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An exceptional condition can circumvent the release of a lock, leading to deadlock. According to the Java API [API 20062014]:

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 prevent other threads from acquiring the same lock. Programs must release all actively held locks on exceptional conditions. Intrinsic locks of class objects used for method and block synchronization are automatically released on exceptional conditions (such as abnormal thread termination).

This guideline is an instance of FIO04-J. Release resources when they are no longer needed. However, most Java lock objects are not closeable, so they cannot be automatically released using Java 7's try-with-resources feature.

Noncompliant Code Example (Checked Exception)

This noncompliant code example protects a resource using a ReentrantLock but , an open file, by using 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() fails to execute never executes.

public final class Client {
  private final Lock lock = new ReentrantLock();

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

      // Perform operations on the open file

      lock.unlock();
    } catch (FileNotFoundException x) {
      // Handle exception
    } finally {
      if (in != null) {
        try {
         lock in.lockclose();
        } 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 code 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 {
  private final Lock lock = new FileInputStream(file);

      // Perform operations on the open file

 ReentrantLock();

  public void doSomething(File file) {
    InputStream in = null;
    try {
      in = new FileInputStream(file);
      lock.unlocklock();
      // Perform operations on the open file
    } catch (FileNotFoundException xfnf) {
      // Forward Handleto exceptionhandler
    } finally {
      lock.unlock();
      if (in != null) {
        try {
          in.close();
        } catch (IOException xe) {
          // Forward Handleto exceptionhandler
        }  
      }
    }
  }
}

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 an exception occurs or not.

public final class Client {
  public void doSomething(File file) {
   private final Lock lock = new ReentrantLock();

  public void doSomething(File file) {
    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
        }
      }
    }
  }
}

...

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 {
  private static final Lock lock = new ReentrantLock();

  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
      }
    });
  }
}

...

This noncompliant code example uses a ReentrantLock to protect a java.util.Date instance �����������€š�š�š�š�š�����������‚�š�š�š�š����������‚�š�š�š��������‚�š�š������‚�š����‚��“ recall instance—recall that java.util.Date is thread-unsafe by design.

final class DateHandler {

  private final Date date = new Date();

  private final Lock lock = new ReentrantLock();

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

    lock.unlock();
  }
}

...

This compliant solution encapsulates all operations that can throw an exception in a try block and releases the lock in the associated finally block. Consequently, the lock is released even in the event of a runtime exception.

final class DateHandler {

  private final Date date = new Date();

  private final Lock lock = new ReentrantLock();

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

    } finally {
      lock.unlock();
    }
  }
}

...

Failure to release locks on exceptional conditions could lead to thread starvation and deadlock.

Automated Detection

Some static analysis tools are capable of detecting violations of this rule.

Related Vulnerabilities

The GERONIMO-2234 issue report describes a vulnerability in the Geronimo application server. If the user single-clicks the keystore portlet, the user will lock the default keystore without warning. This causes a crash and stack trace to be produced. FuthermoreFurthermore, the server cannot be restarted because the lock is never cleared.

Related Guidelines

Bibliography

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