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在Java中，synchronized关键字、ReentrantLock和volatile关键字这三个是编程中常用于实现线程同步的机制，下面结合代码详细说明一下这三个关键字的用法。

1. synchronized关键字：

synchronized关键字是Java语言提供的内置锁机制，用于实现线程之间的同步。它可以修饰方法或代码块，确保同一时刻只有一个线程可以访问被synchronized修饰的代码块或方法。示例

下面使用共同操作一个共享变量count演示一下synchronized的用法，代码如下：

public class SynchronizedExample {private int count = 0;public synchronized void increment() {count++;}public synchronized void decrement() {count--;}public static void main(String[] args) {SynchronizedExample example = new SynchronizedExample();Thread thread1 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.increment();}});Thread thread2 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.decrement();}});thread1.start();thread2.start();try {thread1.join();thread2.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Count: " + example.count);  //Count: 0}
}

2. ReentrantLock：

ReentrantLock是java.util.concurrent包提供的锁机制，相比synchronized关键字更加灵活，可以实现公平锁、可中断锁等特性。使用ReentrantLock需要手动加锁和解锁。

代码如下：

import java.util.concurrent.locks.ReentrantLock;public class ReentrantLockExample {private int count = 0;private ReentrantLock lock = new ReentrantLock();public void decrement() {lock.lock();try {count++;} finally {lock.unlock();}}public void increment() {lock.lock();try {count++;} finally {lock.unlock();}}public static void main(String[] args) {ReentrantLockExample example = new ReentrantLockExample();Thread thread1 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.increment();}});Thread thread2 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.decrement();}});thread1.start();thread2.start();try {thread1.join();thread2.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Count: " + example.count); //Count: 0}
}

3. volatile关键字：

volatile关键字用于保证变量的可见性，即当一个线程修改了volatile变量的值，其他线程可以立即看到最新的值。但它并不具备原子性，不能保证复合操作的原子性。
代码如下：

public class VolatileExample {private volatile boolean flag = false;public void toggleFlag() {flag = !flag;}public static void main(String[] args) {VolatileExample example = new VolatileExample();Thread thread1 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.toggleFlag();}});Thread thread2 = new Thread(() -> {for (int i = 0; i < 1000; i++) {example.toggleFlag();}});thread1.start();thread2.start();try {thread1.join();thread2.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Flag: " + example.flag);}
}