SimpleMongoClientDatabaseFactory does not close MongoClient when it's not registered as a Spring Bean

[jayjaehunchoi]

Summary of why MongoClient is not closed when not registered as a Spring Bean

Problem Description

When SimpleMongoClientDatabaseFactory is created with a MongoClient that is not registered as a Spring Bean, the MongoClient is not properly closed during application shutdown, leading to resource leaks.

Root Cause Analysis

1. SimpleMongoClientDatabaseFactory Constructor Behavior

   public SimpleMongoClientDatabaseFactory(MongoClient mongoClient, String databaseName) {
       this(mongoClient, databaseName, false); // mongoInstanceCreated = false
   }

   When a MongoClient is passed from outside (not created by Spring), mongoInstanceCreated is set to false.

2. Destroy Method Implementation

   @Override
   public void destroy() {
       if (this.mongoInstanceCreated) { // Only closes if true
           this.mongoClient.close();
       }
   }

   The destroy() method only closes the MongoClient if mongoInstanceCreated is true.

3. Spring Lifecycle Management Principle

   • Spring only manages the lifecycle of objects it creates (registered as Beans)
   • External objects injected into Spring Beans are not automatically managed
   • mongoInstanceCreated = false indicates "Spring did not create this MongoClient, so Spring will not close it"

Impact

• Resource Leaks: MongoDB connections remain active on the server side
• Connection Pool Exhaustion: May reach connection pool limits
• Server Resource Waste: MongoDB server continues to maintain inactive connections
• Timeout Issues: Connections eventually timeout after server-side timeout period

Expected Behavior

SimpleMongoClientDatabaseFactory should either:

• Close the MongoClient regardless of mongoInstanceCreated flag, OR
• Provide a clear mechanism to handle externally created MongoClient instances

[mp911de]

The lifecycle of an externally provided MongoClient must be managed outside of SimpleMongoClientDatabaseFactory. This is a core design pattern of dependency injection to avoid that arbitrary components mess with your object state.

Disposable objects, that create other stateful resources have to clean up state themselves as there is not outside mechanism to get hold of the inner resource for outside disposal.

The correct way of expression your principle

mongoInstanceCreated = false indicates "Spring did not create this MongoClient, so Spring will not close it

is:

mongoInstanceCreated = false indicates "SimpleMongoClientDatabaseFactory did not create this MongoClient, so SimpleMongoClientDatabaseFactory will not close it

[mp911de]

FWIW, in many places, DisposableBean is cleaning up resources being provided to the bean:

• Spring LDAP SingleContextSource
• HttpComponentsClientHttpRequestFactory
• JdkHttpClientResourceFactory

The problem with such an approach is that we cannot know whether a resource is managed externally or not. In the case of Spring Boot, the client is managed by Spring Boot. Closing the resource upon destroy() will make it unusable for others and I consider such a behavior a surprise.
While destroy() is typically used in the container lifecycle, such a method can be called independently from the context lifecycle.

[jayjaehunchoi]

@mp911de
Yes, I undestand what you said, and i agree about that 👍 Thank you for your kindness.

Then how about making the following constructor public?

SimpleMongoClientDatabaseFactory(MongoClient mongoClient, String databaseName, boolean mongoInstanceCreated) {
    super(mongoClient, databaseName, mongoInstanceCreated, MongoExceptionTranslator.DEFAULT_EXCEPTION_TRANSLATOR);
}