What should I do to get database response from this vertx code - vert.x

Question

This is the method used for database connection and obtain results:
public Future<String> getDatabaseUsers(JDBCClient client) {
return Future.future(pHandler -> {
client.getConnection(res -> {
if (res.succeeded()) {
SQLConnection con = res.result();
con.query("select u.id, u.name from users u", rHandler -> {
String data;
if (rHandler.succeeded()) {
data = Json.encode(rHandler.result().getResults());
} else {
data = "Database execution error";
}
con.close();
pHandler.complete(data);
});
} else {
pHandler.complete("Cannot connect to database");
}
});
});
}
This is the caller method:
private void handleRequest(RoutingContext routingContext, JDBCClient client,
Handler<List<String>> resultHandler) {
routingContext.vertx().<String>executeBlocking(pHandler -> {
pHandler.complete(getDatabaseUsers(client).result());
}, ar -> {
List<String> responses = new ArrayList<>();
if (ar.succeeded()) {
responses.add(ar.result());
System.out.println(Json.encode(responses)); // Null here
}
resultHandler.handle(responses);
});
}
The resultHandler object is used to add more responses from other processes; but this is not the problem actually.
And this is the endpoint:
router.get("/db").handler(ctx -> handleRequest(
ctx, client, (list) -> ctx.response()
.putHeader("Content-Type", "text/plain")
.end(Json.encode(list))));
The problem with this code, is that service response is [null] and the database method is not accomplished yet.
So, what should I do to wait for database response and then send the response to the client?

Answer 1

There are a number of issues with this code:
getDatabaseUsers swallows errors. You probably want to do pHandler.fail(res.failure()); or something similar.
This code: pHandler.complete(getDatabaseUsers(client).result()); is not doing what you think it's doing. result() doesn't block, so you complete your future with another unfishined future.
What you actually want is the reverse:
getDatabaseUsers(client).handle((r) -> { // This is called when the futures actually completes
if (r.succeeded()) {
pHandler.complete(r.result());
}
...
});
I would really recommend going over this guide, as it covers exactly your case.