to system defaults. {}", MACOS_PROVIDER_CLASS_NAME,
                    "This may result in incorrect DNS resolutions on MacOS.", cause);
            // 这是4.1.55.Final版本下的日志输出
            LOGGER.debug(
                    "Unable to use MacOSDnsServerAddressStreamProvider, fallback to system defaults", cause);
            constructor = null;
        }
    }
    STREAM_PROVIDER_CONSTRUCTOR = constructor;
  }
}

可以看到，4.1.58.Final版本将错误日志级别从debug调整到了warn，所以在老版本内并不是没有报错，只是debug日志默认没有输出而已。

找了一下，发现是在这个pullrequest中调整的https://github.com/netty/netty/pull/10848。

因此，这个异常我认为也大概也可以忽略吧，而且这个问题只会在MacOS平台复现，一般生产环境不会是MacOS，所以也不影响线上运行。如果实在看着别扭，就把DnsServerAddressStreamProviders这个类的日志输出级别调整为ERROR吧。