How can I solve MongoWaitQueueFullException?

You need to check what is the connections per host value which you have given while setting up connection (looking at the exception I think you would have set it to 500).

MongoClientOptions.Builder builder = new MongoClientOptions.Builder();
builder.connectionsPerHost(200);
MongoClientOptions options = builder.build();
mongoClient = new MongoClient(URI, connectionOptions);

An ideal way of setting the connections per host would be by trial and error but you need to make sure that the value which you set should not exceed the number of connections you can have by opening the mongo shell and executing:

db.serverStatus().connections.available

you are in maxWaitQueueSize limit , so increase multiplier ;)

 MongoClientOptions options = MongoClientOptions.builder()
                .threadsAllowedToBlockForConnectionMultiplier(10)
                .build();

 MongoClient mongo = new MongoClient("127.0.0.1:27017", options);
 //run 2000 threads and use database ;)

waitQueueMultiple is the product of maxConnectionPoolSize and threadsAllowedToBlockForConnectionMultiplier hence you can modify one of these three options to tune your app in MongoClientOptions with corresponding values and consume it to your MongoClient as an argument how it was done above here (marked as an answer) https://stackoverflow.com/a/25347310/2852528

BUT

I strongly recommend analysing your code first (where it communicates to the DB), and if no optimization is available (e.g. caching, using aggregation, paging etc.) then go ahead and change the options