一文击穿 JMM 内存模型：从 CPU 底层到 Java 并发实战，99% 的开发都踩过这些坑

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 原子性问题演示
 * @author ken
 */
@Slf4j
publicclass AtomicityDemo {
    privatestaticvolatileint volatileCount = 0;
    privatestaticfinal AtomicInteger atomicCount = new AtomicInteger(0);
    privatestaticint lockCount = 0;
    privatestaticfinalint THREAD_COUNT = 10;
    privatestaticfinalint LOOP_COUNT = 10000;

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch volatileLatch = new CountDownLatch(THREAD_COUNT);
        for (int i = 0; i < THREAD_COUNT; i++) {
            new Thread(() -> {
                for (int j = 0; j < LOOP_COUNT; j++) {
                    volatileCount++;
                }
                volatileLatch.countDown();
            }, "volatile-thread-" + i).start();
        }
        volatileLatch.await();
        log.info("volatile修饰的计数结果:{}，预期结果:{}", volatileCount, THREAD_COUNT * LOOP_COUNT);

        CountDownLatch atomicLatch = new CountDownLatch(THREAD_COUNT);
        for (int i = 0; i < THREAD_COUNT; i++) {
            new Thread(() -> {
                for (int j = 0; j < LOOP_COUNT; j++) {
                    atomicCount.incrementAndGet();
                }
                atomicLatch.countDown();
            }, "atomic-thread-" + i).start();
        }
        atomicLatch.await();
        log.info("原子类修饰的计数结果:{}，预期结果:{}", atomicCount.get(), THREAD_COUNT * LOOP_COUNT);

        CountDownLatch lockLatch = new CountDownLatch(THREAD_COUNT);
        Object lock = new Object();
        for (int i = 0; i < THREAD_COUNT; i++) {
            new Thread(() -> {
                for (int j = 0; j < LOOP_COUNT; j++) {
                    synchronized (lock) {
                        lockCount++;
                    }
                }
                lockLatch.countDown();
            }, "lock-thread-" + i).start();
        }
        lockLatch.await();
        log.info("synchronized保护的计数结果:{}，预期结果:{}", lockCount, THREAD_COUNT * LOOP_COUNT);
    }
}

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;

/**
 * 可见性问题演示
 * @author ken
 */
@Slf4j
publicclass VisibilityDemo {
    privatestaticboolean flag = false;

    public static void main(String[] args) throws InterruptedException {
        Thread threadA = new Thread(() -> {
            int i = 0;
            while (!flag) {
                i++;
            }
            log.info("线程A退出循环，i的值为:{}", i);
        }, "threadA");

        threadA.start();
        Thread.sleep(1000);
        flag = true;
        log.info("主线程已经将flag设置为true");
        threadA.join();
    }
}

package com.jam.demo.jmm;

/**
 * 错误的DCL单例示例
 * @author ken
 */
publicclass WrongDclSingleton {
    privatestatic WrongDclSingleton instance;

    private WrongDclSingleton() {
    }

    public static WrongDclSingleton getInstance() {
        if (instance == null) {
            synchronized (WrongDclSingleton.class) {
                if (instance == null) {
                    instance = new WrongDclSingleton();
                }
            }
        }
        return instance;
    }
}

package com.jam.demo.jmm;

/**
 * 正确的DCL单例示例
 * @author ken
 */
publicclass CorrectDclSingleton {
    privatestaticvolatile CorrectDclSingleton instance;

    private CorrectDclSingleton() {
    }

    /**
     * 获取单例实例
     * @return 单例对象
     */
    public static CorrectDclSingleton getInstance() {
        if (instance == null) {
            synchronized (CorrectDclSingleton.class) {
                if (instance == null) {
                    instance = new CorrectDclSingleton();
                }
            }
        }
        return instance;
    }
}

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

/**
 * VarHandle内存屏障示例
 * @author ken
 */
@Slf4j
publicclass VarHandleBarrierDemo {
    privateint x = 0;
    privatevolatileint y = 0;
    privatestaticfinal VarHandle X_HANDLE;

    static {
        try {
            X_HANDLE = MethodHandles.lookup().findVarHandle(VarHandleBarrierDemo.class, "x", int.class);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            thrownew ExceptionInInitializerError(e);
        }
    }

    public void write() {
        x = 10;
        VarHandle.storeStoreFence();
        y = 1;
    }

    public void read() {
        if (y == 1) {
            VarHandle.loadLoadFence();
            log.info("x的值为:{}", x);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        VarHandleBarrierDemo demo = new VarHandleBarrierDemo();
        for (int i = 0; i < 10000; i++) {
            Thread writer = new Thread(demo::write, "writer-thread-" + i);
            Thread reader = new Thread(demo::read, "reader-thread-" + i);
            writer.start();
            reader.start();
            writer.join();
            reader.join();
            demo.x = 0;
            demo.y = 0;
        }
        log.info("测试完成，所有读操作都能正确获取x的值");
    }
}

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;

/**
 * Happens-Before规则演示
 * @author ken
 */
@Slf4j
publicclass HappensBeforeDemo {
    privatestaticint a = 0;
    privatestaticvolatileint b = 0;

    public static void main(String[] args) throws InterruptedException {
        Thread threadA = new Thread(() -> {
            a = 10;
            b = 1;
        }, "threadA");

        Thread threadB = new Thread(() -> {
            if (b == 1) {
                log.info("a的值为:{}", a);
            }
        }, "threadB");

        threadA.start();
        threadB.start();
        threadA.join();
        threadB.join();
    }
}

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;

/**
 * final字段this溢出错误示例
 * @author ken
 */
@Slf4j
publicclass FinalEscapeDemo {
    privatefinalint value;
    privatestatic FinalEscapeDemo instance;

    public FinalEscapeDemo() {
        instance = this;
        this.value = 10;
    }

    public static void main(String[] args) throws InterruptedException {
        Thread writer = new Thread(FinalEscapeDemo::new, "writer-thread");
        Thread reader = new Thread(() -> {
            if (instance != null) {
                log.info("value的值为:{}", instance.value);
            }
        }, "reader-thread");
        writer.start();
        reader.start();
        writer.join();
        reader.join();
    }
}

package com.jam.demo.jmm;

import lombok.extern.slf4j.Slf4j;
import jdk.internal.vm.annotation.Contended;

/**
 * 伪共享问题演示与解决方案
 * @author ken
 */
@Slf4j
publicclass FalseSharingDemo {
    privatestaticfinalint THREAD_COUNT = 2;
    privatestaticfinallong LOOP_COUNT = 1000_000_000L;

    staticclass NoPaddingObject {
        publicvolatilelong value = 0L;
    }

    staticclass ManualPaddingObject {
        publicvolatilelong value = 0L;
        publiclong p1, p2, p3, p4, p5, p6, p7;
    }

    staticclass ContendedObject {
        @Contended
        publicvolatilelong value = 0L;
    }

    public static void main(String[] args) throws InterruptedException {
        NoPaddingObject[] noPaddingObjects = new NoPaddingObject[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            noPaddingObjects[i] = new NoPaddingObject();
        }
        long start = System.currentTimeMillis();
        Thread[] threads = new Thread[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            int index = i;
            threads[i] = new Thread(() -> {
                for (long j = 0; j < LOOP_COUNT; j++) {
                    noPaddingObjects[index].value = j;
                }
            }, "no-padding-thread-" + i);
        }
        for (Thread thread : threads) {
            thread.start();
        }
        for (Thread thread : threads) {
            thread.join();
        }
        long end = System.currentTimeMillis();
        log.info("无填充的执行时间:{}ms", end - start);

        ManualPaddingObject[] manualPaddingObjects = new ManualPaddingObject[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            manualPaddingObjects[i] = new ManualPaddingObject();
        }
        start = System.currentTimeMillis();
        threads = new Thread[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            int index = i;
            threads[i] = new Thread(() -> {
                for (long j = 0; j < LOOP_COUNT; j++) {
                    manualPaddingObjects[index].value = j;
                }
            }, "manual-padding-thread-" + i);
        }
        for (Thread thread : threads) {
            thread.start();
        }
        for (Thread thread : threads) {
            thread.join();
        }
        end = System.currentTimeMillis();
        log.info("手动填充的执行时间:{}ms", end - start);

        ContendedObject[] contendedObjects = new ContendedObject[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            contendedObjects[i] = new ContendedObject();
        }
        start = System.currentTimeMillis();
        threads = new Thread[THREAD_COUNT];
        for (int i = 0; i < THREAD_COUNT; i++) {
            int index = i;
            threads[i] = new Thread(() -> {
                for (long j = 0; j < LOOP_COUNT; j++) {
                    contendedObjects[index].value = j;
                }
            }, "contended-thread-" + i);
        }
        for (Thread thread : threads) {
            thread.start();
        }
        for (Thread thread : threads) {
            thread.join();
        }
        end = System.currentTimeMillis();
        log.info("@Contended注解的执行时间:{}ms", end - start);
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.jam.demo</groupId>
    <artifactId>jmm-demo</artifactId>
    <version>1.0.0-SNAPSHOT</version>

    <name>jmm-demo</name>
    <description>JMM内存模型实战示例项目</description>

    <properties>
        <java.version>17</java.version>
        <maven.compiler.source>${java.version}</maven.compiler.source>
        <maven.compiler.target>${java.version}</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <lombok.version>1.18.34</lombok.version>
        <spring.version>6.1.14</spring.version>
        <fastjson2.version>2.0.53</fastjson2.version>
        <guava.version>33.2.1-jre</guava.version>
        <mybatis-plus.version>3.5.7</mybatis-plus.version>
        <springdoc.version>2.6.0</springdoc.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>${lombok.version}</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba.fastjson2</groupId>
            <artifactId>fastjson2</artifactId>
            <version>${fastjson2.version}</version>
        </dependency>
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>${guava.version}</version>
        </dependency>
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>${mybatis-plus.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
            <version>${springdoc.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <version>3.3.4</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.13.0</version>
                <configuration>
                    <source>${java.version}</source>
                    <target>${java.version}</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>