volatile 关键字

概述

volatile 关键字是Java提供的轻量级同步机制,用于确保变量的内存可见性和禁止指令重排序。当一个变量被声明为volatile时,对该变量的读写操作都会直接在主内存中进行。

语法格式

volatile int variable;
volatile boolean flag;
volatile Object reference;

核心特性

1. 内存可见性

volatile变量的修改对所有线程立即可见,解决了多线程环境下的数据一致性问题。

2. 禁止指令重排序

编译器和处理器不会对volatile变量的读写操作进行重排序优化。

基本用法

内存可见性示例

public class VisibilityExample {
    // 不使用volatile的情况
    private boolean stopFlag = false;

    // 使用volatile的情况
    private volatile boolean volatileStopFlag = false;

    // 演示普通变量可见性问题
    public void demonstrateVisibilityProblem() {
        Thread writerThread = new Thread(() -> {
            try {
                Thread.sleep(1000);
                stopFlag = true; // 修改标志位
                System.out.println("Writer线程:设置stopFlag = true");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });

        Thread readerThread = new Thread(() -> {
            while (!stopFlag) {
                // 可能会无限循环,因为看不到stopFlag的变化
            }
            System.out.println("Reader线程:检测到stopFlag = true,退出循环");
        });

        writerThread.start();
        readerThread.start();

        try {
            writerThread.join();
            // 注意:readerThread可能不会退出
            readerThread.join(3000); // 最多等待3秒
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    // 演示volatile变量的可见性
    public void demonstrateVolatileVisibility() {
        Thread writerThread = new Thread(() -> {
            try {
                Thread.sleep(1000);
                volatileStopFlag = true; // 修改volatile标志位
                System.out.println("Writer线程:设置volatileStopFlag = true");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });

        Thread readerThread = new Thread(() -> {
            while (!volatileStopFlag) {
                // volatile确保能及时看到变化
            }
            System.out.println("Reader线程:检测到volatileStopFlag = true,退出循环");
        });

        writerThread.start();
        readerThread.start();

        try {
            writerThread.join();
            readerThread.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    public static void main(String[] args) {
        VisibilityExample example = new VisibilityExample();

        System.out.println("=== 演示普通变量可见性问题 ===");
        example.demonstrateVisibilityProblem();

        System.out.println("\n=== 演示volatile变量的可见性 ===");
        example.demonstrateVolatileVisibility();
    }
}

状态标志模式

public class StatusFlagExample {
    private volatile boolean isReady = false;
    private volatile boolean isShutdown = false;

    // 工作线程
    public void workerThread() {
        Thread worker = new Thread(() -> {
            System.out.println("工作线程:等待就绪信号...");

            // 等待就绪信号
            while (!isReady) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    return;
                }
            }

            System.out.println("工作线程:开始工作");

            // 执行工作直到收到关闭信号
            while (!isShutdown) {
                System.out.println("工作线程:正在工作...");
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }

            System.out.println("工作线程:收到关闭信号,停止工作");
        });

        worker.start();

        // 主线程控制工作流程
        try {
            Thread.sleep(1000);
            setReady(true); // 发送就绪信号

            Thread.sleep(3000);
            shutdown(); // 发送关闭信号

            worker.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    public void setReady(boolean ready) {
        this.isReady = ready;
        System.out.println("主线程:设置就绪状态为 " + ready);
    }

    public void shutdown() {
        this.isShutdown = true;
        System.out.println("主线程:发送关闭信号");
    }

    public static void main(String[] args) {
        StatusFlagExample example = new StatusFlagExample();
        example.workerThread();
    }
}

单例模式的双重检查锁定

public class Singleton {
    // 使用volatile确保多线程环境下的正确性
    private static volatile Singleton instance;

    private Singleton() {
        // 私有构造器
    }

    // 双重检查锁定(Double-Checked Locking)
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton(); // volatile确保这里不会被重排序
                }
            }
        }
        return instance;
    }
}

// 演示为什么需要volatile
public class SingletonTest {
    public static void main(String[] args) {
        // 创建多个线程同时获取单例
        Runnable task = () -> {
            Singleton singleton = Singleton.getInstance();
            System.out.println(Thread.currentThread().getName() + 
                ": " + singleton.hashCode());
        };

        for (int i = 0; i < 10; i++) {
            new Thread(task, "Thread-" + i).start();
        }
    }
}

生产者-消费者模式的简单实现

public class SimpleProducerConsumer {
    private volatile int count = 0;
    private volatile boolean hasData = false;
    private final int maxCount = 10;

    // 生产者
    public void producer() {
        Thread producerThread = new Thread(() -> {
            for (int i = 1; i <= maxCount; i++) {
                // 等待消费者处理完数据
                while (hasData) {
                    Thread.yield();
                }

                count = i;
                hasData = true;
                System.out.println("生产者:生产数据 " + count);

                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
        });

        producerThread.start();
    }

    // 消费者
    public void consumer() {
        Thread consumerThread = new Thread(() -> {
            for (int i = 1; i <= maxCount; i++) {
                // 等待生产者生产数据
                while (!hasData) {
                    Thread.yield();
                }

                System.out.println("消费者:消费数据 " + count);
                hasData = false;

                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
        });

        consumerThread.start();
    }

    public static void main(String[] args) {
        SimpleProducerConsumer example = new SimpleProducerConsumer();

        example.producer();
        example.consumer();

        try {
            Thread.sleep(5000); // 等待执行完成
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

配置更新场景

public class ConfigurationManager {
    // 配置信息使用volatile确保更新的可见性
    private volatile String databaseUrl = "jdbc:mysql://localhost:3306/db";
    private volatile int connectionTimeout = 5000;
    private volatile boolean debugMode = false;

    // 读取配置(高频操作)
    public String getDatabaseUrl() {
        return databaseUrl;
    }

    public int getConnectionTimeout() {
        return connectionTimeout;
    }

    public boolean isDebugMode() {
        return debugMode;
    }

    // 更新配置(低频操作)
    public void updateDatabaseUrl(String newUrl) {
        this.databaseUrl = newUrl;
        System.out.println("配置更新:数据库URL = " + newUrl);
    }

    public void updateConnectionTimeout(int timeout) {
        this.connectionTimeout = timeout;
        System.out.println("配置更新:连接超时 = " + timeout);
    }

    public void updateDebugMode(boolean debug) {
        this.debugMode = debug;
        System.out.println("配置更新:调试模式 = " + debug);
    }

    // 模拟配置使用者
    public void startWorkers() {
        // 工作线程1:数据库操作
        Thread dbWorker = new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                String url = getDatabaseUrl();
                int timeout = getConnectionTimeout();
                System.out.println("DB Worker: 连接 " + url + ",超时 " + timeout + "ms");

                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
        }, "DB-Worker");

        // 工作线程2:日志记录
        Thread logWorker = new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                boolean debug = isDebugMode();
                if (debug) {
                    System.out.println("Log Worker: 调试信息 - 操作" + i);
                } else {
                    System.out.println("Log Worker: 普通日志 - 操作" + i);
                }

                try {
                    Thread.sleep(700);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
        }, "Log-Worker");

        // 配置更新线程
        Thread configUpdater = new Thread(() -> {
            try {
                Thread.sleep(2000);
                updateDebugMode(true);

                Thread.sleep(2000);
                updateConnectionTimeout(8000);

                Thread.sleep(2000);
                updateDatabaseUrl("jdbc:mysql://newserver:3306/newdb");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }, "Config-Updater");

        dbWorker.start();
        logWorker.start();
        configUpdater.start();

        try {
            dbWorker.join();
            logWorker.join();
            configUpdater.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    public static void main(String[] args) {
        ConfigurationManager manager = new ConfigurationManager();
        manager.startWorkers();
    }
}

volatile的局限性

1. 非原子操作

public class VolatileLimitation {
    private volatile int counter = 0;

    // 这个操作不是原子的,volatile无法保证线程安全
    public void incrementCounter() {
        counter++; // 相当于:counter = counter + 1(读取、计算、写入三个步骤)
    }

    // 正确的做法:使用synchronized
    public synchronized void safeIncrementCounter() {
        counter++;
    }

    // 或者使用AtomicInteger
    private final AtomicInteger atomicCounter = new AtomicInteger(0);

    public void atomicIncrementCounter() {
        atomicCounter.incrementAndGet();
    }

    // 演示volatile不能保证原子性
    public void demonstrateNonAtomicity() {
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                incrementCounter(); // 不安全的操作
            }
        };

        Thread t1 = new Thread(task, "Thread-1");
        Thread t2 = new Thread(task, "Thread-2");

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

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }

        System.out.println("预期结果: 2000");
        System.out.println("实际结果: " + counter); // 可能小于2000
        System.out.println("原子计数器: " + atomicCounter.get()); // 如果也执行了原子操作
    }

    public static void main(String[] args) {
        VolatileLimitation example = new VolatileLimitation();
        example.demonstrateNonAtomicity();
    }
}

2. 复合操作的问题

public class CompoundOperationIssue {
    private volatile int min = 0;
    private volatile int max = 0;

    // 这种复合操作在多线程环境下可能出现问题
    public void updateRange(int newMin, int newMax) {
        // 问题:这两个赋值操作之间可能被其他线程中断
        min = newMin;
        max = newMax;

        // 如果另一个线程在这两行代码之间读取min和max,
        // 可能看到不一致的状态
    }

    // 正确的做法:使用synchronized保证原子性
    public synchronized void safeUpdateRange(int newMin, int newMax) {
        min = newMin;
        max = newMax;
    }

    public void readRange() {
        int currentMin = min;
        int currentMax = max;
        System.out.println("范围: [" + currentMin + ", " + currentMax + "]");

        // 可能出现min > max的不一致状态
        if (currentMin > currentMax) {
            System.out.println("警告:检测到不一致状态!");
        }
    }
}

使用场景

1. 状态标志

public class StatusFlag {
    private volatile boolean initialized = false;
    private volatile boolean running = false;
    private volatile boolean error = false;

    public void setInitialized() { initialized = true; }
    public void setRunning() { running = true; }
    public void setError() { error = true; }

    public boolean isInitialized() { return initialized; }
    public boolean isRunning() { return running; }
    public boolean hasError() { return error; }
}

2. 配置信息

public class Configuration {
    private volatile String serverUrl;
    private volatile int timeout;
    private volatile boolean enableCache;

    // 配置更新方法
    public void updateConfig(String url, int timeout, boolean cache) {
        this.serverUrl = url;
        this.timeout = timeout;
        this.enableCache = cache;
    }
}

3. 双重检查锁定模式

public class LazyInitialization {
    private volatile ExpensiveObject instance;

    public ExpensiveObject getInstance() {
        if (instance == null) {
            synchronized (this) {
                if (instance == null) {
                    instance = new ExpensiveObject();
                }
            }
        }
        return instance;
    }

    private class ExpensiveObject {
        // 昂贵的对象初始化
    }
}

最佳实践

  1. 仅用于状态标志

    • volatile最适合简单的状态标志
    • 避免在需要原子操作的场景使用
  2. 读多写少的场景

    • 适合配置信息、状态变量等读多写少的场景
    • 写操作通常是单线程的
  3. 与synchronized结合

    • 在需要原子性的地方使用synchronized
    • volatile用于状态发布
  4. 考虑使用Atomic类

    • 对于计数器等场景,使用AtomicInteger等原子类
    • 对于引用类型,使用AtomicReference

注意事项

  1. volatile不能替代synchronized

    • volatile只保证可见性,不保证原子性
    • 复合操作仍需要synchronized
  2. 性能考虑

    • volatile变量的读写比普通变量慢
    • 但比synchronized块快得多
  3. 使用场景限制

    • 不适合复杂的同步需求
    • 不能用于同步多个变量的状态
  4. 编程模型

    • 适合一个线程写,多个线程读的模式
    • 避免多个线程同时写入

volatile关键字是Java并发编程中的重要工具,但需要理解其局限性并在合适的场景中使用。

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