TLDR；答案末尾有一小部分带有摘要。
一点理论
在 Java 中，每个类都必须在使用前进行初始化。初始化意味着执行静态字段初始化器和静态初始化块。当然，在标准 JVM（如 HotSpot）中，这发生在运行时。
但是对于本机图像，您有两种选择。一个类可能仍然在运行时被初始化，或者它的初始化可能在构建时被执行。后者有一个明显的好处，就是在原生镜像启动时避免了这项工作，这使得镜像启动更快。但是对于某些类，在构建时初始化它们是没有意义的。这样的示例可能是一个类，它在其初始化时根据环境（环境变量、配置文件等）做出一些决定（例如，创建这个或那个类的实例）。
在构建/运行时初始化选项之间进行选择有一些限制：
- 如果一个类在构建时初始化，它的所有超类必须在构建时初始化
- 如果一个类在运行时初始化，它的所有子类必须在运行时初始化
- 某些类必须始终在运行时初始化（见下文）
- 在运行时初始化的类的实例不能存在于映像堆中（这意味着没有构建时初始化的类或其实例可以（直接或间接）引用这样的运行时初始化的类实例）
Inet地址问题
从版本 19.3.0 开始，native-image工具要求java.net.InetAddress始终在运行时初始化该类。这（通过限制 2）意味着它的子类，java.net.Inet4Address并且java.net.Inet6Address还必须在运行时初始化，这反过来（通过限制 4）意味着您不能有任何InetAddress被构建时初始化的类引用。
我们在这里遇到的所有构建失败都是由同一个问题引起的：要么在图像堆中，Inet4Address要么Inet6Address
在图像堆中。
但是为什么 Netty 相关的类会在构建时尝试初始化呢？
原来netty-codec-http包含以下内容
Args = --initialize-at-build-time=io.netty \
在其native-image.properties下方META-INF，并且 micronaut 具有netty-codec-http依赖项，因此
io.netty默认情况下所有类都在构建时初始化（因为native-image工具尊重此类
native-image.properties文件）。
样板工程
这里https://github.com/rpuch/netty-InetAddress-native-image-diagnosing是一个模拟问题的项目，我进一步使用它来展示如何解决问题。其main()方法如下：
public static void main(String[] args) throws Exception {
NioEventLoopGroup group = new NioEventLoopGroup(1, new DefaultThreadFactory("netty"));
DnsAddressResolverGroup resolverGroup = new DnsAddressResolverGroup(NioDatagramChannel.class,
DnsServerAddressStreamProviders.platformDefault());
AddressResolver<InetSocketAddress> resolver = resolverGroup.getResolver(group.next());
System.out.println(resolver);
resolver.close();
group.shutdownGracefully().get();
}
它会产生与以下代码相同的效果（关于 Netty）：
Config config = new Config();
config.useSingleServer().setAddress(redisUri);
config.useSingleServer().setPassword(redisPassword);
return Redisson.createReactive(config);
该项目还在--initialize-at-build-time=io.netty其构建脚本中模拟基于 micronaut 的项目行为。
因此，它是解决这个问题的原始项目的有用替代品。
GraalVM 版本
我在这里使用的是 20.2.0 版本（截至撰写本文时最新发布的版本）。
诊断和修复
1
构建失败并出现以下错误：
Error: com.oracle.graal.pointsto.constraints.UnsupportedFeatureException: No instances of java.net.Inet4Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Trace:
at parsing io.netty.resolver.dns.DnsNameResolver.resolveHostsFileEntry(DnsNameResolver.java:659)
Call path from entry point to io.netty.resolver.dns.DnsNameResolver.resolveHostsFileEntry(String):
at io.netty.resolver.dns.DnsNameResolver.resolveHostsFileEntry(DnsNameResolver.java:651)
at io.netty.resolver.dns.DnsNameResolver.doResolve(DnsNameResolver.java:884)
at io.netty.resolver.dns.DnsNameResolver.doResolve(DnsNameResolver.java:733)
at io.netty.resolver.SimpleNameResolver.resolve(SimpleNameResolver.java:61)
at io.netty.resolver.SimpleNameResolver.resolve(SimpleNameResolver.java:53)
at io.netty.resolver.InetSocketAddressResolver.doResolve(InetSocketAddressResolver.java:55)
at io.netty.resolver.InetSocketAddressResolver.doResolve(InetSocketAddressResolver.java:31)
at io.netty.resolver.AbstractAddressResolver.resolve(AbstractAddressResolver.java:106)
at io.netty.bootstrap.Bootstrap.doResolveAndConnect0(Bootstrap.java:206)
at io.netty.bootstrap.Bootstrap.access$000(Bootstrap.java:46)
at io.netty.bootstrap.Bootstrap$1.operationComplete(Bootstrap.java:180)
DnsNameResolver:659是
return LOCALHOST_ADDRESS;
它引用了名为LOCALHOST_ADDRESStype的静态字段InetAddress。让我们通过在命令中添加以下内容来避免它在构建时初始化native-image：
--initialize-at-run-time=io.netty.resolver.dns.DnsNameResolver
错误消失。
2
现在还有一个：
Error: No instances of java.net.Inet6Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Trace: Object was reached by
reading field java.util.HashMap$Node.value of
constant java.util.HashMap$Node@26eb0f30 reached by
indexing into array
constant java.util.HashMap$Node[]@63e95621 reached by
reading field java.util.HashMap.table of
constant java.util.HashMap@563992d1 reached by
reading field java.util.Collections$UnmodifiableMap.m of
constant java.util.Collections$UnmodifiableMap@38a9945c reached by
reading field io.netty.resolver.DefaultHostsFileEntriesResolver.inet6Entries of
constant io.netty.resolver.DefaultHostsFileEntriesResolver@7ef4ba7e reached by
scanning method io.netty.resolver.dns.DnsNameResolverBuilder.<init>(DnsNameResolverBuilder.java:56)
Call path from entry point to io.netty.resolver.dns.DnsNameResolverBuilder.<init>():
at io.netty.resolver.dns.DnsNameResolverBuilder.<init>(DnsNameResolverBuilder.java:68)
at io.netty.resolver.dns.DnsAddressResolverGroup.<init>(DnsAddressResolverGroup.java:54)
at Main.main(Main.java:18)
DnsNameResolverBuilder:56是
private HostsFileEntriesResolver hostsFileEntriesResolver = HostsFileEntriesResolver.DEFAULT;
让我们推迟HostsFileEntriesResolver初始化：
--initialize-at-run-time=io.netty.resolver.HostsFileEntriesResolver
3
现在，还有另一个错误：
Error: com.oracle.graal.pointsto.constraints.UnsupportedFeatureException: No instances of java.net.Inet6Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Trace:
at parsing io.netty.resolver.dns.DnsQueryContextManager.getOrCreateContextMap(DnsQueryContextManager.java:111)
Call path from entry point to io.netty.resolver.dns.DnsQueryContextManager.getOrCreateContextMap(InetSocketAddress):
at io.netty.resolver.dns.DnsQueryContextManager.getOrCreateContextMap(DnsQueryContextManager.java:96)
DnsQueryContextManager:111参考资料NetUtil.LOCALHOST6。NetUtil在构建时初始化，但其字段
LOCALHOST4和分别LOCALHOST6包含Inet4Address和的实例Inet6Address。这可以通过替换来解决。我们只需将以下类添加到我们的项目中：
@TargetClass(NetUtil.class)
final class NetUtilSubstitutions {
@Alias
@InjectAccessors(NetUtilLocalhost4Accessor.class)
public static Inet4Address LOCALHOST4;
@Alias
@InjectAccessors(NetUtilLocalhost6Accessor.class)
public static Inet6Address LOCALHOST6;
private static class NetUtilLocalhost4Accessor {
static Inet4Address get() {
// using https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
return NetUtilLocalhost4LazyHolder.LOCALHOST4;
}
static void set(Inet4Address ignored) {
// a no-op setter to avoid exceptions when NetUtil is initialized at run-time
}
}
private static class NetUtilLocalhost4LazyHolder {
private static final Inet4Address LOCALHOST4;
static {
byte[] LOCALHOST4_BYTES = {127, 0, 0, 1};
// Create IPv4 loopback address.
try {
LOCALHOST4 = (Inet4Address) InetAddress.getByAddress("localhost", LOCALHOST4_BYTES);
} catch (Exception e) {
// We should not get here as long as the length of the address is correct.
PlatformDependent.throwException(e);
throw new IllegalStateException("Should not reach here");
}
}
}
private static class NetUtilLocalhost6Accessor {
static Inet6Address get() {
// using https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
return NetUtilLocalhost6LazyHolder.LOCALHOST6;
}
static void set(Inet6Address ignored) {
// a no-op setter to avoid exceptions when NetUtil is initialized at run-time
}
}
private static class NetUtilLocalhost6LazyHolder {
private static final Inet6Address LOCALHOST6;
static {
byte[] LOCALHOST6_BYTES = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1};
// Create IPv6 loopback address.
try {
LOCALHOST6 = (Inet6Address) InetAddress.getByAddress("localhost", LOCALHOST6_BYTES);
} catch (Exception e) {
// We should not get here as long as the length of the address is correct.
PlatformDependent.throwException(e);
throw new IllegalStateException("Should not reach here");
}
}
}
}
这个想法是用我们控制的方法调用来替换大量有问题的字段。这是通过替换（注@TargetClass和）来实现@Alias的@InjectAccessors。结果，这些InetAddress值不再存储在图像堆中。错误消失。
4
我们现在有另一个：
Error: No instances of java.net.Inet6Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Trace: Object was reached by
reading field io.netty.channel.socket.InternetProtocolFamily.localHost of
constant io.netty.channel.socket.InternetProtocolFamily@5dc39065 reached by
scanning method io.netty.resolver.dns.DnsNameResolver.preferredAddressType(DnsNameResolver.java:487)
Call path from entry point to io.netty.resolver.dns.DnsNameResolver.preferredAddressType(ResolvedAddressTypes):
at io.netty.resolver.dns.DnsNameResolver.preferredAddressType(DnsNameResolver.java:481)
从 的代码中可以看出InternetProtocolFamily，每个枚举常量都存储了一个 的实例InetAddress，因此如果在构建时初始化的任何类初始化，图像堆就会被实例InternetProtocolFamily污染
。InetAddress这也可以通过替换来解决：
@TargetClass(InternetProtocolFamily.class)
final class InternetProtocolFamilySubstitutions {
@Alias
@InjectAccessors(InternetProtocolFamilyLocalhostAccessor.class)
private InetAddress localHost;
private static class InternetProtocolFamilyLocalhostAccessor {
static InetAddress get(InternetProtocolFamily family) {
switch (family) {
case IPv4:
return NetUtil.LOCALHOST4;
case IPv6:
return NetUtil.LOCALHOST6;
default:
throw new IllegalStateException("Unsupported internet protocol family: " + family);
}
}
static void set(InternetProtocolFamily family, InetAddress address) {
// storing nothing as the getter derives all it needs from its argument
}
}
}
错误消失。
5
这次还有一个：
Error: No instances of java.net.Inet4Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Detailed message:
Trace: Object was reached by
reading field java.net.InetSocketAddress$InetSocketAddressHolder.addr of
constant java.net.InetSocketAddress$InetSocketAddressHolder@34913c36 reached by
reading field java.net.InetSocketAddress.holder of
constant java.net.InetSocketAddress@ad1fe10 reached by
reading field io.netty.resolver.dns.SingletonDnsServerAddresses.address of
constant io.netty.resolver.dns.SingletonDnsServerAddresses@79fd599 reached by
scanning method io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.nameServerAddressStream(DefaultDnsServerAddressStreamProvider.java:115)
Call path from entry point to io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.nameServerAddressStream(String):
at io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.nameServerAddressStream(DefaultDnsServerAddressStreamProvider.java:115)
at io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder$1.nameServerAddressStream(DnsServerAddressStreamProviders.java:131)
at io.netty.resolver.dns.DnsNameResolver.doResolveAllUncached0(DnsNameResolver.java:1070)
首先，让我们将初始化移动io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider到运行时：
--initialize-at-run-time=io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider
现在，错误是相似的，但仍然略有不同：
Error: No instances of java.net.Inet4Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Trace: Object was reached by
reading field java.net.InetSocketAddress$InetSocketAddressHolder.addr of
constant java.net.InetSocketAddress$InetSocketAddressHolder@5537c5de reached by
reading field java.net.InetSocketAddress.holder of
constant java.net.InetSocketAddress@fb954f8 reached by
reading field io.netty.resolver.dns.SingletonDnsServerAddresses.address of
constant io.netty.resolver.dns.SingletonDnsServerAddresses@3ec9baab reached by
reading field io.netty.resolver.dns.UnixResolverDnsServerAddressStreamProvider.defaultNameServerAddresses of
constant io.netty.resolver.dns.UnixResolverDnsServerAddressStreamProvider@1b7f0339 reached by
reading field io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder$1.currentProvider of
constant io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder$1@2d249be7 reached by
scanning method io.netty.resolver.dns.DnsServerAddressStreamProviders.unixDefault(DnsServerAddressStreamProviders.java:104)
Call path from entry point to io.netty.resolver.dns.DnsServerAddressStreamProviders.unixDefault():
at io.netty.resolver.dns.DnsServerAddressStreamProviders.unixDefault(DnsServerAddressStreamProviders.java:104)
at io.netty.resolver.dns.DnsServerAddressStreamProviders.platformDefault(DnsServerAddressStreamProviders.java:100)
at Main.main(Main.java:18)
好的，让我们io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder也将初始化移动到运行时：
'--initialize-at-run-time=io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder'
（注意单引号：没有它们$和后面的字符将被解释sh为空字符串并替换为空字符串）。
错误消失。
请注意，订单在这里很重要。当我第一次将
io.netty.resolver.dns.DnsServerAddressStreamProviders$DefaultProviderHolder初始化移到运行时但没有触及io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider初始化时，错误报告并没有改变。所以需要一点耐心和试验（或者一些我没有的知识，唉）。
现在我们有了这个：
Error: com.oracle.graal.pointsto.constraints.UnsupportedFeatureException: No instances of java.net.Inet6Address are allowed in the image heap as this class should be initialized at image runtime. Object has been initialized without the native-image initialization instrumentation and the stack trace can't be tracked.
Detailed message:
Trace:
at parsing io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.<clinit>(DefaultDnsServerAddressStreamProvider.java:87)
Call path from entry point to io.netty.resolver.dns.DefaultDnsServerAddressStreamProvider.<clinit>():
no path found from entry point to target method
好的，它NetUtil.LOCALHOST被引用了，所以让我们也为它添加一个替换（to NetUtilSubstitutions）：
@Alias
@InjectAccessors(NetUtilLocalhostAccessor.class)
public static InetAddress LOCALHOST;
// NOTE: this is the simpliest implementation I could invent to just demonstrate the idea; it is probably not
// too efficient. An efficient implementation would only have getter and it would compute the InetAddress
// there; but the post is already very long, and NetUtil.LOCALHOST computation logic in Netty is rather cumbersome.
private static class NetUtilLocalhostAccessor {
private static volatile InetAddress ADDR;
static InetAddress get() {
return ADDR;
}
static void set(InetAddress addr) {
ADDR = addr;
}
}
这使得最终错误消失。
感谢@NicolasFilotto 对第 5 项的建议，我比原来更喜欢他的解决方案，实际上第 5 项是他想法的实现。
技术总结
- 首先，你可以找到一个类，它被移到运行时初始化阶段，导致失败消失。为此，您可以在提供的堆栈跟踪中跟踪引用。要考虑的最佳候选者是静态字段。
- 如果第 1 项没有帮助，您可以尝试创建替换
- 理论上也有可能使用更轻的变体：
@RecomputeFieldValue，但它受到更多限制，我无法让它在这个与 Netty 相关的任务中工作。
PS。替换相关代码的灵感来自https://github.com/quarkusio/quarkus/pull/5353/files
聚苯乙烯。第 5 项解决方案的灵感来自 @NicolasFilotto