JUC并发编程【八股篇】：并发基础

前言：并发编程的思考

“并发编程不是为了快，而是为了在正确的前提下，尽可能地快。”

在我学习Java的过程中，踩过一些坑：数据随机错乱、程序莫名其妙卡死、CPU飙升100%…每一个问题背后，都指向并发基础的不牢固。今天，我将梳理并发编程基础内容——从进程线程的本质，到线程的创建与控制，再到并发编程面临的核心挑战。

1. 进程与线程：操作系统层面的本质区别

1.1 核心定义与关系

关系本质：线程是进程内部的执行单元，一个进程至少包含一个线程（主线程），可以包含多个线程。线程共享进程的资源，但拥有独立的程序计数器、虚拟机栈和本地方法栈。

1.2 为什么线程需要独立的程序计数器？

这是多线程能够“交替执行”的关键

线程切换时，需要保存当前线程的执行位置，下次切换回来才能继续执行。程序计数器（PC）就是记录“当前执行到哪条字节码指令”的寄存器。如果线程不独立拥有PC，切换后无法恢复执行位置。

1.3 Java线程状态与操作系统线程状态的关系

Java线程状态（java.lang.Thread.State枚举）与操作系统线程状态存在映射关系，但并非完全一一对应。

Java线程状态转换图（基于JVM规范）：

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new Thread()

start()

等待synchronized锁

wait()/join()/park()

sleep(time)/wait(time)/join(time)/parkNanos()

获取到锁

notify()/notifyAll()/unpark()

超时/唤醒

run()执行完毕/未捕获异常

NEW

RUNNABLE

BLOCKED

WAITING

TIMED_WAITING

TERMINATED

关键认知：

• RUNNABLE状态是Java特有的合并状态，包含了操作系统的就绪（Ready）和运行（Running）两个状态。JVM层面不区分是否正在占用CPU，因为这是操作系统调度器的工作。
• BLOCKED状态专指等待synchronized监视器锁时的阻塞。使用Lock接口（如ReentrantLock）等待锁时，线程处于WAITING状态，而非BLOCKED。
• WAITING/TIMED_WAITING是主动进入的等待状态，需要被显式唤醒才能恢复。

思考：为什么JVM不把RUNNABLE拆分为就绪和运行？
因为Java层面无法控制CPU调度，一个线程在Java层面是RUNNABLE，在操作系统层面可能在等待CPU时间片。如果拆分开，状态会频繁变化，但无法被Java程序有效观测，反而增加复杂性。

2. 并发编程的优势与挑战

2.1 并发编程的三大优势

2.2 并发编程的四大挑战

2.2.1 上下文切换

现象：CPU通过时间片分配算法循环执行任务，切换前保存当前任务状态，下次切换回来再加载。即CPU进行线程占用切换。这个过程就是上下文切换。

减少上下文切换的策略：

2.2.2 死锁

死锁四必要条件（必须同时满足）：

死锁检测与预防：

// 死锁检测：使用jstack命令
// 1. jps 找到Java进程PID
// 2. jstack -l <PID> 查看线程栈，会输出Found one Java-level deadlock

// 预防策略对比
public class DeadlockPrevention {
// 策略1：按固定顺序获取锁（打破环路等待）
public void transferMoney(Account from, Account to, int amount) {
// 根据账户ID排序，确保锁获取顺序一致
Account first = from.hashCode() < to.hashCode() ? from : to;
Account second = from.hashCode() < to.hashCode() ? to : from;

synchronized(first) {
synchronized(second) {
// 转账逻辑
}
}
}

// 策略2：使用定时锁（打破不可剥夺）
public boolean tryTransfer(Account from, Account to, int amount) {
while(true) {
if (from.lock.tryLock(50, TimeUnit.MILLISECONDS)) {
try {
if (to.lock.tryLock(50, TimeUnit.MILLISECONDS)) {
try {
// 转账成功
return true;
} finally {
to.lock.unlock();
}
}
} finally {
from.lock.unlock();
}
}
// 获取锁失败，重试或放弃
}
}
}

2.2.3 线程安全

定义：当多个线程访问某个类时，无论运行时环境如何调度，都不需要额外的同步措施，这个类始终表现出正确的行为，则称该类是线程安全的。

线程安全问题的根源：

• 原子性问题：复合操作（如count++）在多线程下被打断
• 可见性问题：一个线程修改了变量，其他线程不可见
• 有序性问题：指令重排序导致意外的执行顺序

2.2.4 资源限制

硬件资源限制：带宽、硬盘读写速度、CPU处理速度
软件资源限制：数据库连接数、Socket连接数

应对策略：

• 集群化：多机并行处理
• 资源池化：连接池复用连接
• 并发度调优：根据资源瓶颈调整线程数

3. 线程创建方式：四种实现及其本质差异

3.1 四种创建方式对比

3.2 实现Runnable与继承Thread的本质区别

// 方式1：继承Thread
class MyThread extends Thread {
@Override
public void run() {
System.out.println("Thread running");
}
}
new MyThread().start();

// 方式2：实现Runnable
class MyTask implements Runnable {
@Override
public void run() {
System.out.println("Task running");
}
}
new Thread(new MyTask()).start();

深度思考：Runnable方式的优势不仅仅是“避免单继承限制”，更重要的是解耦——将“任务”与“执行者”分离。

• 任务：业务逻辑（Runnable）
• 执行者：线程（Thread）

这种分离使得：

3.3 Callable与Future：带返回值的线程

public class CallableDemo {
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(1);

Callable<Integer> task = () -> {
Thread.sleep(1000); // 模拟耗时计算
return 42;
};

Future<Integer> future = executor.submit(task);

try {
// get()会阻塞当前线程，直到任务完成
Integer result = future.get(2, TimeUnit.SECONDS);
System.out.println("计算结果: " + result);
} catch (TimeoutException e) {
System.out.println("计算超时");
future.cancel(true); // 尝试中断任务
} catch (Exception e) {
e.printStackTrace();
}

executor.shutdown();
}
}

Future接口的核心方法：

• get()：阻塞获取结果
• get(timeout, unit)：限时等待
• cancel(boolean mayInterruptIfRunning)：取消任务
• isDone()：任务是否完成
• isCancelled()：任务是否被取消

FutureTask的本质：它既是Runnable（可以被线程执行），又是Future（可以获取结果），实现了任务执行与结果获取的统一。

4. Thread核心方法：控制线程的行为

4.1 start() vs run()

Thread t = new Thread(() -> System.out.println("执行线程: " + Thread.currentThread().getName()));
t.run(); // 输出：执行线程: main
t.start(); // 输出：执行线程: Thread-0

本质区别：

• run()：普通方法调用，在当前线程同步执行
• start()：启动新线程，JVM为新线程分配资源，调用run()方法

思考：为什么start()不能多次调用？
因为线程一旦启动，就加入了线程调度，不能再重置。多次调用start()会抛出IllegalThreadStateException。

4.2 sleep()：暂停而不释放锁

public class SleepDemo {
private static final Object lock = new Object();

public static void main(String[] args) {
Thread t1 = new Thread(() -> {
synchronized(lock) {
try {
System.out.println("T1持有锁，开始睡眠");
Thread.sleep(3000); // 睡眠时不释放锁
System.out.println("T1醒来");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});

Thread t2 = new Thread(() -> {
System.out.println("T2尝试获取锁");
synchronized(lock) {
System.out.println("T2获取到锁");
}
});

t1.start();
t2.start(); // t2会阻塞，直到t1释放锁
}
}

核心特点：

• 使当前线程进入TIMED_WAITING状态
• 不释放任何锁（包括synchronized和Lock）
• 时间到后自动进入RUNNABLE状态，等待CPU调度

4.3 yield()：谦让但不一定有用

Thread.yield(); // 暗示调度器：当前线程愿意让出CPU

思考：yield真的能让出CPU吗？
不一定。yield只是给调度器一个提示，调度器可以忽略它。即使让出CPU，当前线程仍然可能立即又被调度。

实际价值：在调试或特定算法中，可以用yield增加线程切换的概率，暴露潜在的并发问题。

4.4 join()：等待线程终止

public class JoinDemo {
public static void main(String[] args) throws InterruptedException {
Thread worker = new Thread(() -> {
try {
Thread.sleep(2000);
System.out.println("worker执行完毕");
} catch (InterruptedException e) {
e.printStackTrace();
}
});

worker.start();
worker.join(); // 主线程阻塞，等待worker线程结束
System.out.println("主线程继续执行");
}
}

实现原理：join()底层调用wait(0)，等待目标线程执行完notify（实际上线程终止时会自动调用notifyAll）。

5. 线程中断机制：协作式的优雅终止

5.1 中断的本质（必须深刻理解）

核心认知：中断是协作式的，不是抢占式的。调用interrupt()并不会强制停止线程，只是设置一个中断状态标志。

public class InterruptMechanism {
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(() -> {
while (true) {
if (Thread.currentThread().isInterrupted()) {
System.out.println("检测到中断，优雅退出");
// 执行清理工作
break;
}
// 正常任务
System.out.println("working…");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// sleep时被中断，会清除中断标志并抛出异常
System.out.println("sleep时被中断");
// 重要：重新设置中断标志，让循环检测到
Thread.currentThread().interrupt();
}
}
});

t.start();
Thread.sleep(3000);
t.interrupt(); // 设置中断标志
}
}

5.2 三个中断方法的核心区别

public class InterruptMethodsDemo {
public static void main(String[] args) {
Thread.currentThread().interrupt(); // 设置主线程中断

// 第一次调用interrupted()，返回true并清除中断状态
boolean first = Thread.interrupted();
System.out.println("第一次interrupted(): " + first); // true

// 第二次调用interrupted()，中断状态已被清除
boolean second = Thread.interrupted();
System.out.println("第二次interrupted(): " + second); // false

// 再次设置中断状态
Thread.currentThread().interrupt();

// isInterrupted()只返回状态，不清除
boolean third = Thread.currentThread().isInterrupted();
System.out.println("isInterrupted(): " + third); // true

boolean fourth = Thread.currentThread().isInterrupted();
System.out.println("再次isInterrupted(): " + fourth); // true（状态没变）
}
}

为什么要有interrupted()这个会清除状态的方法？

因为中断是一种“信号机制”，当线程通过interrupted()检查到中断后，需要处理中断。处理完后，中断标志应该被清除，避免下次再次检查时重复处理。典型的模式是：

总结：并发基础思维导图

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root（（并发编程基础））

进程与线程

本质区别

资源分配单位 vs 调度单位

线程状态

NEW

RUNNABLE

BLOCKED

WAITING

TIMED_WAITING

TERMINATED

并发挑战

上下文切换

时间片+状态保存恢复

死锁

四条件+预防策略

线程安全

原子性+可见性+有序性

资源限制

硬件限制+软件限制

线程创建

继承Thread

简单但单继承限制

实现Runnable

解耦任务与执行

Callable+Future

带返回值+异常

线程池

资源复用+控制并发

Thread核心方法

start（）

启动线程

run（）

普通方法调用

sleep（）

TIMED_WAITING，不释放锁

yield（）

谦让提示

join（）

等待线程终止

中断机制

interrupt（）

设置中断标志

isInterrupted（）

查询不清除

interrupted（）

查询并清除