类具有2个同步方法。.当一个线程位于第二个方法内时，另一个线程可以进入第一个方法吗？

Question

我知道之前曾问过“同时执行两个同步方法”问题，给出的答案是“否。只有一个线程可以持有对象的锁。然后只有那个线程才能在该对象上输入同步方法。 “。 但是，如果它是真的，那么请帮助我理解以下代码的输出：

public class SynchronizationMistake {

public  int count;

public synchronized int getCount()
{
    return count;
}
public synchronized String incrementCount()
{
    count++;
    return"";
}

public static void main (String args[])
{
    SynchronizationMistake syn = new SynchronizationMistake();
    Thread first = new Thread(syn.new readIncrementCount(syn),"First");
    Thread second = new Thread(syn.new readIncrementCount(syn), "Second");
    Thread third = new Thread(syn.new readIncrementCount(syn), "Third");

    first.start();
    second.start();
    third.start();

}
private class readIncrementCount implements Runnable
{

    SynchronizationMistake syn;
    readIncrementCount(SynchronizationMistake syn)
    {
        this.syn = syn;
    }
    @Override
    public void run() {

        System.out.println("I am "+Thread.currentThread().getName()+".Count is "+ syn.getCount());
        System.out.println("I am "+Thread.currentThread().getName()+".Incrementing count now"+syn.incrementCount());
        System.out.println("I am "+Thread.currentThread().getName()+". After Increment Count is "+ syn.getCount());


    }

}

}

Output is :
I am First.Count is 0
I am First.Incrementing count now
I am First. After Increment Count is 1
I am Third.Count is 0
I am Second.Count is 0
I am Third.Incrementing count now
I am Second.Incrementing count now
I am Third. After Increment Count is 3
I am Second. After Increment Count is 3

每次我运行此程序时，输出都会变化。 我的疑问是当第二/第三线程获得CPU时，为什么它没有将“ count”读为1（由第一线程更新）？ 换句话说，当第一个线程锁定了“ syn”并正在将计数更新为1时，第二个/第三个线程如何设法进入“ getCount”方法。

Answer 1

语句System.out.println("I am "+Thread.currentThread().getName()+".Count is "+ syn.getCount()); 不是以原子方式执行的。 syn.getCount()部分可以在println之前执行。 您的示例中似乎也发生了这种情况。 所有线程首先执行syn.getCount() ，当时计数仍为0，然后再执行其余操作。

Answer 2

您发布的该特定执行的结果是，所有三个线程都读取值为零的计数器，但是线程一有机会在其他线程之前完成其所有输出，即，对计数器的访问已正确同步，但是稍后的打印显示较旧的值归因于输出缓冲。

尝试在第一个线程开始之后进入睡眠状态，这种情况不太可能再次发生。

Answer 3

正如Henry提到的，所有线程都可以首先执行syn.getCount（），然后执行syn.incrementCount。 我已经稍微修改了您的代码以更改println语句，从而获得了更好的画面...

public class SynchronizationMistake {

public  int count;

public synchronized int getCount()
{
    return count;
}

public synchronized String incrementCount() {

    System.out.println("I am " + Thread.currentThread().getName() + ", incrementing the count from " + count);
    count++;
    System.out.println("I am " + Thread.currentThread().getName() + ", incremented the count to " + count);
    return"";
}

public static void main (String args[])
{
    SynchronizationMistake syn = new SynchronizationMistake();
    Thread first = new Thread(syn.new readIncrementCount(syn),"First");
    Thread second = new Thread(syn.new readIncrementCount(syn), "Second");
    Thread third = new Thread(syn.new readIncrementCount(syn), "Third");

    first.start();
    second.start();
    third.start();

}
private class readIncrementCount implements Runnable
{

    SynchronizationMistake syn;
    readIncrementCount(SynchronizationMistake syn)
    {
        this.syn = syn;
    }
    @Override
    public void run() {

        System.out.println("Current count is " + syn.getCount());
        System.out.println(syn.incrementCount());
        System.out.println("Current count is " + syn.getCount());
    }

}

}

不同的输出如下所示...

输出＃1：

 Current count is 0
 I am Second, incrementing the count from 0
 Current count is 0
 I am Second, incremented the count to 1

 Current count is 1
 I am First, incrementing the count from 1
 I am First, incremented the count to 2

 Current count is 2
 Current count is 2
 I am Third, incrementing the count from 2
 I am Third, incremented the count to 3

 Current count is 3

输出＃2：

Current count is 0
Current count is 0
Current count is 0
I am Second, incrementing the count from 0
I am Second, incremented the count to 1

I am Third, incrementing the count from 1
I am Third, incremented the count to 2

I am First, incrementing the count from 2
I am First, incremented the count to 3

Current count is 3
Current count is 3
Current count is 3