feat: add 50 backend interview questions and answers
Generated comprehensive interview preparation materials covering: - Distributed systems (transactions, locks, ID generation, consistency) - Database (indexing, sharding, replication, transactions) - Caching (Redis, cache problems, distributed lock) - Message queues (RocketMQ, Kafka) - Concurrency (ThreadLocal, ConcurrentHashMap, thread pools) - JVM (GC, memory, tuning) - System design (seckill, short URL, IM, feed, LBS) - Algorithms (B+ tree, LRU, Red-Black tree, Skip list, Timing wheel) - Network (TCP/IP, HTTP/HTTPS) - Security (encryption, SQL injection, XSS) - Performance tuning - Design patterns - Microservices (Spring Boot, Gateway, Service Mesh) - Container orchestration (Kubernetes, Docker) - CI/CD, observability Each file includes: - Detailed questions - Comprehensive answers - Code examples - Real project experience - Alibaba P7 level requirements Generated with [Claude Code](https://claude.com/claude-code) via [Happy](https://happy.engineering) Co-Authored-By: Claude <noreply@anthropic.com> Co-Authored-By: Happy <yesreply@happy.engineering>
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yasinshaw
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questions/java-concurrent.md
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questions/java-concurrent.md
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# Java 并发编程基础
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## 问题
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1. 进程和线程的区别？
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2. 创建线程有哪些方式？
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3. synchronized 关键字的原理？
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4. volatile 关键字的作用？
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5. CAS（Compare And Swap）的原理和 ABA 问题？
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6. ThreadLocal 的原理和内存泄漏问题？
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---
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## 标准答案
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### 1. 进程 vs 线程
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| 特性 | 进程 | 线程 |
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|------|------|------|
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| **资源** | 独立内存、文件描述符 | 共享进程资源 |
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| **通信** | IPC（管道、消息队列、共享内存） | 共享内存 |
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| **上下文切换** | 慢（需保存更多状态） | 快 |
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| **开销** | 大 | 小 |
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---
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### 2. 创建线程的方式
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**方式 1：继承 Thread**
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```java
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class MyThread extends Thread {
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@Override
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public void run() {
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System.out.println("Thread running");
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}
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}
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new MyThread().start();
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```
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**方式 2：实现 Runnable**
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```java
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class MyRunnable implements Runnable {
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@Override
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public void run() {
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System.out.println("Runnable running");
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}
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}
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new Thread(new MyRunnable()).start();
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```
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**方式 3：实现 Callable（有返回值）**
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```java
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class MyCallable implements Callable<String> {
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@Override
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public String call() throws Exception {
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return "Callable result";
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}
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}
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FutureTask<String> future = new FutureTask<>(new MyCallable());
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new Thread(future).start();
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String result = future.get();
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```
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---
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### 3. synchronized 原理
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**字节码层面**：
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```java
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// 同步方法
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public synchronized void method() { }
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// 字节码：ACC_SYNCHRONIZED
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// 同步代码块
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synchronized (object) { }
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// 字节码：monitorenter、monitorexit
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```
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**对象头**：
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```
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Mark Word（32 位 JVM）
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┌────────────┬────────────┬──────────────┐
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│ 锁状态 │ 29 位或2位 │ 是否是偏向锁 │
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├────────────┼────────────┼──────────────┤
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│ 无锁 │ 对象 Hash │ 01 │
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│ 偏向锁 │ 线程 ID │ 01 │
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│ 轻量级锁 │ 栈中 Lock Record | 00 │
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│ 重量级锁 │ 管程指针 │ 10 │
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└────────────┴────────────┴──────────────┘
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```
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**锁升级**：
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```
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无锁 → 偏向锁 → 轻量级锁（CAS 自旋） → 重量级锁（阻塞）
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```
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---
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### 4. volatile 关键字
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**作用**：
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1. **保证可见性**（JMM，主内存与工作内存）
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2. **禁止指令重排序**
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**示例**：
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```java
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private volatile boolean running = true;
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public void stop() {
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running = false; // 立即对所有线程可见
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}
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```
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**不能保证原子性**：
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```java
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private volatile int count = 0;
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// ❌ 非线程安全
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count++; // read → modify → write（非原子操作）
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// ✅ 线程安全
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synchronized (this) {
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count++;
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}
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```
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---
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### 5. CAS 和 ABA 问题
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**CAS（Compare And Swap）**：
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```java
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// Unsafe.compareAndSwapInt(object, offset, expect, update)
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// 如果当前值 == 期望值，则更新为 update
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AtomicInteger atomic = new AtomicInteger(0);
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atomic.compareAndSet(0, 1); // CAS 操作
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```
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**ABA 问题**：
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```
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线程 A：读取值 A
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线程 B：A → B → A
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线程 A：CAS 成功（不知道值已变化）
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```
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**解决：版本号（AtomicStampedReference）**：
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```java
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AtomicStampedReference<Integer> ref = new AtomicStampedReference<>(100, 0);
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int oldStamp = ref.getStamp();
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ref.compareAndSet(100, 101, oldStamp, oldStamp + 1); // 版本号也参与 CAS
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```
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---
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### 6. ThreadLocal
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**原理**：
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```java
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public void set(T value) {
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Thread t = Thread.currentThread();
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ThreadLocalMap map = t.threadLocals;
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if (map != null)
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map.set(this, value); // key 是 ThreadLocal 对象，value 是值
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else
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createMap(t, value);
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}
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```
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**内存泄漏**：
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```java
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// ❌ 可能导致内存泄漏
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private static ThreadLocal<byte[]> data = new ThreadLocal<>();
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public void process() {
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data.set(new byte[1024 * 1024]); // 1MB
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// 未调用 remove()
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}
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```
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**原因**：
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- ThreadLocal 是弱引用，但 value 是强引用
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- 线程不销毁，value 不会回收
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**解决**：
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```java
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try {
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data.set(new byte[1024 * 1024]);
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// 业务逻辑
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} finally {
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data.remove(); // 必须
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}
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```
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---
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### 7. 阿里 P7 加分项
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**深度理解**：
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- 理解 JMM（Java 内存模型）
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- 理解 happens-before 原则
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- 理解 synchronized 的优化（偏向锁、轻量级锁）
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**实战经验**：
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- 有并发问题的排查经验
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- 有性能优化经验（减少锁竞争）
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- 有死锁的排查和解决经验
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