CON10-J. Do not override thread-safe methods with thread-unsafe methods

Concurrency related issues 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 safe for concurrent access.

The guideline [CON04-J. Synchronize using an internal private lock object] recommends documenting the locking strategy in use for classes designed for inheritance. This information is useful when deciding the locking strategy of subclasses.

Noncompliant Code Example (synchronized method)

This noncompliant code example defines a synchronized doSomething() method in class Base.

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

class Sub extends Base {
  public void doSomething() {
    // ...		
  }
}

The doSomething() method of class Base can be safely used by multiple threads. However, if a subclass Sub overrides the method but leaves it unsynchronized, its instance cannot be safely used by multiple threads.

This problem is hard to notice because threads that accept instances of Base also accept instances of its subclasses. Consequently, the threads may incorrectly assume that the subclasses are thread-safe.

Compliant Solution (synchronized method)

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

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

class Sub extends Base {
  public synchronized void doSomething() {
    // ...		
  }
}

This compliant solution does not violate [CON04-J. Synchronize using an internal private lock object] because the accessibility of the class is package-private which is allowable when untrusted code cannot infiltrate the package.

Noncompliant Code Example (private lock)

This noncompliant code example defines a doSomething() method in class Base that uses an internal private lock, in accordance with [CON04-J. Synchronize using an internal private lock object].

public class Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      // ...	
    }
  }
}

class Sub extends Base {
  public void doSomething() {
    try {
      super.doSomething();
    } finally() {
      logger.log(Level.FINE, "Did something"); 
    }
  }
}

The doSomething() method of class Sub cannot be safely used by multiple threads because it is not thread-safe.

Compliant Solution (private lock)

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

public class Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      // ...	
    }
  }
}

class Sub extends Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      try {
        super.doSomething();
      } finally() {
        logger.log(Level.FINE, "Did something"); 
      }
    }
  }
}

Note that the Base.lock and Sub.lock objects are distinct and inaccessible from each others' classes. Consequently, Sub can provide thread-safety guarantees independent of Base.

Exceptions

EX1: If the class Base performs a single operation, such as calling the corresponding super class method, or throwing an exception, it need not be synchronized.

// ...
class Sub extends Base {
  public void doSomething() {
    super.doSomething();
  }
}

Risk Assessment

Overriding thread-safe methods with thread-unsafe 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

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