Overriding thread-safe methods with methods that are unsafe for concurrent use can result in improper synchronization when a client that depends on the thread-safety promised by the parent inadvertently operates on an instance of the subclass. For example, an overridden synchronized method's contract can be violated when a subclass provides an implementation that is unsafe for concurrent use. Such overriding can easily result in errors that are difficult to diagnose. Consequently, programs must not override thread-safe methods with methods that are unsafe for concurrent use.

The locking strategy of classes designed for inheritance should always be documented. This information can subsequently be used to determine an appropriate locking strategy for subclasses (see LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code and LCK11-J. Avoid client-side locking when using classes that do not commit to their locking strategy).

Noncompliant Code Example (Synchronized Method)

This noncompliant code example overrides the synchronized doSomething() method in the Base class with an unsynchronized method in the Derived class:

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

class Derived extends Base {
  @Override public void doSomething() {
    // ...
  }
}

The doSomething() method of the Base class can be safely used by multiple threads, but instances of the Derived subclass cannot.

This programming error can be difficult to diagnose because threads that accept instances of Base can also accept instances of its subclasses. Consequently, clients could be unaware that they are operating on a thread-unsafe instance of a subclass of a thread-safe class.

Compliant Solution (Synchronized Method)

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

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

class Derived extends Base {
  @Override public synchronized void doSomething() {
    // ...
  }
}

This solution also complies with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code because the accessibility of the class is package-private. Package-private accessibility is permitted when untrusted code cannot infiltrate the package.

Compliant Solution (Private Final Lock Object)

This compliant solution ensures that the Derived class is thread-safe by overriding the synchronized doSomething() method of the Base class with a method that synchronizes on a private final lock object.

class Base {

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

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

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

This is an acceptable solution, provided the locking policy of the Derived class is consistent with that of the Base class.

Noncompliant Code Example (Private Lock)

This noncompliant code example defines a doSomething() method in the Base class that uses a private final lock in accordance with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.

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

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

class Derived extends Base {
  Logger logger = // Initialize

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

It is possible for multiple threads to cause the entries to be logged in an order that differs from the order in which the tasks are performed. Consequently, the doSomething() method of the Derived class cannot be used safely by multiple threads because it is not thread-safe.

Compliant Solution (Private Lock)

This compliant solution synchronizes the doSomething() method of the subclass using its own private final lock object:

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

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

class Derived extends Base {
  Logger logger = // Initialize

  private final Object lock = new Object();

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

Note that the Base and Derived objects maintain distinct locks that are inaccessible from each other's classes. Consequently, Derived can provide thread-safety guarantees independent of Base.

Risk Assessment

Overriding thread-safe methods with methods that are unsafe for concurrent access can result in unexpected behavior.

Automated Detection

Sound automated detection is infeasible; heuristic checks could be useful.

Bibliography