Vfrdtyky

I am using MongoDB with java. I need a way to check if MongoDB is running (to prevent exceptoins later). I tried it with this code:

mongoClient = new MongoClient();

try {

    mongoClient.getAddress();

    mongoRunning = true;

} catch (Throwable e){

    log.warn("no db connected");

    return;

}

I know throwable is bad but it is not working neither with exception nor with Throwable. I see (the catch block is not entered!):

[INFO ] 2018-11-10 22:33:26.209 [cluster-ClusterId{value='5be74e9f7170312fd4eb1ffe', description='null'}-127.0.0.1:27017] cluster - Exception in monitor thread while connecting to server 127.0.0.1:27017

com.mongodb.MongoSocketOpenException: Exception opening socket

    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:67) ~[mongo-java-driver-3.8.2.jar:?]

    at com.mongodb.internal.connection.InternalStreamConnection.open(InternalStreamConnection.java:126) ~[mongo-java-driver-3.8.2.jar:?]

    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:117) [mongo-java-driver-3.8.2.jar:?]

    at java.lang.Thread.run(Thread.java:745) [?:1.8.0_102]

Caused by: java.net.ConnectException: Connection refused: connect

    at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method) ~[?:1.8.0_102]

    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85) ~[?:1.8.0_102]

    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[?:1.8.0_102]

    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[?:1.8.0_102]

    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[?:1.8.0_102]

    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172) ~[?:1.8.0_102]

    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[?:1.8.0_102]

    at java.net.Socket.connect(Socket.java:589) ~[?:1.8.0_102]

    at com.mongodb.internal.connection.SocketStreamHelper.initialize(SocketStreamHelper.java:64) ~[mongo-java-driver-3.8.2.jar:?]

    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:62) ~[mongo-java-driver-3.8.2.jar:?]

    ... 3 more

And my program terminates. An i thought i know java :-(. But i also tried:

mongoClient.listDatabases();

This behaves also odd: mongoRunning is set to true and an exception is thrown asynchronously.
I need a way to check the connection once. Is this possible without any exception and in a synchronous way?

MongoClient represents a connection pool. It manages the connections, and everything happening in its threads.

I think it is wrong to ask question "if MongoDB is running". Suppose you have a tool or instrument to check if it is running, and you receive no exception at the time of checking, but right after you checked MongoDB is crashed, what would you do. I.e. it is wrong to attempt such check, instead one should focus to write application in reliable way with proper error handling at the time of queries.

Additionally, you may look at ConnectionPoolSettings which has getMaxWaitQueueSize, by default its 500 queries before it starts throwing exceptions. If that is what causes problems to you, you may try to reduce this to lesser value, to start receiving exceptions earlier.