Page History

...

This noncompliant code example protects a resource, an open file, by using a a ReentrantLock. However, the method fails to release the lock when an exception occurs while performing operations on the open file. When an exception is thrown, control transfers to the the catch block  block and the call to to unlock() never  never executes.

public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    InputStream in = null;
    try {
      lock.lock();
      in = new FileInputStream(file);

      // Perform operations on the open file

      lock.unlock();
    } catch (FileNotFoundException x) {
      // Handle exception
    } finally {
      if (in != null) {
        try {
          in.close();
        } catch (IOException x) {
          // Handle exception
        }  
      }
    }
  }
}

Noncompliant Code Example (finally Block)

This noncompliant code example attempts to rectify the problem of the lock not being released, by invoking Lock.unlock() in the finally block. This ensures that the lock is released regardless of whether or not an exception occurs. However, it does not acquire the lock until after trying to open the file. If the file cannot be opened, the lock may be unlocked without ever being locked in the first place.

public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    InputStream in = null;
    try {
      in = new FileInputStream(file);
      lock.lock();
      // 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 (finally Block)

This compliant solution encapsulates operations that could throw an exception in a try block immediately after acquiring the lock (which cannot throw). 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
        }
      }
    }
  }
}

...