There appears to be something fundamentally wrong with @DBusBoundProperty. I can't quite put my finger on it yet, but am getting closer.

All seems well if I am importing an object using these annotations into dbus-java from a service that is also using dbus-java to export it (both ends are using the same interface classes but this doesn't matter).

However, if I try to import interfaces into dbus-java from a service exported by something other than dbus-java, it fails.

The following example fails.

public class AccountsTest {
	
	public static void main(String[] args) throws Exception  {
		try(var conx = DBusConnectionBuilder.forSystemBus().build()) {
			var accounts = conx.getRemoteObject("org.freedesktop.Accounts", "/org/freedesktop/Accounts", Accounts.class);
			System.out.println(accounts.getDaemonVersion());
		}
	}
	
	@DBusInterfaceName("org.freedesktop.Accounts")
	public interface Accounts extends DBusInterface, Properties {
	
		@DBusBoundProperty
		public String getDaemonVersion();
	}
	
}

Results in the exception ...

Exception in thread "main" org.freedesktop.dbus.errors.UnknownMethod: No such method “Get”
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:480)
	at org.freedesktop.dbus.messages.Error.getException(Error.java:111)
	at org.freedesktop.dbus.messages.Error.throwException(Error.java:138)
	at org.freedesktop.dbus.RemoteInvocationHandler.executeRemoteMethod(RemoteInvocationHandler.java:237)
	at org.freedesktop.dbus.RemoteInvocationHandler.executeRemoteMethod(RemoteInvocationHandler.java:250)
	at org.freedesktop.dbus.propertyref.PropRefRemoteHandler.handleDBusBoundProperty(PropRefRemoteHandler.java:69)
	at org.freedesktop.dbus.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:94)
	at jdk.proxy2/jdk.proxy2.$Proxy4.getDaemonVersion(Unknown Source)
	at test.AccountsTest.main(AccountsTest.java:14)

Now I'm sure this used to work during the initial development, but I cannot find any changes since that may have broken it, so maybe it never actually worked and I only ever tested with dbus-java on both ends.

I wondered if part of the problem was what was addressed by this PR. #252. However, fixing that made no difference.

Digging deeper, I found the following two changes seem to fix it. I've not made a PR for this yet, as this was just me stabbing randomly at the keyboard until it worked, and it probably breaks other stuff.

This is in PropRefRemoteHandler. It basically changes the interface that is held by RemoteObject to Properties. I am fairly happy this is a correct change, and safe, as it only applies when the annotation is in use.

The second change was in Marshalling. The was necessary because the return value was always coming back as a Variant in this case. This one was a bit brute force, and looks likely to break something else. But it should hopefully be a clue as to what is actually going on.

The fact that it works with dbus-java exporting the service is a bit odd. What is it doing differently I wonder? It seems like it might not be following the specification of Properties properly, and the importing side was incorrectly designed to match that.