An exceptional condition can circumvent the release of a lock, leading to deadlock. According to the Java API [API 20062014]:
A
ReentrantLockis owned by the thread last successfully locking, but not yet unlocking it. A thread invokinglockwill 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. HoweverHowever, 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, 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 catch block and the call to unlock() never executes.
...
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.
...
This noncompliant code example uses a ReentrantLock to protect a java.util.Date instance – recall instance—recall that java.util.Date is thread-unsafe by design.
...
Failure to release locks on exceptional conditions could lead to thread starvation and deadlock.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
LCK08-J | lowLow | likelyLikely | lowLow | P9 | L2 |
Automated Detection
Some static analysis tools are capable of detecting violations of this rule.
| Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| ThreadSafe |
| CCE_LK_UNRELEASED_ON_EXN | Implemented |
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
...