程序在Mongodb异步查询完成前终止

Question

我正在学习以下教程:http://mongodb.github.io/mongo-java-driver/3.2/driver-async/reference/crud/。我只是试图连接到数据库，并读取我创建的一个集合，其中包含两个文档：

import com.mongodb.async.SingleResultCallback;
import com.mongodb.async.client.MongoClient;
import com.mongodb.async.client.MongoClients;
import com.mongodb.async.client.MongoCollection;
import com.mongodb.async.client.MongoDatabase;
import java.util.ArrayList;
import java.util.List;
import org.bson.Document;

public class Main {

    MongoClient client = MongoClients.create();
    MongoDatabase database = client.getDatabase("mydb");

    public Main() {
        readUsers();
    }


    public void readUsers() {

        MongoCollection<Document> collection = database.getCollection("user");

        // find documents
        collection.find().into(new ArrayList<Document>(),
                new SingleResultCallback<List<Document>>() {
            @Override
            public void onResult(final List<Document> result, final Throwable t) {
                System.out.println("Found Documents: #" + result.size());
            }
        });
    }

    public static void main(String[] args) throws Exception {
        new Main();
    }

}

但我不断得到以下错误：

Answer 1

您需要确保main()代码在readusers返回如下内容之前没有完成：

...
import java.util.concurrent.Semaphore;

public class Main {
    MongoClient client = MongoClients.create();
    MongoDatabase database = client.getDatabase("mydb");
    Semaphore semaphore = new Semaphore(0);

    public Main() throws Exception {
        readUsers();
        semaphore.acquire();
    }


    public void readUsers() {

        MongoCollection<Document> collection = database.getCollection("users");

        // find documents
        collection.find().into(new ArrayList<Document>(),
                new SingleResultCallback<List<Document>>() {
                    @Override
                    public void onResult(final List<Document> result, final Throwable t) {
                        System.out.println("Found Documents: #" + result.size());
                        semaphore.release();
                    }
                });
    }

    public static void main(String[] args) throws Exception {
        new Main();
    }

}