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什么时候用synchronized，什么时候用Reentrantlock

article 2024/9/23 7:14:37

文章目录

使用 synchronized 的场景
使用 ReentrantLock 的场景
综合考虑

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 {
        @Override
        public 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 {
        @Override
        public void run() {
            while (true) {
                try {
                    lock.lock();
                    while (count % 2 == 0) {
                        conditionOdd.await();  // Wait for odd number condition
                    }
                    // Process odd number
                    System.out.println(Thread.currentThread().getName() + " consumed: " + count);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                } finally {
                    lock.unlock();
                }
            }
        }
    }

    static class ConsumerEven implements Runnable {
        @Override
        public void run() {
            while (true) {
                try {
                    lock.lock();
                    while (count % 2 != 0) {
                        conditionEven.await();  // Wait for even number condition
                    }
                    // Process even number
                    System.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 {
        @Override
        public 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 {
        @Override
        public void run() {
            while (true) {
                synchronized (lock) {
                    try {
                        while (count % 2 == 0) {
                            lock.wait(); // Wait for odd number condition
                        }
                        // Process odd number
                        System.out.println(Thread.currentThread().getName() + " consumed: " + count);
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                    }
                }
            }
        }
    }

    static class ConsumerEven implements Runnable {
        @Override
        public void run() {
            while (true) {
                synchronized (lock) {
                    try {
                        while (count % 2 != 0) {
                            lock.wait(); // Wait for even number condition
                        }
                        // Process even number
                        System.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。