高級Java面試問題

高級Java面試問題主要考察JVM內部機制、並發編程、性能調優、設計模式和系統架構的理解。 1. Java內存模型（JMM）定義了線程間內存操作的可見性、原子性和順序性，volatile關鍵字和happens-before規則確保正確同步，避免因CPU緩存導致的更新不可見問題；G1GC適用於大堆和可預測停頓場景，通過分區域回收優先清理垃圾多的區域，而ZGC利用著色指針和加載屏障實現亞毫秒級停頓且停頓時間與堆大小無關，適合低延遲系統。 2. 設計線程安全的LRU緩存可使用ConcurrentHashMap結合ConcurrentLinkedQueue維護訪問順序，但queue.remove為O(n)需優化，生產環境推薦Caffeine；synchronized方法鎖整個對象可能阻塞無關操作，而同步塊可細粒度控制，過度同步降低吞吐量，不足則引發競態條件。 3. 高CPU診斷需通過top/jstack獲取線程棧，定位熱點方法如正則回溯導致的無限循環，使用Async-Profiler等工具分析；減少GC開銷應固定-Xms/-Xmx避免動態擴容，選用ZGC或Shenandoah降低停頓，減少對象創建、使用對像池或直接內存存儲，並通過GC日誌監控優化。 4. Reactive編程適用於I/O密集型高並發場景，基於事件循環以少量線程支撐高並發，支持背壓但調試複雜，不適用於CPU密集任務；微服務彈性保障採用Resilience4j實現熔斷、重試、艙壁隔離、限流，並結合Micrometer和OpenTelemetry提升可觀測性。 5. Java泛型在運行時經歷類型擦除，List和List均變為List，無法new T()或instanceof泛型類型，可通過Class或TypeToken繞過；密封類（Java 17）限制繼承體系，模式匹配（Java 16 ）簡化instanceof和switch的類型轉換邏輯，提升代碼安全性與可讀性。綜上，資深開發者需具備JVM行為理解、並發模型權衡、系統擴展能力及現代語言特性的綜合判斷力，不僅知其然更需知其所以然，體現對真實係統問題的實踐經驗與決策能力。

Advanced Java Interview Questions for Senior Developers

When interviewing senior Java developers, the focus shifts from basic syntax and OOP principles to deep understanding of JVM internals, concurrency, performance optimization, design patterns, and system design. Below are some advanced Java interview questions that test real-world expertise and architectural thinking.

1. JVM Internals and Memory Management

Question: Explain the Java Memory Model (JMM) and how it affects concurrent programming.

The Java Memory Model defines how threads interact through memory and establishes guarantees around visibility, atomicity, and ordering of memory operations.

• Main components : Heap (shared among threads), Stack (thread-local), Method Area, PC Registers, Native Method Stack.
• Visibility issues : Without proper synchronization (eg, volatile , synchronized , or java.util.concurrent constructs), one thread may not see updates made by another.
• The volatile keyword ensures visibility and prevents reordering via memory barriers.
• Happens-before relationships are key: actions in one thread that happen-before another ensure proper ordering.

Example: Two threads accessing a shared boolean flag without volatile might result in one thread never seeing the update due to CPU caching.

Follow-up: How does garbage collection work in Java? Discuss G1GC vs ZGC.

• G1GC (Garbage-First) : Designed for large heaps with predictable pause times. Divides heap into regions, prioritizes collection of regions with the most garbage ("garbage first").
• ZGC (Z Garbage Collector) : Ultra-low pause times (
• ZGC is pause-time independent of heap size; G1's pauses grow slightly with heap size.

Senior devs should understand trade-offs: ZGC for latency-sensitive systems, G1 for throughput and moderate latency requirements.

2. Concurrency and Multithreading

Question: How would you design a thread-safe cache with eviction policies (eg, LRU)?

Key considerations:

• Use ConcurrentHashMap for thread-safe access.
• Wrap access to ordering structures with ReentrantLock or use synchronized blocks.
• For LRU: combine LinkedHashMap with synchronized or better, use ConcurrentLinkedQueue to track access order.

 public class ThreadSafeLRUCache<K, V> {
    private final int capacity;
    private final Map<K, V> cache = new ConcurrentHashMap<>();
    private final ConcurrentLinkedQueue<K> queue = new ConcurrentLinkedQueue<>();

    public V get(K key) {
        V value = cache.get(key);
        if (value != null) {
            queue.remove(key); // Not ideal – O(n)
            queue.add(key);
        }
        return value;
    }

    public void put(K key, V value) {
        if (cache.size() >= capacity) {
            K oldestKey = queue.poll();
            cache.remove(oldestKey);
        }
        cache.put(key, value);
        queue.add(key);
    }
}

Optimization : Use ConcurrentSkipListMap or external libraries like Caffeine for production-grade caches.

Follow-up: What are the risks of using synchronized on a method vs. a block?

• Synchronizing on the entire method locks the object (for instance methods), potentially blocking unrelated operations.
• Block-level synchronization allows finer control, eg, lock only the critical section.
• Risk: over-synchronization reduces throughput; under-synchronization causes race conditions.

3. Performance Tuning and Diagnostics

Question: How do you diagnose and resolve high CPU usage in a Java application?

Steps:

• Monitor : Use top , htop , or jtop to identify high CPU process.
• Thread dump : Use jstack <pid> or kill -3 <pid> to get stack traces.
• Convert PID to hexadecimal thread ID for the offending thread.
• Analyze which method is consuming CPU (eg, infinite loop, tight regex, inefficient algorithm).
• Use profiling tools: Async-Profiler , JVisualVM , YourKit , or JFR (Java Flight Recorder) .

Example: A regex like "(.*?)*@" can cause catastrophic backtracking → high CPU.

Follow-up: How do you reduce GC overhead in a high-throughput service?

• Tune heap size: -Xms and -Xmx set to same value to avoid expansion pauses.
• Choose appropriate GC: ZGC or Shenandoah for low latency; G1 for balanced workloads.
• Minimize object creation: object pooling, primitive wrappers, string interning.
• Use off-heap storage for large datasets (eg, via ByteBuffer.allocateDirect or frameworks like Chronicle).
• Monitor GC logs: Enable -Xlog:gc*:file=gc.log and analyze with tools like GCViewer.

4. Design Patterns and System Architecture

Question: When would you use the Reactive Programming model (eg, Project Reactor) over traditional threading?

Reactive programming (eg, Flux , Mono ) is ideal when:

• You have I/O-bound operations (eg, DB calls, HTTP requests).
• Need high concurrency with limited threads (eg, Netty-based servers like WebFlux).
• Want non-blocking backpressure support.

Traditional threading (Tomcat Servlet) scales linearly with threads → high memory use.
Reactive model scales with event loops → efficient resource usage.

But: reactive code is harder to debug, test, and reason about. Not always worth it for CPU-bound tasks.

Follow-up: How do you ensure resilience in microservices using Java?

Use patterns and libraries:

• Circuit Breaker : Resilience4j or Hystrix (deprecated) to fail fast.
• Retry Mechanisms : With exponential backoff.
• Bulkheads : Limit concurrency per service.
• Rate Limiting & Throttling : Prevent overload.
• Integrate with observability: logging, metrics (Micrometer), tracing (OpenTelemetry).

5. Advanced Language Features

Question: Explain how Java generics work at runtime and what is type erasure.

• Generics are compile-time only; type information is erased during compilation.
• At runtime, List<String> and List<Integer> are both List .
• Implications :
  • Cannot instantiate generic types: new T() is invalid.
  • Cannot check instanceof List<String> .
  • Bridge methods are generated to maintain polymorphism.
• Workarounds: Pass Class<T> or use TypeToken (Google Gson).

Follow-up: What are sealed classes and pattern matching in modern Java?

• Sealed classes (Java 17 ): Restrict which classes can extend/implement a class/interface.

 public sealed interface Shape permits Circle, Rectangle, Triangle {}

• Pattern matching (Java 16 instanceof , Java 21 switch ):

 if (shape instanceof Circle c) {
    return c.radius();
} else if (shape instanceof Rectangle r) {
    return r.width() * r.height();
}

These features enable algebraic data types and reduce boilerplate, improving code safety and readability.

Final Thoughts

Senior Java roles require more than coding skill — they demand deep understanding of:

• JVM behavior under load
• Trade-offs in concurrency models
• System design at scale
• Modern Java features and best practices

These questions probe not just knowledge, but judgment. A strong candidate doesn't just know what to do — they explain why , and when not to do it.

Basically, it's not about memorizing answers — it's about showing you've wrestled with real systems and learned from them.

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