Java monitorenter instructions cannot be interrupted

A Java application I'm helping with sometimes needs to terminate untrusted threads that got out of hand. Our sandbox seemed to be working great until I tried running two tasks that tried to synchronize on the same object and spin indefinitely once the lock was acquired. Trying to terminate the locked-out thread failed until the thread that acquired the lock was stopped. Apparently the monitorexit JVM instruction used to enter a synchronized block cannot be interrupted by stopping the thread, much less with the normal interrupt mechanism. We were already doing some bytecode editing for our sandbox, so I extended that to rewrite uses of synchronization-related instructions or methods to call static sandbox functions that simulate the originals using ReentrantLock, locking functions of which can be halted by thread shutdown.