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诊断并修复原生兼容性问题

跟踪并解决原生构建中的提示缺失故障和不受支持的构造。

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本课时的部分内容尚未翻译，以英文显示。

Why Native Builds Break

A Spring Boot 4 app that runs perfectly on the JVM can still fail at native runtime. The GraalVM native-image compiler performs closed-world analysis: it must see, at build time, every class, method, and resource the program will ever touch. Anything discovered only at runtime is invisible to it.

Reflection on a class the analyzer never saw → ClassNotFoundException or a missing method.
Resources loaded by name that weren't registered → null stream.
Proxies / serialization created dynamically → unsupported feature errors.

These are missing-hint failures: the AOT engine simply lacked a hint telling it to include the dynamic element.

Reading the Crash

Native failures usually surface as a runtime exception once the executable starts. The most common signature is reflection that the image stripped out.

Read the stack trace top-down and ask: what dynamic operation triggered this? A missing constructor, a missing field, or a class that 'doesn't exist' even though it's on your classpath all point to a reachability gap.

Caused by: java.lang.NoSuchMethodException:
  com.example.OrderDto.<init>()
  at java.base/java.lang.Class.getConstructor0(...)
  at java.base/java.lang.Class.getDeclaredConstructor(...)
  at o.s.beans.BeanUtils.instantiateClass(BeanUtils.java)

// JVM: works. Native: the no-arg constructor was
// never registered for reflection, so it was removed.

Turning On AOT Diagnostics

Before guessing, make the build talk. Spring Boot's process-aot goal and GraalVM's native-image plugin both emit diagnostics you can opt into.

Build with extra native flags to surface analysis problems early.
--report-unsupported-elements-at-runtime is now the default: unsupported code only fails if actually reached.
Use -H:+PrintClassInitialization and verbose output to see what gets initialized at build time vs run time.

<!-- pom.xml: pass diagnostic flags to native-image -->
<plugin>
  <groupId>org.graalvm.buildtools</groupId>
  <artifactId>native-maven-plugin</artifactId>
  <configuration>
    <buildArgs>
      <buildArg>-H:+ReportExceptionStackTraces</buildArg>
      <buildArg>--verbose</buildArg>
    </buildArgs>
  </configuration>
</plugin>

The Tracing Agent

The fastest way to discover what dynamic features your app actually uses is the GraalVM tracing agent. You run your app on a normal JVM with the agent attached, exercise every code path (run your tests!), and it records all reflection, JNI, proxy, resource, and serialization calls.

The output is a set of JSON config files that the native build automatically picks up from META-INF/native-image.

# Run the app/tests on the JVM with the agent attached.
# It writes reflect-config.json, resource-config.json, etc.

java -agentlib:native-image-agent=\
config-output-dir=src/main/resources/META-INF/native-image \
  -jar target/app.jar

# Tip: use config-merge-dir to accumulate hints across
# multiple runs that cover different code paths.

What a Reflection Hint Looks Like

The tracing agent produces a reflect-config.json. Understanding its shape lets you read and hand-edit hints when the agent misses something. Each entry names a type and which members to keep.

queryAllDeclaredConstructors / allDeclaredFields keep metadata reachable.
You can also register a single constructor or method explicitly.

[
  {
    "name": "com.example.OrderDto",
    "allDeclaredConstructors": true,
    "allDeclaredFields": true,
    "allDeclaredMethods": true
  },
  {
    "name": "com.example.Status",
    "allPublicMethods": true
  }
]

The Programmatic Fix: RuntimeHints

Spring Boot 4's preferred approach is code, not JSON. Implement RuntimeHintsRegistrar to register reflection, resources, and serialization hints in a type-safe, refactor-friendly way. Reference it with @ImportRuntimeHints so the AOT engine contributes it.

This keeps hints next to the code that needs them and survives package renames.

public class OrderHints implements RuntimeHintsRegistrar {
  @Override
  public void registerHints(RuntimeHints hints, ClassLoader cl) {
    hints.reflection().registerType(
        OrderDto.class,
        MemberCategory.INVOKE_DECLARED_CONSTRUCTORS,
        MemberCategory.DECLARED_FIELDS);
    hints.resources().registerPattern("templates/*.html");
  }
}

@Configuration
@ImportRuntimeHints(OrderHints.class)
class AppConfig { }

Fixing Missing Resources

A frequent native failure is a resource that resolves to null because it was never bundled. Native-image only embeds resources you explicitly register by pattern.

If getResourceAsStream returns null only in the native binary, the resource is missing a hint. Register it with a pattern via RuntimeHints or resource-config.json.

// Works on JVM, returns null in native unless registered.
var in = getClass().getResourceAsStream("/data/rules.csv");
if (in == null) {
  throw new IllegalStateException("rules.csv not bundled");
}

// Fix (in a RuntimeHintsRegistrar):
// hints.resources().registerPattern("data/rules.csv");

Build-Time Initialization Traps

Native-image initializes many classes at build time for speed. That bites you when a class captures state that must be fresh at runtime — a cached Random seed, a hostname, a system time, or an open file handle baked into the image heap.

Symptom: every native run produces the same 'random' value, or a stale timestamp.
Fix: force that class to initialize at run time.

// This static seed would be frozen into the image
// if the class is initialized at build time.
public final class TokenGen {
  static final long SEED = System.nanoTime();
}

// Fix via build arg:
//   --initialize-at-run-time=com.example.TokenGen

Unsupported Constructs

Some constructs are genuinely unsupported in native images, not just unhinted. Recognizing them saves hours of fruitless hint-hunting.

Dynamic class loading of bytecode generated at runtime (some old AOP/proxy libs).
Unregistered dynamic proxies — you must declare proxy interfaces up front.
InvokeDynamic-heavy scripting engines and arbitrary runtime bytecode weaving.

The fix is usually to register the proxy or switch to a native-friendly alternative.

// Register a JDK dynamic proxy so it survives AOT.
hints.proxies().registerJdkProxy(
    com.example.AuditService.class,
    org.springframework.aop.SpringProxy.class);

// Unregistered runtime proxies throw at startup:
//   com.oracle.svm.core.jdk.proxy...
//   No proxy class defined for interfaces [...]

Serialization and JNI Gaps

Java serialization and JNI both rely on reflection-like metadata that the analyzer strips. If you serialize DTOs (e.g. via a cache or session store) or call native libraries, register them explicitly.

Spring's RuntimeHints has dedicated builders for serialization and JNI so you don't hand-write serialization-config.json.

// Register a type for Java serialization in native.
hints.serialization().registerType(OrderDto.class);

// JNI access (e.g. for a native crypto lib):
hints.jni().registerType(
    com.example.NativeCrypto.class,
    MemberCategory.INVOKE_DECLARED_METHODS);

A Repeatable Diagnosis Loop

Put it together into a workflow you can run every time native breaks:

1. Reproduce — run the native binary, capture the exact exception.
2. Classify — reflection? resource? proxy? build-time init? unsupported?
3. Trace — rerun on the JVM with the tracing agent over the failing path.
4. Register — add a RuntimeHintsRegistrar (preferred) or generated JSON.
5. Rebuild & verify — native test suite, not just startup.

Always verify by running the native binary, since JVM tests will never reveal the gap.

Quick Check

Your Spring Boot 4 native binary throws NoSuchMethodException for a DTO's no-arg constructor, but the same code runs fine on the JVM. What is the most appropriate first fix?

在交互式课程中回答

Recap

Native compatibility issues are almost always missing hints caused by GraalVM's closed-world analysis stripping dynamic features.

Classify the failure: reflection, resource, proxy, serialization/JNI, build-time init, or truly unsupported.
Discover needed hints with the tracing agent by exercising every path on the JVM.
Register hints programmatically with RuntimeHintsRegistrar + @ImportRuntimeHints — type-safe and refactor-proof.
Use --initialize-at-run-time for stale-state bugs and register proxies up front.
Always verify against the native binary, never just the JVM.

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