Java操作MongoDB：创建用户前存在性检查逻辑失效问题求助

Hey there! Let's break down why your user existence check isn't working before creating a MyTrazUser, even though you can see the document in MongoDB. First, I notice you didn't include the code for the createMyTrazUser() method itself—that's critical for pinpointing the exact issue—but let's walk through the most common pitfalls based on what you've shared.

Common Causes & Fixes #

1. Your Existence Check Uses the Wrong Identifier #

Looking at your MyTrazUser class, you're fetching a user's UUID via UUIDFetcher, which is great because UUIDs are stable (unlike usernames, which players can change). If your existence check is querying MongoDB by name instead of uuid, that's almost certainly the problem.

For example, if a player changes their username, your check for the new name won't find the existing document (which has the old name), leading you to create a duplicate entry—even though the user already exists in MongoDB under their old name.

Fix: Always query MongoDB using the user's UUID for existence checks. Here's a quick example of what that might look like with the MongoDB Java driver:

public boolean userExists(UUID userUuid) {
    return mongoCollection.find(eq("uuid", userUuid.toString())).first() != null;
}

2. UUIDFetcher Caching or Accuracy Issues #

You mentioned UUIDFetcher caches UUIDs—if the cached UUID doesn't match what's stored in MongoDB, your check will miss the existing document. Maybe the cache has stale data, or UUIDFetcher is returning an incorrect UUID for some users.

Fix:

• Add debug logs to print the UUID returned by UUIDFetcher before your existence check, then compare it to the uuid field in the MongoDB document (use db.yourCollection.findOne({name: "username"}) in the Mongo Shell to verify).
• If caching is the issue, add a way to invalidate the cache for specific users, or bypass the cache temporarily to test if the check starts working.

3. Race Conditions (Multi-Threaded Scenarios) #

If your plugin handles multiple concurrent requests (like multiple players trying to create accounts at the same time), a race condition could be causing the issue: one thread checks for the user (doesn't find it), then before it finishes inserting, another thread also checks and inserts a duplicate.

Fix: Replace the "check-then-insert" pattern with MongoDB's atomic upsert operation. This ensures that either the existing document is found, or a new one is created—no duplicates, no race conditions. Here's an example:

public MyTrazUser createOrGetMyTrazUser(String userName) {
    UUID userUuid = UUIDFetcher.getUUID(userName);
    Document query = new Document("uuid", userUuid.toString());
    
    // Build the document to insert if the user doesn't exist
    Document newUserDoc = new Document("name", userName)
            .append("worldList", new ArrayList<>());
    
    // Use upsert to atomically create or retrieve the user
    UpdateResult result = mongoCollection.updateOne(
        query,
        new Document("$setOnInsert", newUserDoc),
        new UpdateOptions().upsert(true)
    );
    
    // If a new document was inserted, create a MyTrazUser instance
    if (result.getUpsertedId() != null) {
        return new MyTrazUser(userName);
    } else {
        // Fetch the existing document and map it to MyTrazUser
        Document existingDoc = mongoCollection.find(query).first();
        String existingName = existingDoc.getString("name");
        return new MyTrazUser(existingName);
    }
}

4. MongoDB Field Mapping Mismatches #

MongoDB is case-sensitive with field names. If your MyTrazUser class uses a field name that doesn't exactly match what's stored in MongoDB (e.g., Name instead of name), your existence check will fail to find the document.

Fix:

• Verify the field names in your MongoDB document match the ones in your MyTrazUser class. Use db.yourCollection.findOne() in the Mongo Shell to check the exact keys.
• If you're using an ORM like Spring Data MongoDB, double-check that you've annotated fields correctly (e.g., @Field("uuid") if the Mongo field is named uuid).

Next Steps #

To get to the bottom of this quickly, share the code for your createMyTrazUser() method—seeing exactly how you're performing the existence check and inserting the document will let us nail down the issue immediately.

内容的提问来源于stack exchange，提问作者FileInputStream