Use of Singleton Pattern Without Synchronization in a Multithreaded Context

Incomplete Variant

Structure: Simple

Description

The product uses the singleton pattern when creating a resource within a multithreaded environment.

Extended Description

The use of a singleton pattern may not be thread-safe.

Common Consequences 1

Scope: OtherIntegrity

Impact: OtherModify Application Data

Potential Mitigations 3

Phase: Architecture and Design

Use the Thread-Specific Storage Pattern. See References.

Phase: Implementation

Do not use member fields to store information in the Servlet. In multithreading environments, storing user data in Servlet member fields introduces a data access race condition.

Phase: Implementation

Avoid using the double-checked locking pattern in language versions that cannot guarantee thread safety. This pattern may be used to avoid the overhead of a synchronized call, but in certain versions of Java (for example), this has been shown to be unsafe because it still introduces a race condition (Generation of Error Message Containing Sensitive Information).

Effectiveness: Limited

Demonstrative Examples 1

This method is part of a singleton pattern, yet the following singleton() pattern is not thread-safe. It is possible that the method will create two objects instead of only one.

Code Example:Bad
Java
java

Consider the following course of events:

- Thread A enters the method, finds singleton to be null, begins the NumberConverter constructor, and then is swapped out of execution. - Thread B enters the method and finds that singleton remains null. This will happen if A was swapped out during the middle of the constructor, because the object reference is not set to point at the new object on the heap until the object is fully initialized. - Thread B continues and constructs another NumberConverter object and returns it while exiting the method. - Thread A continues, finishes constructing its NumberConverter object, and returns its version.

At this point, the threads have created and returned two different objects.

References 1

Thread-Specifc Storage for C/C++

Douglas C. Schmidt, Timothy H. Harrison, and Nat Pryce

https://www.dre.vanderbilt.edu/~schmidt/PDF/TSS-pattern.pdf(2025-07-24)

ID: REF-474

Applicable Platforms

Languages:

Java : UndeterminedC++ : Undetermined

Modes of Introduction

Implementation

Related Weaknesses