CON15-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 06]]:

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. In an exception is thrown, control transfers to the catch block and the call to unlock() is not executed.

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.

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. The lock is released in the corresponding finally block, which ensures the lock is released regardless of whether or not an exception occurs.

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

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.

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.

Note that the doSomethingSafely() method must catch Throwable to be compliant with [EXC06-J. Do not allow exceptions to transmit sensitive information].

Compliant Solution (finally block)

This compliant solution encapsulates operations that can thrown an exception in an 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.

Automated Detection

TODO

Related Vulnerabilities

GERONIMO-2234

References

[[API 06]] Class ReentrantLock

[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!]