LCK08-J. Ensure actively held locks are released on exceptional conditions

An exceptional condition can circumvent the release of a lock, leading to deadlock. According to the Java API [[API 2006]]:

A ReentrantLock is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking lock will return, successfully acquiring the lock, when the lock is not owned by another thread.

Consequently, an unreleased lock in any thread will stop other threads from acquiring the same lock. Intrinsic locks of class objects used for method and block synchronization are automatically released on exceptional conditions (such as abnormal thread termination).

Noncompliant Code Example (Checked Exception)

This noncompliant code example protects a resource using a ReentrantLock but fails to release the lock if an exception occurs while performing operations on the open file. If an exception is thrown, control transfers to the catch block and the call to unlock() is not executed.

public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    try {
      lock.lock();
      InputStream in = new FileInputStream(file);
      // Perform operations on the open file
      lock.unlock();
    } catch (FileNotFoundException fnf) {
      // Handle the exception
    }
  }
}

Note that the lock is not released, even when the doSomething() method returns.

This noncompliant code example does not close the input stream and, consequently, also violates guideline FIO06-J. Ensure all resources are properly closed when they are no longer needed.

Compliant Solution (finally block)

This compliant solution encapsulates operations that may throw an exception in a try block immediately after acquiring the lock. The lock is acquired just before the try block, which guarantees that it is held when the finally block executes. Invoking Lock.unlock() in the finally block ensures that the lock is released, regardless of whether or not an exception occurs.

public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    InputStream in = null;
    lock.lock();
    try {
      in = new FileInputStream(file);
      // Perform operations on the open file
    } catch (FileNotFoundException fnf) {
      // Forward to handler
    } finally {
      lock.unlock();

      if (in != null) {
        try {
          in.close();
        } catch (IOException e) {
           // Forward to handler
        }
      }
    }
  }
}

Compliant Solution (Execute-around Idiom)

The execute-around idiom provides a generic mechanism to perform resource allocation and clean-up operations so that the client can focus on specifying only the required functionality. This idiom reduces clutter in client code and provides a secure mechanism for resource management.

In this compliant solution, the client's doSomething() method provides only the required functionality by implementing the doSomethingWithFile() method of the LockAction interface, without having to manage the acquisition and release of locks or the open and close operations of files. The ReentrantLockAction class encapsulates all resource management actions.

public interface LockAction {
  void doSomethingWithFile(InputStream in);
}

public final class ReentrantLockAction {
  public static void doSomething(File file, LockAction action)  {
    Lock lock = new ReentrantLock();
    InputStream in = null;
    lock.lock();
    try {
      in = new FileInputStream(file);
      action.doSomethingWithFile(in);
    } catch (FileNotFoundException fnf) {
      // Forward to handler
    } finally {
      lock.unlock();

      if (in != null) {
        try {
          in.close();
        } catch (IOException e) {
          // Forward to handler
        }
      }
    }
  }
}

public final class Client {
  public void doSomething(File file) {
    ReentrantLockAction.doSomething(file, new LockAction() {
      public void doSomethingWithFile(InputStream in) {
        // Perform operations on the open file
      }
    });
  }
}

Noncompliant Code Example (unchecked exception)

This noncompliant code example uses a ReentrantLock to protect a java.util.Date instance, which is not thread-safe by design. The doSomethingSafely() method must catch Throwable to comply with guideline EXC06-J. Do not allow exceptions to transmit sensitive information.

final class DateHandler {
  private final Date date = new Date();
  final Lock lock = new ReentrantLock();

  public void doSomethingSafely(String str) {
    try {
      doSomething(str);
    } catch(Throwable t) {
      // Forward to handler
    }
  }

  public void doSomething(String str) {
    lock.lock();
    String dateString = date.toString();
    if (str.equals(dateString)) {
      // ...
    }
    lock.unlock();
  }
}

Because the doSomething() method fails to check if str is null, a runtime exception can occur, preventing the lock from being released.

Compliant Solution (finally block)

This compliant solution encapsulates all operations that can throw an exception in a try block and release the lock in the associated finally block.

final class DateHandler {
  private final Date date = new Date();
  final Lock lock = new ReentrantLock();

  public void doSomethingSafely(String str) {
    try {
      doSomething(str);
    } catch(Throwable t) {
      // Forward to handler
    }
  }

  public void doSomething(String str) {
    lock.lock();
    try {
      String dateString = date.toString();
      if (str != null && str.equals(dateString)) {
        // ...
      }
    } finally {
      lock.unlock();
    }
  }
}

Consequently, the lock is released even in the event of a runtime exception. The doSomething() method also ensures that the string is not null to avoid throwing a NullPointerException.

Risk Assessment

Failing to release locks on exceptional conditions may lead to thread starvation and deadlock.

References

[[API 2006]] Class ReentrantLock

TSM01-J. Do not let the (this) reference escape during object construction      12. Locking (LCK)      THI00-J. Do not assume that the sleep(), yield() or getState() methods provide synchronization semantics