Which JAX-RS endpoints? Headless APIs or your custom?

Thanks Javier, actually it is a basic api created by CLI tool : blade create -t rest and deployed as a jar into liferay.

If we add a line "throw new Exception("test expection)" into the code of the typical "It works" endpoint, the exception is throw only inside Liferay logs, but any frontend consuming the endpoint receives code success 200

@GET
    @Produces("text/plain")
    public String working() {
        return "It works!";
    }

Thanks

Assuming that you build your own endpoint:
You need to return a Response instead of a String 

@GET
@Produces("text/plain")
public Response working() {
  if (everything fine) {
     return Response.status(Response.Status.OK).entity("It works").build();
  } else {
    return Response.status(500).build();
  }
}

The problem is: An exception is thrown, what do you want t send to the frontend? 500? Or was the exception caused by something the frontend did wrong? So, you want to send an appropriate message back.

The easiest way is to handle the exceptions yourself and create an appropriate response.

BUT what you want, should be possible. The question is, which Liferay version do you use? Or to be more precise: Which rest implementation do you use. 7.0 has only CXF, 7.1+ usually uses OSGI Whiteboard.

And these implementations of the specifications provide ways to handle exceptions. OSGI Whiteboard allows you to implement Exception Mappers to handle and "translate" Exceptions into an appropriate response. But I never did that before and I can only point you to the specification:

https://docs.osgi.org/specification/osgi.cmpn/7.0.0/service.jaxrs.html

It looks like a bug, not sure if CXF or ours. I've filed it here: https://issues.liferay.com/browse/LPS-131519.

You can avoid it by registering an exception mapper that catches all exceptions/errors. That's what the Liferay APIs are doing by default here. That code has a lot of stuff because it returns exceptions formatted like Problem JSON and is registered with a JAX-RS feature. But you can just return easier responses like this:

 @Override
  public Response toResponse(Exception exception) {
    return Response.status(500)
        .entity(ex.getMessage())
        .build();
  }

You can register it to your application like this:

@Component(
	property = {
		"osgi.jaxrs.application.select=(osgi.jaxrs.name=YOUR_APPLICATION)",
		"osgi.jaxrs.extension=true",
		"osgi.jaxrs.name=YOUR_EXCEPTION_NAME"
	},
	service = ExceptionMapper.class
)