并发的基石：深入浅出 Java 线程同步机制

并发的基石：深入浅出 Java 线程同步机制

• • 1. 引言：从多线程的混乱到有序
  • 2. 一图看懂：什么是线程同步？
  • 3. 为什么需要同步：问题演示
  • • 3.1 一个经典的并发问题
    • 3.2 count++ 的非原子性分析
  • 4. Java 同步机制全景图
  • 5. synchronized：Java 最基础的同步机制
  • • 5.1 synchronized 的三种用法
    • 5.2 锁的获取与释放流程
    • 5.3 解决之前的计数器问题
  • 6. ReentrantLock：更灵活的锁
  • • 6.1 基本用法
    • 6.2 synchronized vs ReentrantLock
  • 7. 原子类：无锁同步
  • • 7.1 原子类的原理
    • 7.2 原子类示例
    • 7.3 常用原子类对比
  • 8. 条件变量：线程间的协作
  • • 8.1 wait/notify 机制
    • 8.2 Condition 接口（ReentrantLock）
  • 9. 其他同步工具
  • • 9.1 volatile：轻量级同步
    • 9.2 ThreadLocal：线程隔离
  • 10. 同步策略选择指南
  • • 10.1 选择决策表
  • 11. 常见问题与最佳实践
  • • 11.1 死锁及其避免
    • 11.2 锁粒度控制
    • 11.3 最小化同步块
  • 12. 总结

1. 引言：从多线程的混乱到有序

想象这样一个场景：你和家人共用一张银行卡，你去 ATM 取钱的同时，你的家人也在柜台取钱。如果没有任何协调机制，账户余额可能变成负数，这显然是不允许的。

Java 多线程编程中同样存在这个问题——多个线程同时访问共享资源时，会产生数据不一致的问题。而线程同步，就是解决这个问题的核心手段。

本文将全面介绍 Java 中的线程同步机制，从基本概念到高级工具，助你彻底掌握并发编程的精髓。

2. 一图看懂：什么是线程同步？

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✅ 同步后的状态

等待

等待

同步机制

锁/信号量

线程1

线程2

线程3

安全访问

❌ 未同步的状态

线程1

共享资源

线程2

线程3

数据混乱

脏读

丢失更新

核心概念：

线程同步是指当多个线程访问共享资源时，通过某种机制确保资源在同一时刻只能被一个线程使用，从而保证数据的正确性和一致性。

3. 为什么需要同步：问题演示

3.1 一个经典的并发问题

public class UnsafeCounter {
private int count = 0;

public void increment() {
count++; // 这不是原子操作！
}

public int getCount() {
return count;
}

public static void main(String[] args) throws InterruptedException {
UnsafeCounter counter = new UnsafeCounter();
Thread[] threads = new Thread[1000];

for (int i = 0; i < 1000; i++) {
threads[i] = new Thread(() -> {
for (int j = 0; j < 1000; j++) {
counter.increment();
}
});
threads[i].start();
}

for (Thread t : threads) {
t.join();
}

System.out.println("Expected: 1000000, Actual: " + counter.getCount());
// 实际输出可能是 998765 之类的，小于预期值
}
}

3.2 count++ 的非原子性分析

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并发干扰场景

线程1: 读取 count=5

线程1: 计算 5+1=6

线程2: 读取 count=5

线程2: 计算 5+1=6

线程1: 写回 count=6

线程2: 写回 count=6

结果: 两次自增后 count=6

count++ 的三个步骤

读取 count 值

count + 1

写回 count

结果：两个线程各执行一次 increment，期望值应该是 7，实际却是 6——这就是典型的丢失更新问题。

4. Java 同步机制全景图

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Java线程同步

互斥锁

synchronized

synchronized方法

synchronized块

静态同步方法

ReentrantLock

公平锁/非公平锁

可中断锁

超时获取

条件变量

原子操作

AtomicInteger

AtomicLong

AtomicReference

LongAdder

信号量

Semaphore

CountDownLatch

CyclicBarrier

线程局部变量

ThreadLocal

其他工具

volatile

ReadWriteLock

StampedLock

5. synchronized：Java 最基础的同步机制

5.1 synchronized 的三种用法

public class SynchronizedDemo {

// 1. 修饰实例方法：锁的是当前实例对象
public synchronized void instanceMethod() {
// 同步代码
}

// 2. 修饰静态方法：锁的是类的 Class 对象
public static synchronized void staticMethod() {
// 同步代码
}

// 3. 修饰代码块：可以指定锁对象
public void blockMethod() {
synchronized(this) { // 锁当前实例
// 同步代码
}

synchronized(SynchronizedDemo.class) { // 锁 Class 对象
// 同步代码
}

private final Object lock = new Object();
synchronized(lock) { // 锁自定义对象
// 同步代码
}
}
}

5.2 锁的获取与释放流程

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是

否

锁升级过程

无锁

偏向锁

轻量级锁

重量级锁

线程尝试获取锁

锁是否可用?

线程获得锁

执行同步代码块

释放锁

其他线程可获取

线程进入阻塞队列

等待锁释放

被唤醒

5.3 解决之前的计数器问题

public class SafeCounter {
private int count = 0;

// 加上 synchronized 即可解决
public synchronized void increment() {
count++;
}

public synchronized int getCount() {
return count;
}

// 或者使用同步块，更精细的控制
private final Object lock = new Object();
public void increment2() {
synchronized(lock) {
count++;
}
}
}

6. ReentrantLock：更灵活的锁

6.1 基本用法

public class ReentrantLockDemo {
private final ReentrantLock lock = new ReentrantLock(); // 非公平锁（默认）
private final ReentrantLock fairLock = new ReentrantLock(true); // 公平锁
private int count = 0;

public void increment() {
lock.lock(); // 获取锁
try {
count++;
} finally {
lock.unlock(); // 务必在 finally 中释放
}
}

// 尝试获取锁（非阻塞）
public boolean tryIncrement() {
if (lock.tryLock()) {
try {
count++;
return true;
} finally {
lock.unlock();
}
}
return false;
}

// 尝试获取锁（带超时）
public boolean tryIncrementWithTimeout() throws InterruptedException {
if (lock.tryLock(1, TimeUnit.SECONDS)) {
try {
count++;
return true;
} finally {
lock.unlock();
}
}
return false;
}
}

6.2 synchronized vs ReentrantLock

7. 原子类：无锁同步

7.1 原子类的原理

原子类基于 CAS (Compare-And-Swap) 实现，是一种乐观锁机制。

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是

否

读取当前值 V

计算新值

CAS 操作内存值是否仍为 V?

更新为新值

重试/失败

7.2 原子类示例

public class AtomicCounter {
private final AtomicInteger count = new AtomicInteger(0);

public void increment() {
count.incrementAndGet(); // 原子操作，无需锁
}

public int getCount() {
return count.get();
}

// 更复杂的原子操作
public void updateIfCondition() {
count.updateAndGet(x -> {
if (x < 100) return x + 1;
return x;
});
}

public static void main(String[] args) throws InterruptedException {
AtomicCounter counter = new AtomicCounter();
// 多线程测试，结果正确
}
}

7.3 常用原子类对比

8. 条件变量：线程间的协作

8.1 wait/notify 机制

生产者线程

共享队列

消费者线程

生产者线程

共享队列

消费者线程

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发现队列为空

wait() 进入等待

生产数据

notifyAll()

唤醒

消费数据

public class ProducerConsumerDemo {
private final Queue<Integer> queue = new LinkedList<>();
private final int MAX_SIZE = 10;

// 生产者
public synchronized void produce() throws InterruptedException {
while (queue.size() == MAX_SIZE) {
wait(); // 队列满，等待
}
queue.offer(1);
notifyAll(); // 唤醒消费者
}

// 消费者
public synchronized void consume() throws InterruptedException {
while (queue.isEmpty()) {
wait(); // 队列空，等待
}
queue.poll();
notifyAll(); // 唤醒生产者
}
}

8.2 Condition 接口（ReentrantLock）

public class ConditionDemo {
private final ReentrantLock lock = new ReentrantLock();
private final Condition notFull = lock.newCondition();
private final Condition notEmpty = lock.newCondition();
private final Queue<Integer> queue = new LinkedList<>();
private final int MAX_SIZE = 10;

public void produce() throws InterruptedException {
lock.lock();
try {
while (queue.size() == MAX_SIZE) {
notFull.await(); // 等待队列不满
}
queue.offer(1);
notEmpty.signal(); // 通知队列不空
} finally {
lock.unlock();
}
}

public void consume() throws InterruptedException {
lock.lock();
try {
while (queue.isEmpty()) {
notEmpty.await(); // 等待队列不空
}
queue.poll();
notFull.signal(); // 通知队列不满
} finally {
lock.unlock();
}
}
}

9. 其他同步工具

9.1 volatile：轻量级同步

public class VolatileDemo {
private volatile boolean flag = true; // volatile 保证可见性
private int value = 0; // 普通变量

public void writer() {
value = 42; // 普通写
flag = false; // volatile 写，会触发内存屏障
}

public void reader() {
if (flag) { // volatile 读
System.out.println(value); // 能保证看到 42
}
}
}

volatile 的语义：

• 可见性：对一个 volatile 变量的写，立即对其他线程可见
• 禁止指令重排序：volatile 写之前的操作不会重排序到之后
• 不保证原子性：volatile count++ 仍然是线程不安全的

9.2 ThreadLocal：线程隔离

public class ThreadLocalDemo {
// 每个线程都有自己的 SimpleDateFormat 实例
private static final ThreadLocal<SimpleDateFormat> dateFormat =
ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

public String formatDate(Date date) {
return dateFormat.get().format(date);
}

// 使用后记得清理，避免内存泄漏
public void cleanup() {
dateFormat.remove();
}
}

10. 同步策略选择指南

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简单计数

只有一个共享变量

复杂共享状态

简单易用

需要可中断/超时

读多写少

高性能读写

需要线程同步

共享资源的特征?

AtomicLong/LongAdder

volatile + CAS

需要什么特性?

synchronized

ReentrantLock

ReadWriteLock

StampedLock

推荐

10.1 选择决策表

11. 常见问题与最佳实践

11.1 死锁及其避免

// ❌ 死锁示例
public class DeadlockDemo {
private final Object lock1 = new Object();
private final Object lock2 = new Object();

public void method1() {
synchronized(lock1) {
synchronized(lock2) {
// 业务逻辑
}
}
}

public void method2() {
synchronized(lock2) { // 相反的加锁顺序
synchronized(lock1) {
// 业务逻辑
}
}
}
}

避免死锁的原则：

11.2 锁粒度控制

public class LockGranularity {
private final List<String> list = new ArrayList<>();
private final Map<String, Integer> map = new HashMap<>();

// ❌ 粗粒度锁：锁了整个对象
public synchronized void addBad(String key, String value) {
list.add(value);
map.put(key, value.length());
}

// ✅ 细粒度锁：使用不同的锁对象
private final Object listLock = new Object();
private final Object mapLock = new Object();

public void addGood(String key, String value) {
synchronized(listLock) {
list.add(value);
}
synchronized(mapLock) {
map.put(key, value.length());
}
}
}

11.3 最小化同步块

public class SyncMinimize {
private final List<String> data = new ArrayList<>();

// ❌ 整个方法同步
public synchronized void addBad(String item) {
// 耗时操作也在锁内
validateItem(item);
processItem(item);
data.add(item);
}

// ✅ 只同步必要的代码
public void addGood(String item) {
// 耗时操作在锁外
validateItem(item);
String processed = processItem(item);
// 只同步关键操作
synchronized(this) {
data.add(processed);
}
}
}

12. 总结

一句话总结：

线程同步是多线程编程的基石，Java 提供了从 synchronized 到 ReentrantLock、从 volatile 到 Atomic 类的多层次同步机制。选择合适的同步方案需要权衡性能、安全性和代码复杂度，核心原则是：只同步必要的代码，最小化锁持有时间，避免死锁和锁竞争。

📌 记住：同步不是万能的，过度同步会导致性能下降甚至死锁；设计之初就考虑线程安全性，比后期调试要重要得多。

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