類具有2個同步方法。.當一個線程位於第二個方法內時，另一個線程可以進入第一個方法嗎？

Question

我知道之前曾問過“同時執行兩個同步方法”問題，給出的答案是“否。只有一個線程可以持有對象的鎖。然后只有那個線程才能在該對象上輸入同步方法。 “。 但是，如果它是真的，那么請幫助我理解以下代碼的輸出：

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

每次我運行此程序時，輸出都會變化。 我的疑問是當第二/第三線程獲得CPU時，為什么它沒有將“ count”讀為1（由第一線程更新）？ 換句話說，當第一個線程鎖定了“ syn”並正在將計數更新為1時，第二個/第三個線程如何設法進入“ getCount”方法。

Answer 1

語句System.out.println("I am "+Thread.currentThread().getName()+".Count is "+ syn.getCount()); 不是以原子方式執行的。 syn.getCount()部分可以在println之前執行。 您的示例中似乎也發生了這種情況。 所有線程首先執行syn.getCount() ，當時計數仍為0，然后再執行其余操作。

Answer 2

您發布的該特定執行的結果是，所有三個線程都讀取值為零的計數器，但是線程一有機會在其他線程之前完成其所有輸出，即，對計數器的訪問已正確同步，但是稍后的打印顯示較舊的值歸因於輸出緩沖。

嘗試在第一個線程開始之后進入睡眠狀態，這種情況不太可能再次發生。

Answer 3

正如Henry提到的，所有線程都可以首先執行syn.getCount（），然后執行syn.incrementCount。 我已經稍微修改了您的代碼以更改println語句，從而獲得了更好的畫面...

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());
    }

}

}

不同的輸出如下所示...

輸出＃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

輸出＃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