How do we obtain the Google App Engine safe-URL key in Java for a different appId and namespace? - google-app-engine

Question

When I just need to do it for my application without namespace I can use the following code:
final Key myKey = KeyFactory.createKey(kind, id);
final String safeUrlKey = KeyFactory.keyToString(myKey);
Unfortunately when I need to do it for a different appId or namespace I don't find any way to do it in Java.
In python for example I can use the following code:
new_key = db.Key.from_path(entity, id, _app=application_id, namespace=namespace)
return str(new_key)
But in Java this doesn't seem to be available.
Any idea on how I can do this?

Answer 1

The App Engine SDK does indeed try to prohibit this, as evidenced by the lack of public classes/methods that can handle app IDs and namespaces. Even in python this is discouraged by the underscore prefix on the _app keyword argument. This is probably because App Engine apps are meant to be well-contained within their project.
It is possible to use reflection to workaround these barriers, but only on the Standard Java 8 runtime (which is currently in beta). The Standard Java 7 runtime prohibits reflecting non-accessible methods. (If you're using App Engine Flex I suspect you'll be ok too, although I haven't tested that.)
If you are already using Java 8 or willing to switch, I was able to create keys for arbitrary app IDs/namespaces with the following:
Key createKey(String appId, String namespace, String kind, long id) {
try {
Class<?> appNsClazz = Class.forName("com.google.appengine.api.datastore.AppIdNamespace");
Constructor<?> constructor = appNsClazz.getConstructor(String.class, String.class);
constructor.setAccessible(true);
Constructor<Key> keyFactory = Key.class.getDeclaredConstructor(String.class,
Key.class, long.class, String.class, appNsClazz);
keyFactory.setAccessible(true);
Object appNs = constructor.newInstance(appId, namespace);
return keyFactory.newInstance(kind, /* parent key */ null, id, /* name */ null, appNs);
} catch (ClassNotFoundException | NoSuchMethodException |
InvocationTargetException | InstantiationException |
IllegalAccessException e) {
throw new RuntimeException(e);
}
}
If you will be running this code often it would be good to cache the Constructor instances, and the appNs instance if possible to avoid the performance overhead of reflection.
Please do note that this code will not work on the Standard Java 7 runtime.

Answer 2

Finally I was able to make it work with the following code (looking at how KeyFactory was doing it internally):
public static String getSafeUrlFromId(final String kind, final Long id, final String applicationId, final String namespace) {
final com.google.storage.onestore.v3.OnestoreEntity.Reference myMessage = new com.google.storage.onestore.v3.OnestoreEntity.Reference();
final Element pathElement = new Element().setType(kind).setId(id);
final Path path = myMessage.getMutablePath();
path.addElement(pathElement);
myMessage.setPath(path);
if (namespace != null){
myMessage.setNameSpace(namespace);
}
myMessage.setApp(applicationId);
final BaseEncoding encoder = BaseEncoding.base64Url();
final String alphanumericKey = encoder.omitPadding().encode(myMessage.toByteArray());
return alphanumericKey;
}