How to properly handle exceptions in MongoClient for VertX - mongodb

Question

In the startup method of my application I want to check that the credentials for MongoDB provided to the application are OK. If they are OK, I continue the startup, if not, the application is supposed to exit as it cannot connect to the DB. The code snippet is as below:
// Create the client
MongoClient mongodb = null;
try {
mongodb = MongoClient.createShared(vertx, mongo_cnf, mongo_cnf.getString("pool_name"));
}
catch(Exception e) {
log.error("Unable to create MongoDB client. Cause: '{}'. Bailing out", e.getMessage());
System.exit(-1);
}
If I provide wrong credentials, the catch block is not called. Yet I get the following on the console:
19:35:43.017 WARN org.mongodb.driver.connection - Exception thrown during connection pool background maintenance task
com.mongodb.MongoSecurityException: Exception authenticating MongoCredential{mechanism=null, userName='user', source='admin', password=<hidden>, mechanismProperties={}}
at com.mongodb.connection.SaslAuthenticator.wrapException(SaslAuthenticator.java:162)
at com.mongodb.connection.SaslAuthenticator.access$200(SaslAuthenticator.java:39)
... many lines
The question is: how to intercept this exception in my code and be able to handle it properly ?

Answer 1

The exception is happening in the mongodb's java driver daemon thread so you cannot catch it.
Vertx MongoClient abstracts you from direct interaction with MongoDB Java driver so you can't modify anything related to the client.
You could access mongo client instance via reflection, but as it's already created you cannot pass additional configuration to it.
If you used com.mongodb.async.client.MongoClient you could pass ServerListener which could access the exception and you could examine it (please see this answer for more details - https://stackoverflow.com/a/46526000/1126831).
But it's only possible to specify the ServerListener in the moment of construction of the mongo client, which happens inside the Vertx MongoClient wrapper and there's no way to pass this additional configuration.

Answer 2

Currently the exception is not thrown, which in my opinion a mistake in design, since you receive an object that you cannot work with. Feel free to open a bug: https://github.com/vert-x3/vertx-mongo-client/issues
What you can do to detect that your client is "dead or arrival" is to wait for connection timeout:
// Default is 30s, which is quite long
JsonObject config = new JsonObject().put("serverSelectionTimeoutMS", 5_000);
MongoClient client = MongoClient.createShared(vertx, config, "pool_name");
client.findOne("some_collection", json1, json2, (h) -> {
if (h.succeeded()) {
//...
}
else {
// Notify that the client is dead
}
});