Synchronizing a class is unnecessary when it is designed for single-threaded use. Such classes are required to document their lack of thread-safety. For instance, the documentation of class {{java.lang.StringBuilder}} states \[[API 06|AA. Java References#API 06]\] :

This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.

Multithreaded clients of classes that are not thread-safe must externally synchronize any accesses if the documentation of the classes specify the lack of thread-safety, or fail to provide any conclusive information.

Classes that use mutable static fields must always internally synchronize accesses to their fields. This is because there is no guarantee that all clients will synchronize externally when accessing the field. Because a static field is shared by all clients, unrelated clients may violate the contract by not performing adequate synchronization.

Noncompliant Code Example

This noncompliant code example does not synchronize access to the static field counter.

final class CountHits {
  private static int counter;
  
  public void incrementCounter() {
    counter++;
  }
}

It relies on clients to externally synchronize the object and specifies its lack of thread-safety in the documentation. However, there is no guarantee that all unrelated (trusted or untrusted) clients will follow this advice.

Compliant Solution

This compliant solution internally synchronizes the counter field and consequently, does not depend on any external synchronization.

final class CountHits {
  private static int counter;
  
  public synchronized void incrementCounter() {
    counter++;
  }
}

Risk Assessment

Failing to internally synchronize classes containing accessible static members can result in unexpected results when a client fails to obey the classes' synchronization policy.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] 
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 67: "Avoid excessive synchronization"

VOID CON06-J. Do not defer a thread that is holding a lock      11. Concurrency (CON)