apache/beam · error · RuntimeException

Unable to generate a getter for field

Error message

Unable to generate a getter for field '{}'.

What it means

createGetter generates a bytecode-implemented FieldValueGetter for a schema field and then warms it up by constructing an instance of the POJO via its no-arg constructor. If code generation or that warm-up instantiation throws, this RuntimeException is raised with the field in the message (cause chain preserved via `e`).

Solutions

1. Read the chained cause `e` to determine whether it is a generation error or an instantiation error; fix accordingly.
2. Add a public zero-argument constructor so the getter warm-up instantiation succeeds.
3. Simplify or correctly parameterize the field type (e.g. List<String> rather than raw List) so a getter can be generated.
4. Check field visibility/accessors: ensure a standard bean-style getter exists or the field is accessible.

Example fix

// before
public class Item { private List values; } // raw generic type, no no-arg ctor
// after
public class Item {
  public Item() { }
  private List<String> values;
  public List<String> getValues() { return values; }
}

Field f = MyPojo.class.getDeclaredField("values");
if (f.getGenericType() instanceof Class) {
  throw new IllegalStateException("Field '" + f + "' uses a raw type; Beam cannot generate a getter");
}

try {
  FieldValueGetter<T, Object> getter = POJOUtils.createGetter(type, typeConversionsFactory);
} catch (RuntimeException e) {
  if (e.getMessage() != null && e.getMessage().startsWith("Unable to generate a getter")) {
    // inspect e.getCause(): fix field type parameterization or add a no-arg ctor
  } else throw e;
}

When it happens

Trigger: Calling createGetter for a field whose type has no supported getter mapping, or when the transient instantiation of the POJO inside the try block fails (no zero-arg constructor, constructor throws), or ByteBuddy generation fails on the class.

Common situations: Fields of exotic types (e.g. generic collections without proper type info) that fail getter generation, POJOs without no-arg constructors tripping the warm-up instantiation, or final classes/methods interfering with bytecode generation.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• A method marked with SchemaCreate in class
• Collection parameter is not parameterized!
• Could not determine array parameter type for field.
• Cound not determine array parameter type for field.
• Failed to locate DefaultGetSize.validateSize()

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/28a4c5989ac58008. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/utils/POJOUtils.java:320

            BYTE_BUDDY,
            field.getDeclaringClass(),
            typeConversionsFactory.createTypeConversion(false).convert(typeInformation.getType()));
    builder = implementGetterMethods(builder, typeInformation, typeConversionsFactory);
    try {
      return builder
          .visit(new AsmVisitorWrapper.ForDeclaredMethods().writerFlags(ClassWriter.COMPUTE_FRAMES))
          .make()
          .load(
              ReflectHelpers.findClassLoader(field.getDeclaringClass().getClassLoader()),
              getClassLoadingStrategy(field.getDeclaringClass()))
          .getLoaded()
          .getDeclaredConstructor()
          .newInstance();
    } catch (InstantiationException
        | IllegalAccessException
        | NoSuchMethodException
        | InvocationTargetException e) {
      throw new RuntimeException("Unable to generate a getter for field '" + field + "'.", e);
    }
  }

  private static <ObjectT, ValueT>
      DynamicType.Builder<FieldValueGetter<@NonNull ObjectT, ValueT>> implementGetterMethods(
          DynamicType.Builder<FieldValueGetter<@NonNull ObjectT, ValueT>> builder,
          FieldValueTypeInformation typeInformation,
          TypeConversionsFactory typeConversionsFactory) {
    return builder
        .visit(new AsmVisitorWrapper.ForDeclaredMethods().writerFlags(ClassWriter.COMPUTE_FRAMES))
        .method(ElementMatchers.named("name"))
        .intercept(FixedValue.reference(typeInformation.getName()))
        .method(ElementMatchers.named("get"))
        .intercept(new ReadFieldInstruction(typeInformation, typeConversionsFactory));
  }

  // Implements a method to read a public field out of an object.
  static class ReadFieldInstruction implements Implementation {

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