Use of Singleton Pattern in a Non-thread-safe Manner
Weakness ID: 543 (Weakness Variant)Status: Incomplete
+ Description

Description Summary

The use of a singleton pattern may not be thread-safe.
+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

Java

+ Demonstrative Examples

Example 1

This method is part of a singleton pattern, yet the following singleton() pattern is not thread-safe. It fails to ensure the creation of only one object.

(Bad Code)
Example Language: Java 
private static NumberConverter singleton;
public static NumberConverter get_singleton() {
if (singleton == null) singleton = new NumberConverter();
return singleton;
}

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, for 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. It created and returned two different objects. Many programmers turned to the double-check pattern to avoid the overhead of a synchronized call, which is an extension of the one employed, until it too was shown to be not thread-safe.

+ Potential Mitigations

Use Thread-Specific Storage Pattern

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

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness VariantWeakness Variant383J2EE Bad Practices: Direct Use of Threads
Development Concepts (primary)699
ChildOfWeakness BaseWeakness Base662Insufficient Synchronization
Research Concepts (primary)1000
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Anonymous Tool Vendor (under NDA)Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential Mitigations, Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Taxonomy Mappings
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