Java 17 Records to serialize in GWT

[tekkyru]

Many thanks for supporting Java 17 language features

But in my setup the Java records (new feature) are supported on client side but cannot be passed between server and client:

public record TestRecord(String name, int age) implements IsSerializable {}

interface MyService extends RemoteService {

public TestRecord getTestRecord();

}

The GWT Code server displays an error:

[ERROR] 'com.mycompany.TestRecord' has no instantiable subtypes

Am I doing something wrong or the records cannot be de/serialized yet?

I know they don't have the default constructors, but creating them in javascript should not be affected, and de-serializing a record instance as a call parameter can be (probably) done via reflection.

Best regards

Alex

[Jens]

GWT uses either a CustomFieldSerializer implementation to serialize a concrete class or it uses bean serialization. Bean serialization requires a default constructor, serializable fields and final fields are only supported if you enable it.

A CustomFieldSerializer must be written for a concrete class so you cannot provide a serializer for class Record and then expect to send all kinds of records. You can write your own TestRecord_CustomFieldSerializer if you want to send a record via GWT-RPC. It is the same with any other class that you define in your application that need special treatment because it does not have a default constructor. Records just allow you to write a little less code.

https://www.gwtproject.org/doc/latest/DevGuideServerCommunication.html#DevGuideCustomSerialization

-- J.

[Colin Alworth]

Note also that Java itself doesn't serialize records in the same way as other objects, there are a few interesting hiccups there that might impact some use cases. Search for "serializable" at https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Record.html for more discussion on this.

This is not to say that GWT-RPC, if it were updated to support this, would also reflect those exact semantics, but that this shouldn't be implemented lightly, and no one has taken the time to consider how it would be done.

I would also point out that the com.google.gwt.core.ext.typeinfo.JType hierarchy also has not grown past Java 7 (no "default" from Java 8, "module" from Java 9, "sealed" in java 17 etc) so Generators cannot see these flags and react accordingly. This was by design at the time, encouraging any code that needs them to move to annotation processors.

There is a gwt-rpc annotation processor implementation, and while it is used in production, it isn't used widely, but this feature could be explicitly added there.

By the same token, if you have a bunch of records, you could generate custom field serializers with an annotation processor as well.

[tekkyru]

thank you for the detailed answer (as always)