Java 线程基础：进程、线程、并发并行、创建方式与生命周期

Java 并发面试的第一层，通常不是上来就问 AQS 和锁升级，而是先看你能不能把几个基础概念说清楚。

进程是什么，线程是什么，并发和并行有什么区别，线程怎么创建，run() 和 start() 有什么区别，线程有哪些状态，wait() 和 sleep() 又差在哪里。

这些问题看起来碎，其实都在问一件事：一个任务从代码变成 CPU 上执行的指令，中间到底经历了什么。

进程和线程怎么区分

程序只是躺在磁盘上的文件。它要运行起来，就要把指令加载到 CPU，把数据加载到内存，还要管理文件、网络、磁盘这些资源。

这个正在运行的程序实例，就是进程。

线程则更轻。一个线程可以理解成一条指令流，它把自己的指令一条一条交给 CPU 执行。一个进程里面可以有一个线程，也可以有多个线程。
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进程
线程 1
线程 2
线程 3
独立内存空间 / IO 资源
共享进程内存

面试里可以这样说：

对比点	进程	线程
本质	正在运行的程序实例	进程内的一条执行路径
内存	不同进程通常有独立内存空间	同一进程内线程共享内存
切换成本	较高	较低
关系	一个进程可以包含多个线程	线程依附于进程存在

一句话总结就是：进程负责资源边界，线程负责执行路径。

并发和并行不是一回事

并发是同一时间段内应对多件事情，并行是同一时刻真正同时做多件事情。

单核 CPU 也能并发，因为操作系统会把 CPU 时间片分给不同线程。线程切换得很快，人看起来像同时执行，但微观上还是一个线程一个线程轮流跑。

多核 CPU 才能真正并行。比如 4 核 CPU 在某个时刻可以同时执行 4 个线程。
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多核
多个任务
CPU 核心数
时间片轮转
宏观看起来同时运行
并发
多个核心同时执行
并行

可以用一句很稳的面试话术：

并发强调处理多个任务的能力，并行强调同一时刻同时执行多个任务。单核 CPU 可以并发，多核 CPU 才能在物理上并行。

线程有哪些创建方式

PPT 里列了四种：

1. 继承 Thread 类。
2. 实现 Runnable 接口。
3. 实现 Callable 接口，配合 FutureTask 拿返回值。
4. 使用线程池创建线程。

继承 Thread 最直接：

public class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("MyThread...run...");
    }

    public static void main(String[] args) {
        new MyThread().start();
    }
}

实现 Runnable 更常见，因为任务和线程对象分开了：

Runnable task = () -> System.out.println("running...");
new Thread(task, "t1").start();

Callable 比 Runnable 多两个能力：有返回值，可以抛出异常。

Callable<String> task = () -> {
    System.out.println(Thread.currentThread().getName());
    return "ok";
};

FutureTask<String> futureTask = new FutureTask<>(task);
new Thread(futureTask, "t1").start();
String result = futureTask.get();

但真实项目里，不建议频繁 new Thread()。更常见的方式是把任务交给线程池。

ExecutorService threadPool = Executors.newFixedThreadPool(3);
threadPool.submit(() -> System.out.println("running..."));
threadPool.shutdown();

线程池这块后面会单独写一篇，因为它比"创建线程"复杂得多。

Runnable 和 Callable 有什么区别

这个问题很适合用表格回答：

对比点	Runnable	Callable
方法	run()	call()
返回值	没有返回值	有返回值
异常	不能向外抛受检异常	可以抛异常
获取结果	不支持	配合 Future / FutureTask

所以面试里不要只背"一个有返回值一个没有返回值"。更完整的回答是：

Runnable 更适合只执行任务、不关心结果的场景；Callable 更适合异步计算后还要拿结果或处理异常的场景。

run 和 start 有什么区别

run() 只是普通方法调用。

start() 才是真正启动一个新线程，让 JVM 创建线程并异步执行 run() 里的代码。

Thread t1 = new Thread("t1") {
    @Override
    public void run() {
        System.out.println("running...");
    }
};

t1.start();

如果直接调用 t1.run()，代码仍然在当前线程里同步执行，不会创建新线程。

还有一个细节，start() 只能调用一次。对同一个线程对象重复调用 start()，会抛 IllegalThreadStateException。
#mermaid-svg-ObM0FvDxoMTjGH3H{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-ObM0FvDxoMTjGH3H .error-icon{fill:#552222;}#mermaid-svg-ObM0FvDxoMTjGH3H .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-ObM0FvDxoMTjGH3H .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-ObM0FvDxoMTjGH3H .marker{fill:#333333;stroke:#333333;}#mermaid-svg-ObM0FvDxoMTjGH3H .marker.cross{stroke:#333333;}#mermaid-svg-ObM0FvDxoMTjGH3H svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-ObM0FvDxoMTjGH3H p{margin:0;}#mermaid-svg-ObM0FvDxoMTjGH3H .label{font-family:"trebuchet ms",verdana,arial,sans-serif;color:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster-label text{fill:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster-label span{color:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster-label span p{background-color:transparent;}#mermaid-svg-ObM0FvDxoMTjGH3H .label text,#mermaid-svg-ObM0FvDxoMTjGH3H span{fill:#333;color:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H .node rect,#mermaid-svg-ObM0FvDxoMTjGH3H .node circle,#mermaid-svg-ObM0FvDxoMTjGH3H .node ellipse,#mermaid-svg-ObM0FvDxoMTjGH3H .node polygon,#mermaid-svg-ObM0FvDxoMTjGH3H .node path{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-ObM0FvDxoMTjGH3H .rough-node .label text,#mermaid-svg-ObM0FvDxoMTjGH3H .node .label text,#mermaid-svg-ObM0FvDxoMTjGH3H .image-shape .label,#mermaid-svg-ObM0FvDxoMTjGH3H .icon-shape .label{text-anchor:middle;}#mermaid-svg-ObM0FvDxoMTjGH3H .node .katex path{fill:#000;stroke:#000;stroke-width:1px;}#mermaid-svg-ObM0FvDxoMTjGH3H .rough-node .label,#mermaid-svg-ObM0FvDxoMTjGH3H .node .label,#mermaid-svg-ObM0FvDxoMTjGH3H .image-shape .label,#mermaid-svg-ObM0FvDxoMTjGH3H .icon-shape .label{text-align:center;}#mermaid-svg-ObM0FvDxoMTjGH3H .node.clickable{cursor:pointer;}#mermaid-svg-ObM0FvDxoMTjGH3H .root .anchor path{fill:#333333!important;stroke-width:0;stroke:#333333;}#mermaid-svg-ObM0FvDxoMTjGH3H .arrowheadPath{fill:#333333;}#mermaid-svg-ObM0FvDxoMTjGH3H .edgePath .path{stroke:#333333;stroke-width:2.0px;}#mermaid-svg-ObM0FvDxoMTjGH3H .flowchart-link{stroke:#333333;fill:none;}#mermaid-svg-ObM0FvDxoMTjGH3H .edgeLabel{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-ObM0FvDxoMTjGH3H .edgeLabel p{background-color:rgba(232,232,232, 0.8);}#mermaid-svg-ObM0FvDxoMTjGH3H .edgeLabel rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-ObM0FvDxoMTjGH3H .labelBkg{background-color:rgba(232, 232, 232, 0.5);}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster rect{fill:#ffffde;stroke:#aaaa33;stroke-width:1px;}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster text{fill:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H .cluster span{color:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:12px;background:hsl(80, 100%, 96.2745098039%);border:1px solid #aaaa33;border-radius:2px;pointer-events:none;z-index:100;}#mermaid-svg-ObM0FvDxoMTjGH3H .flowchartTitleText{text-anchor:middle;font-size:18px;fill:#333;}#mermaid-svg-ObM0FvDxoMTjGH3H rect.text{fill:none;stroke-width:0;}#mermaid-svg-ObM0FvDxoMTjGH3H .icon-shape,#mermaid-svg-ObM0FvDxoMTjGH3H .image-shape{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-ObM0FvDxoMTjGH3H .icon-shape p,#mermaid-svg-ObM0FvDxoMTjGH3H .image-shape p{background-color:rgba(232,232,232, 0.8);padding:2px;}#mermaid-svg-ObM0FvDxoMTjGH3H .icon-shape .label rect,#mermaid-svg-ObM0FvDxoMTjGH3H .image-shape .label rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-ObM0FvDxoMTjGH3H .label-icon{display:inline-block;height:1em;overflow:visible;vertical-align:-0.125em;}#mermaid-svg-ObM0FvDxoMTjGH3H .node .label-icon path{fill:currentColor;stroke:revert;stroke-width:revert;}#mermaid-svg-ObM0FvDxoMTjGH3H :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} run()
start()
Thread 对象
调用方式
普通方法调用
当前线程执行
JVM 启动新线程
新线程执行 run()

线程有哪些状态

JDK 的 Thread.State 定义了六种状态：

状态	含义
NEW	新建，还没有调用 start()
RUNNABLE	可运行，可能正在运行，也可能等待 CPU 时间片
BLOCKED	等待获取 monitor 锁
WAITING	无限期等待，需要其他线程唤醒
TIMED_WAITING	限时等待，到时间或被唤醒后继续
TERMINATED	执行结束

状态流转可以这样看：
#mermaid-svg-6vV84wnyf1Zvly5C{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-6vV84wnyf1Zvly5C .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-6vV84wnyf1Zvly5C .error-icon{fill:#552222;}#mermaid-svg-6vV84wnyf1Zvly5C .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-6vV84wnyf1Zvly5C .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-6vV84wnyf1Zvly5C .marker{fill:#333333;stroke:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C .marker.cross{stroke:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-6vV84wnyf1Zvly5C p{margin:0;}#mermaid-svg-6vV84wnyf1Zvly5C defs #statediagram-barbEnd{fill:#333333;stroke:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C g.stateGroup text{fill:#9370DB;stroke:none;font-size:10px;}#mermaid-svg-6vV84wnyf1Zvly5C g.stateGroup text{fill:#333;stroke:none;font-size:10px;}#mermaid-svg-6vV84wnyf1Zvly5C g.stateGroup .state-title{font-weight:bolder;fill:#131300;}#mermaid-svg-6vV84wnyf1Zvly5C g.stateGroup rect{fill:#ECECFF;stroke:#9370DB;}#mermaid-svg-6vV84wnyf1Zvly5C g.stateGroup line{stroke:#333333;stroke-width:1;}#mermaid-svg-6vV84wnyf1Zvly5C .transition{stroke:#333333;stroke-width:1;fill:none;}#mermaid-svg-6vV84wnyf1Zvly5C .stateGroup .composit{fill:white;border-bottom:1px;}#mermaid-svg-6vV84wnyf1Zvly5C .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px;}#mermaid-svg-6vV84wnyf1Zvly5C .state-note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-6vV84wnyf1Zvly5C .state-note text{fill:black;stroke:none;font-size:10px;}#mermaid-svg-6vV84wnyf1Zvly5C .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.5;}#mermaid-svg-6vV84wnyf1Zvly5C .edgeLabel .label rect{fill:#ECECFF;opacity:0.5;}#mermaid-svg-6vV84wnyf1Zvly5C .edgeLabel{background-color:rgba(232,232,232, 0.8);text-align:center;}#mermaid-svg-6vV84wnyf1Zvly5C .edgeLabel p{background-color:rgba(232,232,232, 0.8);}#mermaid-svg-6vV84wnyf1Zvly5C .edgeLabel rect{opacity:0.5;background-color:rgba(232,232,232, 0.8);fill:rgba(232,232,232, 0.8);}#mermaid-svg-6vV84wnyf1Zvly5C .edgeLabel .label text{fill:#333;}#mermaid-svg-6vV84wnyf1Zvly5C .label div .edgeLabel{color:#333;}#mermaid-svg-6vV84wnyf1Zvly5C .stateLabel text{fill:#131300;font-size:10px;font-weight:bold;}#mermaid-svg-6vV84wnyf1Zvly5C .node circle.state-start{fill:#333333;stroke:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C .node .fork-join{fill:#333333;stroke:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C .node circle.state-end{fill:#9370DB;stroke:white;stroke-width:1.5;}#mermaid-svg-6vV84wnyf1Zvly5C .end-state-inner{fill:white;stroke-width:1.5;}#mermaid-svg-6vV84wnyf1Zvly5C .node rect{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-6vV84wnyf1Zvly5C .node polygon{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-6vV84wnyf1Zvly5C #statediagram-barbEnd{fill:#333333;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-cluster rect{fill:#ECECFF;stroke:#9370DB;stroke-width:1px;}#mermaid-svg-6vV84wnyf1Zvly5C .cluster-label,#mermaid-svg-6vV84wnyf1Zvly5C .nodeLabel{color:#131300;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-cluster rect.outer{rx:5px;ry:5px;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-state .divider{stroke:#9370DB;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-state .title-state{rx:5px;ry:5px;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-cluster.statediagram-cluster .inner{fill:white;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-cluster.statediagram-cluster-alt .inner{fill:#f0f0f0;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-cluster .inner{rx:0;ry:0;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-state rect.basic{rx:5px;ry:5px;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#f0f0f0;}#mermaid-svg-6vV84wnyf1Zvly5C .note-edge{stroke-dasharray:5;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-note rect{fill:#fff5ad;stroke:#aaaa33;stroke-width:1px;rx:0;ry:0;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-note rect{fill:#fff5ad;stroke:#aaaa33;stroke-width:1px;rx:0;ry:0;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-note text{fill:black;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram-note .nodeLabel{color:black;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagram .edgeLabel{color:red;}#mermaid-svg-6vV84wnyf1Zvly5C #dependencyStart,#mermaid-svg-6vV84wnyf1Zvly5C #dependencyEnd{fill:#333333;stroke:#333333;stroke-width:1;}#mermaid-svg-6vV84wnyf1Zvly5C .statediagramTitleText{text-anchor:middle;font-size:18px;fill:#333;}#mermaid-svg-6vV84wnyf1Zvly5C :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} new Thread()
start()
等待 synchronized 锁
获得锁
wait() / join()
notify() / notifyAll()
sleep(ms) / wait(ms) / join(ms)
到时间或被唤醒
run() 执行结束
NEW
RUNNABLE
BLOCKED
WAITING
TIMED_WAITING
TERMINATED

有一个容易说错的点：Java 里的 RUNNABLE 包括操作系统层面的"就绪"和"运行"。所以线程进入 RUNNABLE，不代表它此刻一定正在占用 CPU。

join 怎么保证线程顺序执行

如果新建 T1、T2、T3 三个线程，要保证它们按顺序执行，可以用 join()。

join() 的含义是：当前线程等待目标线程执行结束。

Thread t1 = new Thread(() -> System.out.println("t1"));

Thread t2 = new Thread(() -> {
    try {
        t1.join();
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
    System.out.println("t2");
});

Thread t3 = new Thread(() -> {
    try {
        t2.join();
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
    System.out.println("t3");
});

t1.start();
t2.start();
t3.start();

线程 T3 线程 T2 线程 T1 线程 T3 线程 T2 线程 T1 #mermaid-svg-z1ORY3TCajBFzj16{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-z1ORY3TCajBFzj16 .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-z1ORY3TCajBFzj16 .error-icon{fill:#552222;}#mermaid-svg-z1ORY3TCajBFzj16 .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-z1ORY3TCajBFzj16 .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-z1ORY3TCajBFzj16 .marker{fill:#333333;stroke:#333333;}#mermaid-svg-z1ORY3TCajBFzj16 .marker.cross{stroke:#333333;}#mermaid-svg-z1ORY3TCajBFzj16 svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-z1ORY3TCajBFzj16 p{margin:0;}#mermaid-svg-z1ORY3TCajBFzj16 .actor{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-z1ORY3TCajBFzj16 text.actor>tspan{fill:black;stroke:none;}#mermaid-svg-z1ORY3TCajBFzj16 .actor-line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-z1ORY3TCajBFzj16 .innerArc{stroke-width:1.5;stroke-dasharray:none;}#mermaid-svg-z1ORY3TCajBFzj16 .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333;}#mermaid-svg-z1ORY3TCajBFzj16 .messageLine1{stroke-width:1.5;stroke-dasharray:2,2;stroke:#333;}#mermaid-svg-z1ORY3TCajBFzj16 #arrowhead path{fill:#333;stroke:#333;}#mermaid-svg-z1ORY3TCajBFzj16 .sequenceNumber{fill:white;}#mermaid-svg-z1ORY3TCajBFzj16 #sequencenumber{fill:#333;}#mermaid-svg-z1ORY3TCajBFzj16 #crosshead path{fill:#333;stroke:#333;}#mermaid-svg-z1ORY3TCajBFzj16 .messageText{fill:#333;stroke:none;}#mermaid-svg-z1ORY3TCajBFzj16 .labelBox{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-z1ORY3TCajBFzj16 .labelText,#mermaid-svg-z1ORY3TCajBFzj16 .labelText>tspan{fill:black;stroke:none;}#mermaid-svg-z1ORY3TCajBFzj16 .loopText,#mermaid-svg-z1ORY3TCajBFzj16 .loopText>tspan{fill:black;stroke:none;}#mermaid-svg-z1ORY3TCajBFzj16 .loopLine{stroke-width:2px;stroke-dasharray:2,2;stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-z1ORY3TCajBFzj16 .note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-z1ORY3TCajBFzj16 .noteText,#mermaid-svg-z1ORY3TCajBFzj16 .noteText>tspan{fill:black;stroke:none;}#mermaid-svg-z1ORY3TCajBFzj16 .activation0{fill:#f4f4f4;stroke:#666;}#mermaid-svg-z1ORY3TCajBFzj16 .activation1{fill:#f4f4f4;stroke:#666;}#mermaid-svg-z1ORY3TCajBFzj16 .activation2{fill:#f4f4f4;stroke:#666;}#mermaid-svg-z1ORY3TCajBFzj16 .actorPopupMenu{position:absolute;}#mermaid-svg-z1ORY3TCajBFzj16 .actorPopupMenuPanel{position:absolute;fill:#ECECFF;box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);filter:drop-shadow(3px 5px 2px rgb(0 0 0 / 0.4));}#mermaid-svg-z1ORY3TCajBFzj16 .actor-man line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-z1ORY3TCajBFzj16 .actor-man circle,#mermaid-svg-z1ORY3TCajBFzj16 line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;stroke-width:2px;}#mermaid-svg-z1ORY3TCajBFzj16 :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} 执行任务join 等待 T1 结束T1 结束执行任务join 等待 T2 结束T2 结束执行任务

notify 和 notifyAll 有什么区别

notify() 和 notifyAll() 都是唤醒在同一个对象监视器上 wait() 的线程。

区别很简单：

方法	效果
notify()	随机唤醒一个等待线程
notifyAll()	唤醒所有等待线程，让它们重新竞争锁

注意，唤醒不等于立刻执行。被唤醒的线程要先从 WAITING 回到可竞争状态，等当前线程释放锁之后，才能重新抢锁。

wait 和 sleep 有什么区别

这也是高频题，最好按三个维度回答。

对比点	wait	sleep
所属类	Object 成员方法	Thread 静态方法
使用前提	必须先获得对象锁	不要求持有锁
是否释放锁	会释放对象锁	不会释放已持有的锁
唤醒方式	notify / notifyAll / 超时 / 中断	超时 / 中断

最关键的一句是：

wait 是对象监视器上的等待，会释放锁；sleep 只是让当前线程暂停一会儿，不会释放锁。
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sleep(ms)
线程持有锁
调用方法
释放锁
进入 WAITING / TIMED_WAITING
被 notify 或超时后重新竞争锁
不释放锁
进入 TIMED_WAITING
时间到后继续执行

如何停止一个正在运行的线程

不要用 Thread.stop()。它已经被废弃，因为它会粗暴终止线程，可能导致锁释放和数据状态都不可靠。

常见做法有两种：

1. 使用退出标记，让线程自己在合适的位置结束。
2. 使用 interrupt() 中断线程，由线程自己决定如何响应。

阻塞中的线程，比如正在 sleep()、wait()、join()，收到中断后会抛 InterruptedException。

普通运行中的线程，interrupt() 不会强行停止它，只是把中断标记设置为 true。线程需要自己检查这个标记。

Thread worker = new Thread(() -> {
    while (!Thread.currentThread().isInterrupted()) {
        // do work
    }
});

worker.start();
worker.interrupt();

这也是并发编程里很重要的思想：停止线程不是外部强杀，而是协作式退出。

面试怎么答

如果面试官从基础题开始问，可以把回答组织成一条线：

线程是进程内的执行路径，同一进程内的线程共享内存，所以多线程能提升吞吐，也会带来线程安全问题。

并发是多个任务交替推进，并行是多个任务同一时刻同时执行。Java 里创建线程可以继承 Thread、实现 Runnable、实现 Callable 或使用线程池，项目里更推荐线程池。

线程启动要调用 start()，直接调用 run() 只是普通方法调用。线程状态主要有 NEW、RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED。其中 wait() 会释放锁，sleep() 不会释放锁。停止线程时不建议用 stop()，更推荐退出标记或 interrupt() 这种协作式方式。