synchronized锁和lock锁队列中线程执行顺序对比

柔情只为你懂 2021-07-21 00:15 280阅读 0赞

### 背景 ###

最近复习一下多线程，上面说到synchronized是将被阻塞的现场放到一个虚拟队列（Lock-Free无锁机制）中，而Lock是通过AQS队列存放阻塞的线程。那synchronized里的线程默认是怎么竞争锁的，Lock又是如何？

*之前记录的结论：*

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JpYmlib3l4_size_16_color_FFFFFF_t_70][]

### 思路 ###

开启一个死循环，然后让多个线程竞争通一把锁，被阻塞的线程孜然就会被放到对应的阻塞队列里，执行时打印线程的名字，然后根据打印的结果判断顺序

先验证synchronized的情况，创建一个runnable实现类，然后共享成员变量map

public class MyThread implements Runnable{
        Map<String,String> map;
    
        public Map<String, String> getMap() {
            return map;
        }
    
        public void setMap(Map<String, String> map) {
            this.map = map;
        }
    
        @Override
        public void run() {
            while (true){
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (map){
                    System.out.println(Thread.currentThread().getName()+"拿到了锁");
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+"结束了");
                }
            }
        }
    }

在主方法中执行

public static void main(String[] args) {
            Map<String, String> map = new HashMap<>();
            for (int i = 0; i < 10; i++) {
                MyThread myThread = new MyThread();
                myThread.setMap(map);
                Thread thread = new Thread(myThread);
                System.out.println(thread.getName()+"开始启动了");
                thread.start();
            }
            System.out.println("主线程结束了");
        }

结果：线程竞争结果无顺序，可以认为当一个线程释放锁资源时，队列中的其他线程都在竞争？

Thread-0开始启动了
    Thread-1开始启动了
    Thread-2开始启动了
    Thread-3开始启动了
    Thread-4开始启动了
    Thread-5开始启动了
    Thread-6开始启动了
    Thread-7开始启动了
    Thread-8开始启动了
    Thread-9开始启动了
    主线程结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-9拿到了锁
    Thread-9结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-6拿到了锁
    Thread-6结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-8拿到了锁
    Thread-8结束了
    Thread-7拿到了锁
    Thread-7结束了
    Thread-5拿到了锁
    Thread-5结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-5拿到了锁
    Thread-5结束了
    Thread-7拿到了锁
    Thread-7结束了
    Thread-8拿到了锁
    Thread-8结束了
    Thread-2拿到了锁

验证Lock锁的情况，通过共享一个ReentrantLock对象。

public class MyThread2 implements  Runnable{
        ReentrantLock reentrantLock ;
    
        public ReentrantLock getReentrantLock() {
            return reentrantLock;
        }
    
        public void setReentrantLock(ReentrantLock reentrantLock) {
            this.reentrantLock = reentrantLock;
        }
    
        @Override
        public void run() {
            while (true){
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                reentrantLock.lock();
                System.out.println(Thread.currentThread().getName()+"拿到了锁");
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"结束了");
                reentrantLock.unlock();
            }
        }
    }

主线程

结果可见，线程执行是有顺序的，确实是按照顺序不停更换头结点，循环，有序拿到锁的。

Thread-0开始启动了
    Thread-1开始启动了
    Thread-2开始启动了
    Thread-3开始启动了
    Thread-4开始启动了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-4拿到了锁

这里有个疑问就是公平锁的问题，公平锁反应的应该是新来的线程是否应该排队吧，已经在CLH队列里的线程就是按照顺序执行，跟是否公平无关吧？

这里证明一下，先让5个线程都打印完，得以判断出最初的队列顺序，然后再突然让5个新的线程去竞争，看看这5个线程是否会排到后面。

public static void main(String[] args) {
            ReentrantLock reentrantLock = new ReentrantLock(false);
            for (int i = 0; i < 5; i++) {
                MyThread2 myThread2 = new MyThread2();
                myThread2.setReentrantLock(reentrantLock);
                Thread thread = new Thread(myThread2);
                System.out.println(thread.getName()+"开始启动了");
                thread.start();
            }
            try {
                Thread.sleep(3500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("开始加入额外线程");
            for (int i = 0; i < 10; i++) {
                MyThread2 myThread2 = new MyThread2();
                myThread2.setReentrantLock(reentrantLock);
                Thread thread = new Thread(myThread2);
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                thread.start();
            }
            System.out.println("主线程结束了");
    
        }

设置的Lock锁为false，为不公平锁

最初顺序为3,2,4,1,0，最后结果成了3,10,5,11,2,12,6,13,4,14,7,1,8等等

根据结果可以发现确实有插队情况出现了，确实符合理论。

Thread-0开始启动了
    Thread-1开始启动了
    Thread-2开始启动了
    Thread-3开始启动了
    Thread-4开始启动了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-3拿到了锁
    开始加入额外线程
    Thread-3结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-5拿到了锁
    Thread-5结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-6拿到了锁
    Thread-6结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-7拿到了锁
    主线程结束了
    Thread-7结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-8拿到了锁
    Thread-8结束了
    Thread-0拿到了锁
    Thread-0结束了
    Thread-9拿到了锁
    Thread-9结束了
    Thread-3拿到了锁
    Thread-3结束了
    Thread-10拿到了锁
    Thread-10结束了
    Thread-5拿到了锁
    Thread-5结束了
    Thread-11拿到了锁
    Thread-11结束了
    Thread-2拿到了锁
    Thread-2结束了
    Thread-12拿到了锁
    Thread-12结束了
    Thread-6拿到了锁
    Thread-6结束了
    Thread-13拿到了锁
    Thread-13结束了
    Thread-4拿到了锁
    Thread-4结束了
    Thread-14拿到了锁
    Thread-14结束了
    Thread-7拿到了锁
    Thread-7结束了
    Thread-1拿到了锁
    Thread-1结束了
    Thread-8拿到了锁

现在改成ture，也就是公平锁，看看结果如何

结果显示不符合预期效果，想了一下，因为CLH他是一个带头结点的双向链表，也就是说他的链表是一直在变化的，所以顺序肯定不会是固定的，头结点释放以后，多个线程同时竞争尾部，然后再依次往前排，顺序势必会打乱。

进一步改造一下ReentrantLock ，打印出他的队列内的元素。

public class ReentrantLockMine extends ReentrantLock {
    
        public ReentrantLockMine(boolean fair) {
         super(fair);
     }
    
                     @Override
             protected Collection<Thread> getQueuedThreads() {   //获取同步队列中的线程
                         List<Thread> arrayList = new ArrayList<Thread>(super.getQueuedThreads());
                         Collections.reverse(arrayList);
                         return arrayList;
                     }
     }

这边也是改一下内部的成员变量

public class MyThread2 implements  Runnable{
        ReentrantLockMine reentrantLock ;
    
        public ReentrantLock getReentrantLock() {
            return reentrantLock;
        }
    
        public void setReentrantLock(ReentrantLockMine reentrantLock) {
            this.reentrantLock = reentrantLock;
        }
    
        @Override
        public void run() {
            while (true){
                reentrantLock.lock();
                System.out.println(Thread.currentThread().getName()+"拿到了锁,當前隊列："+((ReentrantLockMine)reentrantLock).getQueuedThreads());
    
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"结束了");
                reentrantLock.unlock();
            }
        }
    }

测试方法

public static void main(String[] args) {
            ReentrantLockMine reentrantLock = new ReentrantLockMine(false);
            for (int i = 0; i < 5; i++) {
                MyThread2 myThread2 = new MyThread2();
                myThread2.setReentrantLock(reentrantLock);
                Thread thread = new Thread(myThread2);
                System.out.println(thread.getName()+"开始启动了");
                thread.start();
            }
            try {
                Thread.sleep(1500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("开始加入额外线程");
            for (int i = 0; i < 10; i++) {
                MyThread2 myThread2 = new MyThread2();
                myThread2.setReentrantLock(reentrantLock);
                Thread thread = new Thread(myThread2);
                thread.start();
            }
            System.out.println("主线程结束了");
    
        }

非公平结果：

可以发现，之后加入的线程全部按照顺序排在了后面，0号线程拿到线程后又插队获取到锁，继续执行，其他线程拿不到锁。

Thread-0开始启动了
    Thread-1开始启动了
    Thread-2开始启动了
    Thread-3开始启动了
    Thread-4开始启动了
    Thread-0拿到了锁,當前隊列：[]
    Thread-0结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main]]
    Thread-0结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main]]
    开始加入额外线程
    主线程结束了
    Thread-0结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-12,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-9,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main]]
    Thread-0结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-12,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-9,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main]]
    Thread-0结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-12,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-9,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main]]

公平锁结果：

可以发现全部按照队列顺序去执行了，不存在某个线程一直获取资源，符合预期效果。

Thread-0开始启动了
    Thread-1开始启动了
    Thread-2开始启动了
    Thread-3开始启动了
    Thread-4开始启动了
    Thread-0拿到了锁,當前隊列：[]
    Thread-0结束了
    Thread-1拿到了锁,當前隊列：[Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main]]
    Thread-1结束了
    Thread-4拿到了锁,當前隊列：[Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main]]
    开始加入额外线程
    主线程结束了
    Thread-4结束了
    Thread-2拿到了锁,當前隊列：[Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main]]
    Thread-2结束了
    Thread-3拿到了锁,當前隊列：[Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main]]
    Thread-3结束了
    Thread-0拿到了锁,當前隊列：[Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main]]
    Thread-0结束了
    Thread-1拿到了锁,當前隊列：[Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main]]
    Thread-1结束了
    Thread-5拿到了锁,當前隊列：[Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main]]
    Thread-5结束了
    Thread-7拿到了锁,當前隊列：[Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main]]
    Thread-7结束了
    Thread-6拿到了锁,當前隊列：[Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main]]
    Thread-6结束了
    Thread-8拿到了锁,當前隊列：[Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main]]
    Thread-8结束了
    Thread-9拿到了锁,當前隊列：[Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main]]
    Thread-9结束了
    Thread-10拿到了锁,當前隊列：[Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main]]
    Thread-10结束了
    Thread-11拿到了锁,當前隊列：[Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main]]
    Thread-11结束了
    Thread-12拿到了锁,當前隊列：[Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main]]
    Thread-12结束了
    Thread-13拿到了锁,當前隊列：[Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main]]
    Thread-13结束了
    Thread-14拿到了锁,當前隊列：[Thread[Thread-4,5,main], Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main]]
    Thread-14结束了
    Thread-4拿到了锁,當前隊列：[Thread[Thread-2,5,main], Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main]]
    Thread-4结束了
    Thread-2拿到了锁,當前隊列：[Thread[Thread-3,5,main], Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main]]
    Thread-2结束了
    Thread-3拿到了锁,當前隊列：[Thread[Thread-0,5,main], Thread[Thread-1,5,main], Thread[Thread-5,5,main], Thread[Thread-7,5,main], Thread[Thread-6,5,main], Thread[Thread-8,5,main], Thread[Thread-9,5,main], Thread[Thread-10,5,main], Thread[Thread-11,5,main], Thread[Thread-12,5,main], Thread[Thread-13,5,main], Thread[Thread-14,5,main], Thread[Thread-4,5,main], Thread[Thread-2,5,main]]

### 结论 ###

synchronized的队列似乎并不有序，根据Lock-Free的解释，它属于一个虚拟的队列，队列里的各个数据都在竞争锁。

Lock严格按照队列顺序执行，若非公平锁状态则可能被非队列中的现场插队执行，公平锁状态下按照队列顺序，只有头结点才回去尝试获取锁，和synchronized都在竞争锁情况相比可能更节省资源？但是同一时刻多线程入队的顺序是不确定的（雾）。

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