CON06-J. Methods that override synchronized methods must also possess synchronization capabilities

Concurrency related vulnerabilities can manifest themselves when assumptions are made about the multithreaded behavior of derived classes. An overridden synchronized method's contract may be violated if a subclass defines an implementation that is not thread safe.

Noncompliant Code Example

This noncompliant code example defines a synchronized doSomething() method in a class called SynchronizedBase. The method can be safely used by many threads. However, if a subclass UnsynchronizedSubclass overrides the method but leaves it unsynchronized, its instance cannot be safely passed to multiple threads. This problem is hard to notice because threads that accept instances of SynchronizedBase also freely accept instances of its subclasses. Consequently, the threads may incorrectly assume that the subclasses are thread-safe.

public class SynchronizedBase {
  public synchronized void doSomething() {
    // ...	
  }
}
public class UnsynchronizedSubclass extends SynchronizedBase {
  public void doSomething() {
    // ...		
  }
}

Compliant Solution

This compliant solution synchronizes the doSomething() method of the subclass.

public class SynchronizedBase {
  public synchronized void doSomething() {
    // ...	
  }
}

public class SynchronizedSubclass extends SynchronizedBase {
  public synchronized void doSomething() {
    // ...		
  }
}

Exceptions

EX1: If the subclass implements block synchronization instead of method synchronization, it is quite possible that thread-safety has been accounted for. This condition should be sufficiently documented.

Risk Assessment

Failure to provide synchronization in overriding methods can result in unexpected behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]]
[[SDN 08]] Sun bug database, Bug ID 4294756

FIO36-J. Do not create multiple buffered wrappers on an InputStream      09. Input Output (FIO)      09. Input Output (FIO)