无法使用Heroku上的Java驱动程序与MongoDB连接

我正在Heroku上使用Java和MongoDB构建Web应用程序。我从这里学到了这个项目。我一直努力尝试了两天。

我用3.2.2 mongo-java-driver和3.5.1 maven-compiler-plugin

这是在我的Main课上：

public static void main(String[] args) throws MongoException, UnknownHostException, Exception {
    MongoClientURI uri = new MongoClientURI(System.getenv("MONGOHQ_URL"));
    MongoClient mongoClient = new MongoClient(uri);

    String dbname = uri.getDatabase();

    //mongoClient.setWriteConcern(WriteConcern.JOURNALED);
    DB db = mongoClient.getDB(dbname);

    staticFileLocation("/public");
    new TodoResource(new TodoService(db));

    MongoCredential credential = MongoCredential.createCredential(uri.getUsername(),dbname,uri.getPassword());
    MongoClientOptions mongoClientOptions = MongoClientOptions.builder().build();
}

我得到了这个错误：

1:38:42 AM web.1 | Exception in thread "main"
1:38:42 AM web.1 | java.lang.NullPointerException
1:38:42 AM web.1 | at com.mongodb.MongoClientURI.<init>   (MongoClientURI.java:174…
1:38:42 AM web.1 | at com.mongodb.MongoClientURI.<init> (MongoClientURI.java:159…
1:38:42 AM web.1 | at Main.main(Main.java:32)

这是空指针异常，所以我试图取代MONGOHQ_URL的mongodb://user:passwd@host1:port1,host2:port2/dbname，而且我还试图用MLAB的MongoDB在Heroku上，但同样的错误：

11:17:05 AM web.1 |  SLF4J: Class path contains multiple SLF4J bindings.
11:17:05 AM web.1 |  SLF4J: Found binding in [jar:file:/Users/zhugejunwei/full-sta…
11:17:05 AM web.1 |  SLF4J: Found binding in [jar:file:/Users/zhugejunwei/full-sta…
11:17:05 AM web.1 |  SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings …
11:17:05 AM web.1 |  SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerF…
11:17:05 AM web.1 |  Exception in thread "main" java.lang.NullPointerException
11:17:05 AM web.1 |     at com.mongodb.ConnectionString.<init>(ConnectionString.java…
11:17:05 AM web.1 |     at com.mongodb.MongoClientURI.<init>(MongoClientURI.java:176…
11:17:05 AM web.1 |     at com.mongodb.MongoClientURI.<init>(MongoClientURI.java:158…
11:17:05 AM web.1 |     at Main.main(Main.java:28)

从原始版本到当前版本，我确实做了很多尝试，但我只想使其在Heroku上可运行，以另一种方式学习该项目。

当我学习了另一个教程时，下面的代码还可以，但是我不知道为什么它在该项目中不起作用。

MongoURI mongoURI = new MongoURI(System.getenv("MONGOHQ_URL"));
//get connected
DB db = mongoURI.connectDB();
mongoURI.getPassword());
MongoCredential credential = MongoCredential.createCredential(mongoURI.getUsername(), mongoURI.getDatabase(), mongoURI.getPassword());
MongoClient mongoClient = new MongoClient(new ServerAddress(), Arrays.asList(credential));

我还在github和jira上打开了一个问题

请帮我解决这个问题。提前致谢。