google/gson · error · JsonIOException

Unable to invoke no-args constructor of ${rawType}; construc

Error message

Unable to invoke no-args constructor of ${rawType}; constructor is not accessible and ReflectionAccessFilter does not permit making it accessible. Register an InstanceCreator or a TypeAdapter for this type, change the visibility of the constructor or adjust the access filter.

What it means

Thrown when a no-args constructor exists but is not accessible (private/protected/package-private) and the ReflectionAccessFilter does not permit making it accessible (BLOCK_INACCESSIBLE or BLOCK_ALL). The filter prevents the setAccessible(true) call that Gson would otherwise use.

Source

Thrown at gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java:424

   * ObjectConstructor}, which would then choose another way of creating the object. And it supports
   * types which are only serialized but not deserialized (compared to directly throwing an
   * exception when the {@code ObjectConstructor} is requested), e.g. when the runtime type of an
   * object is inaccessible, but the compile-time type is accessible.
   */
  private static final class ThrowingObjectConstructor<T> implements ObjectConstructor<T> {
    private final String exceptionMessage;

    ThrowingObjectConstructor(String exceptionMessage) {
      this.exceptionMessage = exceptionMessage;
    }

    @Override
    public T construct() {
      // New exception is created every time to avoid keeping a reference to an exception with
      // potentially long stack trace, causing a memory leak
      // (which would happen if the exception was already created when the
      // `ThrowingObjectConstructor` is created)
      throw new JsonIOException(exceptionMessage);
    }
  }

  private static final class InstanceCreatorConstructor<T> implements ObjectConstructor<T> {
    private final InstanceCreator<T> instanceCreator;
    private final Type type;

    InstanceCreatorConstructor(InstanceCreator<T> instanceCreator, Type type) {
      this.instanceCreator = instanceCreator;
      this.type = type;
    }

    @Override
    public T construct() {
      return instanceCreator.createInstance(type);
    }
  }
}

View on GitHub (pinned to 310ac341f2)

Solutions

1. Change the no-args constructor visibility to public so no setAccessible is needed
2. Register an InstanceCreator for the type to bypass reflective construction
3. Adjust the ReflectionAccessFilter to ALLOW the specific type
4. Add a module 'opens' directive so Gson can access the package

Example fix

// before
class Foo { private Foo() {} }
// + filter returning BLOCK_INACCESSIBLE

// after
class Foo { public Foo() {} }

// Check constructor accessibility given the active filter
Constructor<?> ctor = MyType.class.getDeclaredConstructor();
FilterResult result = myFilter.check(MyType.class);
boolean accessibleByDefault = ReflectionAccessFilterHelper.canAccess(ctor, null);
if (!accessibleByDefault && result != FilterResult.ALLOW) {
  throw new IllegalStateException(
    MyType + " no-args ctor inaccessible and filter is " + result + "; make ctor public or register InstanceCreator");
}

try {
  return gson.fromJson(json, MyType.class);
} catch (JsonIOException e) {
  if (e.getMessage().startsWith("Unable to invoke no-args constructor") && e.getMessage().contains("not accessible")) {
    // make ctor public, widen filter, or register InstanceCreator
    throw new MyParseException("Constructor inaccessible for " + MyType.class, e);
  }
  throw e;
}

When it happens

Trigger: A class has a non-public no-args constructor; a ReflectionAccessFilter with BLOCK_INACCESSIBLE or BLOCK_ALL is registered for the type; the canAccess check fails or the filter blocks the accessibility escalation.

Common situations: Java 9+ JPMS modules where the package isn't open to Gson; private constructors with strict reflection filters; using the built-in BLOCK_INACCESSIBLE_JAVA filter; library classes with package-private constructors.

Related errors

• Unable to create instance of ${rawType}; ReflectionAccessFil
• ${memberDescription} is not accessible and ReflectionAccessF
• cannot deserialize ${baseType} subtype named ${label}; did y
• Failed parsing JSON source to Json
• Invalid EnumSet type: ${type}

AI-assisted analysis of google/gson@310ac341f2 (2026-08-10). Data as JSON: /api/errors/bbb521d296b891cf. Report an issue: GitHub.