Java Concurrency Interview Questions

Threads encounter various lock-related issues during execution, including lock contention, lock waiting, and lock failure. These phenomena are typically caused by factors such as lock granularity, fairness, acquisition order, and memory visibility. The following is a detailed explanation of these problems and their corresponding optimization strategies:

1. Lock Contention (锁竞争)

  • Phenomenon: Multiple threads compete for the same lock simultaneously, leading to performance degradation due to context switching and serialization.
  • Cause: Coarse-grained locks or frequent locking/unlocking operations on small data units.
  • Optimization: Use lock coarsening (merging multiple fine-grained locks into a single coarse-grained lock) or non-fair locks to reduce the overhead of acquiring and releasing locks under high contention.

2. Lock Waiting (锁等待)

  • Phenomenon: A thread is blocked because it cannot acquire the lock immediately.
  • Cause: Failed lock acquisition attempts or excessively long wait times due to poor queue management.
  • Optimization: Utilize a lock pool mechanism to manage waiting queues efficiently, or adopt more efficient locking strategies (e.g., biased locking for single-threaded access) to minimize waiting time.

3. Lock Failure / Deadlock (锁失效/死锁)

  • Phenomenon: A lock remains held indefinitely, preventing other threads from acquiring it, potentially causing the system to hang.
  • Cause: Threads fail to release locks correctly, often due to unhandled exceptions or circular dependency in lock acquisition order.
  • Optimization: Ensure proper lock release mechanisms (e.g., using try-finally blocks) and implement deadlock detection/prevention strategies to avoid resource leaks.

4. Lock Fairness (锁的公平性)

  • Phenomenon: Fair locks grant access strictly in the order of request arrival, which can lead to performance drops if a thread holding the lock for a long time blocks many subsequent requests.
  • Cause: Non-fair locks allow "thread stealing" (interleaving), increasing contention but improving throughput under high load.
  • Optimization: Select between fair locks and non-fair locks based on the specific scenario to balance fairness with performance.

5. Lock Coarsening (锁的降级/粗化)

  • Phenomenon: Upgrading from fine-grained locks to coarse-grained locks to reduce contention.
  • Cause: Fine-grained locks incur significant overhead due to frequent acquisition and release operations.
  • Optimization: Use a lock pool or lock coarsening techniques to manage granularity, thereby improving overall efficiency by reducing the number of synchronization points.

6. Lock Fineness (锁的升级/细化)

  • Phenomenon: Upgrading from coarse-grained locks to fine-grained locks to increase flexibility and parallelism.
  • Cause: Coarse-grained locks may limit concurrency by serializing access to larger data structures unnecessarily.
  • Optimization: Dynamically adjust lock granularity based on demand to balance performance with the need for flexible data access patterns.

7. Lock Visibility (锁的可见性)

  • Phenomenon: Other threads cannot see modifications made by the thread that released the lock.
  • Cause: Improper memory barriers or lack of synchronization guarantees leading to stale reads.
  • Optimization: Use efficient locking strategies combined with appropriate memory barriers (e.g., volatile, happens-before relationships) to ensure visibility across threads.

8. Lock Ordering / Reordering (锁的有序性)

  • Phenomenon: The order of lock acquisition and release affects the execution sequence, potentially leading to inconsistent states or deadlocks.
  • Cause: Instruction reordering at the CPU level can disrupt the logical order of operations if not properly synchronized.
  • Optimization: Use volatile variables or atomic classes (e.g., AtomicInteger, ReentrantLock) to enforce memory ordering and ensure consistent execution sequences.

Summary

When using locks, developers must pay attention to granularity, fairness, acquisition order, visibility, and ordering. By reasonably selecting lock strategies—such as lock coarsening, non-fair locks, and lock pool mechanisms—it is possible to effectively reduce contention, waiting times, and failures. This approach enhances system performance while avoiding deadlocks and resource leaks, ensuring robust thread safety.