how to get specific fields from MongoDB Atlas in java?

Question

Following Java code return the JSON object which contains all fields of "users" collection in my MongoDB Atlas database, but I want only one field value, say "password". What should I do?

package com.servlets;

import com.google.gson.Gson;
import com.google.gson.JsonObject;
import com.mongodb.MongoClient;
import com.mongodb.MongoClientURI;
import com.mongodb.MongoException;
import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.Filters;
import org.bson.Document;
public class mongoDB 
{
    public static void main(String[] args)
    {
        String username = "gary29198";
        Gson gson = new Gson();
        MongoClientURI uri = new MongoClientURI("mongodb+srv://gary29198:0001221149@sih2019-yp3zc.mongodb.net/test?retryWrites=true");
        MongoClient mongoClient = new MongoClient(uri);
        MongoDatabase database = mongoClient.getDatabase("SIH2019");
        MongoCollection<Document> collections = database.getCollection("users");
        FindIterable<Document> find = collections.find(Filters.eq("username", username));
        try( MongoCursor<Document> cursor = find.iterator() ) 
        {
            while(cursor.hasNext())
            {
                System.out.println(gson.toJson(cursor.next()));
            }
        }
        catch(MongoException e)
        {
            System.out.println(e.getMessage());
        }
    }

}

Answer 1

see this example:

package com.jcg.java.mongodb;
import org.apache.log4j.Logger;
import org.bson.Document;

import com.mongodb.MongoClient;
import com.mongodb.MongoClientURI;
import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.Filters;

public class MongoDemo {

    private static Logger log = Logger.getLogger(MongoDemo.class);

    // Fetching all documents from the mongo collection.
    private static void getAllDocuments(MongoCollection<Document> col) {
        log.info("Fetching all documents from the collection");

        // Performing a read operation on the collection.
        FindIterable<Document> fi = col.find();
        MongoCursor<Document> cursor = fi.iterator();
        try {
            while(cursor.hasNext()) {
                log.info(cursor.next().toJson());
            }
        } finally {
            cursor.close();
        }
    }

    // Fetch a selective document from the mongo collection.
    private static void getSelectiveDocument(MongoCollection<Document> col) {
        log.info("Fetching a particular document from the collection");

        // Performing a read operation on the collection.
        String col_name = "name", srch_string = "Charlotte Neil";
        FindIterable<Document> fi = col.find(Filters.eq(col_name, srch_string));        
        MongoCursor<Document> cursor = fi.iterator();
        try {
            while(cursor.hasNext()) {
                log.info(cursor.next().toJson());
            }
        } finally {
            cursor.close();
        }
    }

    @SuppressWarnings("resource")
    public static void main(String[] args) {

        // Mongodb initialization parameters.
        int port_no = 27017;
        String host_name = "localhost", db_name = "sampledb", db_coll_name = "emp";

        // Mongodb connection string.
        String client_url = "mongodb://" + host_name + ":" + port_no + "/" + db_name;
        MongoClientURI uri = new MongoClientURI(client_url);

        // Connecting to the mongodb server using the given client uri.
        MongoClient mongo_client = new MongoClient(uri);

        // Fetching the database from the mongodb.
        MongoDatabase db = mongo_client.getDatabase(db_name);

        // Fetching the collection from the mongodb.
        MongoCollection<Document> coll = db.getCollection(db_coll_name);

        // Fetching all the documents from the mongodb.
        getAllDocuments(coll);

        log.info("\n");

        // Fetching a single document from the mongodb based on a search_string.
        getSelectiveDocument(coll);
    }
}

Answer 2

Can use projections

collection.find().projection(Projections.include("password"));