java - Java 同步(锁定、条件)的奇怪行为

Question

我只是想模拟一个简单的读者/作者场景。

这是代码:

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ReadersWriters implements Solution {
    private final static Lock readerLock = new ReentrantLock();
    private final static Lock writerLock = new ReentrantLock();
    private final static Condition noReader = readerLock.newCondition();
    private static CountDownLatch countDown;
    private static volatile int readerCount=0;

    public static class Reader implements Runnable {
        private static int count=1;
        private int id = count++;

        @Override
        public void run() {
            int readCount = (int) (Math.random()*20);
            while (readCount > 0) {
                readCount--;
                writerLock.lock();
                try {
                    readerLock.lock();
                    try {
                        readerCount++;
                    } finally {
                        readerLock.unlock();
                    }
                } finally {
                    writerLock.unlock();
                }
                System.out.println("Reader "+id+" reading ("+readerCount+" readers)");
                try {
                    Thread.sleep((long) (Math.random()*500));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Reader "+id+" done");
                readerLock.lock();
                try {
                    readerCount--;
                    noReader.signalAll();
                } finally {
                    readerLock.unlock();
                }
                try {
                    Thread.sleep((long) (Math.random()*500));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            countDown.countDown();
        }
    }

    public static class Writer implements Runnable {
        private static int count=1;
        private int id = count++;

        @Override
        public void run() {
            int writeCount = (int) (Math.random()*20);
            while (writeCount>0) {
                writeCount--;
                writerLock.lock();
                try {
                    readerLock.lock();
                    try {
                        while (readerCount>0) {
                            noReader.await();
                        }
                        System.out.println("Writer "+id+" writing");
                        try {
                            Thread.sleep((long) (Math.random()*500));
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println("Writer "+id+" done");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        readerLock.unlock();
                    }
                } finally {
                    writerLock.unlock();
                }
                try {
                    Thread.sleep((long) (Math.random()*500));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            countDown.countDown();
        }
    }

    public static void main(String []args) {
        Executor exec = Executors.newCachedThreadPool();
        int numReaders = 10;
        int numWriters = 4;
        countDown = new CountDownLatch(numReaders+numWriters);
        for (int i=0; i<numReaders; i++) {
            exec.execute(new Reader());
        }
        for (int i=0; i<numWriters; i++) {
            exec.execute(new Writer());
        }
        try {
            countDown.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

我知道我也可以使用ReadWriteLock，但这不是重点......

我的问题是在日志中，我看到这样的内容:

Writer 4 writing
Writer 4 done
Reader 9 reading (1 readers)
Reader 8 reading (3 readers)
Reader 5 reading (2 readers)
Reader 8 done
Reader 5 done
Reader 9 done
Writer 3 writing
Writer 3 done

我真的不明白这是怎么发生的......这只是控制台打印混淆了还是我真的在这里遗漏了一些东西？

Answer 1

我相信您的问题与这几行有关，对吗？

Reader 9 reading (1 readers)
Reader 8 reading (3 readers)
Reader 5 reading (2 readers)

您预计它是 1,2 个读者，然后是 3 个读者。

简而言之，问题是因为您的打印不是同步块(synchronized block)的一部分。

根据您的代码给出可能原因的示例，如下:

writerLock.lock();
try {
    readerLock.lock();
    try {
        readerCount++;
    } finally {
        readerLock.unlock();
    }
} finally {
    writerLock.unlock();
}
System.out.println("Reader "+id+" reading ("+readerCount+" readers)");

对于上面的代码，简单来说，readerCount的更新是由writerLock来保护的。然而，有可能:

READER8                         READER5
(readerCount = 1 at this point)

lock writerLock
readerCount++  (=2)
unlock writerLock
                                lock writerLock               
                                update readerCount to 3
                                unlock writerLock

sysout of readerCount (3)
lock writerLock
readerCount-- (=2)

                                sysout of readerCount (2)
unlock writerLock
                                lock writerLock
                                readerCount-- (=1)
                                unlock writerLock

不难想象为什么这个数字看起来很奇怪。

将系统 out 语句放在锁定范围内，就在 readerCount++ 之后，将给出预期的结果。