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使用 synchronized 的场景

synchronized 是 Java 内置的同步机制，使用起来比较简单且常用于如下场景：

1、简单的同步逻辑：当你的同步逻辑非常简单，比如只需要对某个方法或代码块进行互斥访问时，synchronized 非常适用，因为它的语法简洁且易于维护。

public synchronized void simpleMethod() {// critical section
}

2、隐式监视器锁：synchronized 隐式地使用对象的监视器锁来控制同步，不需要显式的锁定和解锁操作，因此避免了锁忘记释放的问题。

3、异常安全：synchronized 块在异常发生时，会自动释放锁，确保不会因为未释放锁而导致死锁问题。

使用 ReentrantLock 的场景

ReentrantLock 是 java.util.concurrent.locks 包中提供的，更高级的锁机制，适用于以下场景：

1、需要更灵活的锁控制：ReentrantLock 提供了更多的锁控制功能，如 tryLock()（尝试锁定）和 lockInterruptibly()（可中断锁定），使得能够响应中断或超时。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class TryLockExample {private final Lock lock = new ReentrantLock();public void tryMethod() {if (lock.tryLock()) {try {// critical section} finally {lock.unlock();}} else {// handle the case where lock was not acquired}}
}

2、需要公平锁机制：ReentrantLock 支持公平锁（即先请求先获得锁），这在某些需要防止线程饥饿的场景中特别有用。

Lock fairLock = new ReentrantLock(true); // true for fairness

3、需要条件变量：ReentrantLock 提供了条件变量（Condition），可以更精细地控制线程等待和通知机制，适用于需要多个条件队列的复杂同步场景。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;// 假设我们有一个界面程序，其中一个生产者线程和两个消费者线程，
// 并且我们希望消费者1只处理奇数编号的任务，而消费者2只处理偶数编号的任务。
public class MultiConditionExample {private static final Lock lock = new ReentrantLock();private static final Condition conditionOdd = lock.newCondition();private static final Condition conditionEven = lock.newCondition();private static int count = 0;public static void main(String[] args) {Thread producer = new Thread(new Producer());Thread consumer1 = new Thread(new ConsumerOdd(), "Consumer-Odd");Thread consumer2 = new Thread(new ConsumerEven(), "Consumer-Even");producer.start();consumer1.start();consumer2.start();}static class Producer implements Runnable {@Overridepublic void run() {while (true) {try {lock.lock();count++;System.out.println("Produced: " + count);if (count % 2 == 0) {conditionEven.signal();  // Signal consumers waiting on even condition} else {conditionOdd.signal();  // Signal consumers waiting on odd condition}} finally {lock.unlock();}try {Thread.sleep(1000);} catch (InterruptedException e) {Thread.currentThread().interrupt();}}}}static class ConsumerOdd implements Runnable {@Overridepublic void run() {while (true) {try {lock.lock();while (count % 2 == 0) {conditionOdd.await();  // Wait for odd number condition}// Process odd numberSystem.out.println(Thread.currentThread().getName() + " consumed: " + count);} catch (InterruptedException e) {Thread.currentThread().interrupt();} finally {lock.unlock();}}}}static class ConsumerEven implements Runnable {@Overridepublic void run() {while (true) {try {lock.lock();while (count % 2 != 0) {conditionEven.await();  // Wait for even number condition}// Process even numberSystem.out.println(Thread.currentThread().getName() + " consumed: " + count);} catch (InterruptedException e) {Thread.currentThread().interrupt();} finally {lock.unlock();}}}}
}

class MultiConditionExampleWithWaitNotify {private static final Object lock = new Object();private static int count = 0;public static void main(String[] args) {Thread producer = new Thread(new Producer());Thread consumerOdd = new Thread(new ConsumerOdd(), "Consumer-Odd");Thread consumerEven = new Thread(new ConsumerEven(), "Consumer-Even");producer.start();consumerOdd.start();consumerEven.start();}static class Producer implements Runnable {@Overridepublic void run() {while (true) {synchronized (lock) {count++;System.out.println("Produced: " + count);if (count % 2 == 0) {lock.notifyAll(); // Notify consumers waiting on even condition} else {lock.notifyAll(); // Notify consumers waiting on odd condition}}try {Thread.sleep(1000);} catch (InterruptedException e) {Thread.currentThread().interrupt();}}}}static class ConsumerOdd implements Runnable {@Overridepublic void run() {while (true) {synchronized (lock) {try {while (count % 2 == 0) {lock.wait(); // Wait for odd number condition}// Process odd numberSystem.out.println(Thread.currentThread().getName() + " consumed: " + count);} catch (InterruptedException e) {Thread.currentThread().interrupt();}}}}}static class ConsumerEven implements Runnable {@Overridepublic void run() {while (true) {synchronized (lock) {try {while (count % 2 != 0) {lock.wait(); // Wait for even number condition}// Process even numberSystem.out.println(Thread.currentThread().getName() + " consumed: " + count);} catch (InterruptedException e) {Thread.currentThread().interrupt();}}}}}
}

wait 和 notify：只能使用单一的监视器锁，并通过 notify 或 notifyAll 来通知等待线程。由于所有线程都在同一个锁对象上等待，即使只需要唤醒部分线程（例如只唤醒等待奇数的线程），也必须通知所有等待的线程（使用 notifyAll），这样会导致非目标线程频繁被唤醒和再次等待。

Condition：可以创建多个 Condition 实例，在不同的条件队列上管理等待和通知。只会唤醒指定条件下的线程，从而提高效率。

综合考虑

简洁性和安全性：如果你的同步需求简单，且不需要复杂的锁定逻辑，选择 synchronized 更为简洁和安全。

灵活性和功能：如果需要更灵活的锁控制、更高的性能、更多的特性（如条件变量、尝试锁定、响应中断等），ReentrantLock 提供更强大的功能。