My MongoDB .find() method is returning null even thought the data exists

Question

I am trying to find a user's UUID in my database, but it's returning null for no reason. It's in the connect() method. I can find this manually, and in my project in javaScript. But for some reason, I'm getting this error.

public final class DiscordLink extends JavaPlugin implements Listener {
public MongoCollection<Document> users;
public MongoDatabase mongoDatabase;
    @Override
    public void onEnable() {
        System.setProperty("DEBUG.GO", "true");
        System.setProperty("DB.TRACE", "true");
        Logger mongoLogger = Logger.getLogger("org.mongodb.driver");
        mongoLogger.setLevel(Level.WARNING);
        connect();

        getServer().getConsoleSender().sendMessage(ChatColor.GREEN + "[DiscordLink] Plugin enabled...");

        getServer().getPluginManager().registerEvents(this, this);

    }

    @Override
    public void onDisable() {
        getServer().getConsoleSender().sendMessage(ChatColor.RED + "[DiscordLink] Plugin disabled...");
    }

    public void connect() {
        MongoClient mongoClient = MongoClients.create("MONGOURI");
        MongoDatabase mongoDatabase = mongoClient.getDatabase("myFirstDatabase");
        MongoCollection<Document> users = mongoDatabase.getCollection("users");
        Bukkit.getConsoleSender().sendMessage(ChatColor.GREEN + "[DiscordLink] Connected to database!");
    }

    @EventHandler
    public void onPlayerJoin(PlayerJoinEvent event) {
        Player player = event.getPlayer();
        Document document = users.find(Filters.eq("mcUUID", player.getUniqueId().toString())).first();
        if(document != null) {
            player.kickPlayer(ChatColor.RED + "Discord Authentication Failed!");
        } else { return; }

Error:

[15:43:01 ERROR]: Could not pass event PlayerJoinEvent to DiscordLink v1.0-SNAPSHOT
java.lang.NullPointerException: Cannot invoke "com.mongodb.client.MongoCollection.find(org.bson.conversions.Bson)" because "this.users" is null

Does this answer your question? [Avoiding NullPointerException in Java](https://stackoverflow.com/questions/271526/avoiding-nullpointerexception-in-java) — Elikill58, Jan 12 '22 at 20:56
Woudn't declaring `MongoCollection users` inside the `connect` function mask the class property? Does it behave the same if you start that line with `users = `? — Joe, Jan 13 '22 at 04:54

Elikill58 · Accepted Answer · 2022-01-14T09:23:25.787

2

In the connect method, you are declaring the field MongoCollection<Document> users.

If you IDE is well configurated, it should tell you that this field is not used. Here, you are creating a local field (that keep only in the method scope) instead of setting the global field, which is declared just below the public final class DiscordLink.

To fix it, do like this :

this.mongoDatabase = mongoClient.getDatabase("myFirstDatabase")
this.users = this.mongoDatabase.getCollection("users");

Here, the this key-word is to say you are looking for the global variable, declared outside of the method.

edited Jan 14 '22 at 09:23

answered Jan 13 '22 at 11:09

Elikill58

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This is the error I'm getting now `Exception in thread "Thread-10" java.lang.NullPointerException: Cannot invoke "com.mongodb.client.MongoDatabase.getCollection(String)" because "this.mongoDatabase" is null` – Aklikesramen Jan 14 '22 at 02:35
I just edit my answer. That's exactly the same problem, So you can apply same suggestion to the mongoDatabase object – Elikill58 Jan 14 '22 at 09:23

My MongoDB .find() method is returning null even thought the data exists

1 Answers1