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Comments on Convert Shaded Java class to its original

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Convert Shaded Java class to its original

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How do I cast or convert a shadow/shaded class (a package relocated class which is generally generated via tools like gradle/maven) to its original class in Java?

The assumption is that both the shaded version and the original version are simultaneously available on the classpath

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Suggestion for improvements to the question (1 comment)
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Casting the shaded class to its original is not possible, even though they are exactly similar. One will hit classCastExceptions while doing so

The simplest and easiest way is to serialize the shaded class is, using a serializer that doesn't encode the original class information in the serialized version and the deserialize it back. The default Java serializer cannot be used for this. However, gson is a good option.

import shade.prefix.com.company.Employee;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class ShadedEmployeeToVanillaEmployeeConverter {
  private static final Gson gson = createGson();
  private static final String packagePrefix = "shade.prefix.";
  
  public <U extends com.company.Employee> U convert(Employee shadedType) {
    String json = gson.toJson(shadedType);
    try {
      Class<?> clazz = Class.forName(shadedType.getClass().getTypeName().replaceAll(packagePrefix, ""));
      Object o = gson.fromJson(json, clazz);
      return (U) o;
    } catch (ClassNotFoundException e) {
      throw new RuntimeException(e);
    }
  }

  private static Gson createGson() {
    GsonBuilder builder = new GsonBuilder();
    return builder.create();
  }
}

Limitations: (Pointed out by @derek elkins)

This approach will only work for "value" objects, and even then it will have caveats, e.g. loss of numerical precision and the inability to handle objects with cyclic reference graphs. For example, this clearly won't work for a "service" object that holds, say, an open network connection.

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Many caveats (5 comments)
Many caveats
Derek Elkins‭ wrote 11 days ago

This approach will only work for "value" objects, and even then it will have caveats, e.g. loss of numerical precision and the inability to handle objects with cyclic reference graphs. For example, this clearly won't work for a "service" object that holds, say, an open network connection.

This is a partial answer at best. Given that, it would be better to 1) make that explicit, and 2) discuss the limitations of this approach.

Abbas Gadhia‭ wrote 11 days ago

Thanks for pointing them out. I've added the the limitations into the answer

Andreas demands justice for humanity‭ wrote 9 days ago · edited 9 days ago

Once the FFM API receives record mappers, a much quicker solution would be to just go via a MemorySegment. That would achieve pretty much the same as this answer, with more clarity, less overhead, more optimizations, but the same limitations.

That does of course open up the question; can you use a native library to get a pointer to the object, then use the FFM API to reinterpret it? Or otherwise just reinterpret it in the native code? No, because there's an object header in front of the class data, and that header contains the class pointer. So the object header must be overwritten, else Java will continue to interpret the class as what it was, not what you want. So with the exception of inlined value objects (Valhalla) (or any inlined object for that matter), there's no way to reinterpret the class of an object without copying it.

And all that is without mentioning that the JVM is free to organize class data as it sees fit. Two seemingly identical classes could have a different layout, with fields rearranged. I don't think the specification mandates a certain way to lay out classes.

Let's say you did use a native library to forcibly overwrite the class header of the object by overwriting the memory address, would that work? No idea. Possibly, but seems unreliable, especially long term. The garbage collectors and other JVM components rely on the object header, and the non-classpointer bits could be preserved in such an overwrite, though there's no guarantee you won't break something critical.